Category Archive Anatomy A – Z

The somatosensory system is a part of the sensory nervous system. The somatosensory system is a complex system of sensory neurons and neural pathways that responds to changes at the surface or inside the body. The axons (as afferent nerve fibers) of sensory neurons connect with or respond to, various receptor cells.

The somatosensory system comprises those elements of the peripheral nervous system (PNS) and the central nervous system (CNS) subserving the modalities of touch, vibration, temperature, pain, and kinesthesia. Specific modalities can be associated with unique peripheral receptors, peripheral axons of stereotyped diameter, and specific central projection pathways. Several features of the somatosensory system render regions of it vulnerable to a wide variety of toxicants. The present report highlights these features and, furthermore, suggests that analysis of these regions is invaluable in studying the three most common varieties of toxic neuropathy: toxic distal axonopathy, toxic myelinopathy, and toxic sensory neuronopathy.

Structure of

Somatic Sensory Pathways

The somatosensory pathway is composed of three neurons located in the dorsal root ganglion, the spinal cord, and the thalamus.

Key Points

A somatosensory pathway will typically have three neurons: primary, secondary, and tertiary.

The cell bodies of the three neurons in a typical somatosensory pathway are located in the dorsal root ganglion, the spinal cord, and the thalamus.

A major target of somatosensory pathways is the postcentral gyrus in the parietal lobe of the cerebral cortex.

A major somatosensory pathway is the dorsal column–medial lemniscal pathway.

The postcentral gyrus is the location of the primary somatosensory area that takes the form of a map called the sensory homunculus.

Key Terms

parietal lobe: A part of the brain positioned superior to the occipital lobe and posterior to the frontal lobe that integrates sensory information from different modalities and is particularly important for determining spatial sense and navigation.

reticular activating system: A set of connected nuclei in the brain responsible for regulating wakefulness and sleep-to-wake transitions.

postcentral gyrus: A prominent structure in the parietal lobe of the human brain that is the location of the primary somatosensory cortex, the main sensory receptive area for the sense of touch.

thalamus: Either of two large, ovoid structures of gray matter within the forebrain that relay sensory impulses to the cerebral cortex.

A somatosensory pathway will typically have three long neurons: primary, secondary, and tertiary. The first always has its cell body in the dorsal root ganglion of the spinal nerve.

Dorsal root ganglion: Sensory nerves of a dorsal root ganglion are depicted entering the spinal cord.

The second has its cell body either in the spinal cord or in the brainstem; this neuron’s ascending axons will cross to the opposite side either in the spinal cord or in the brainstem. The axons of many of these neurons terminate in the thalamus, and others terminate in the reticular activating system or the cerebellum.

In the case of touch and certain types of pain, the third neuron has its cell body in the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal lobe.

In the periphery, the somatosensory system detects various stimuli by sensory receptors, such as by mechanoreceptors for tactile sensation and nociceptors for pain sensation. The sensory information (touch, pain, temperature, etc.,) is then conveyed to the central nervous system by afferent neurons, of which there are a number of different types with varying size, structure, and properties.

Generally, there is a correlation between the type of sensory modality detected and the type of afferent neuron involved. For example, slow, thin, unmyelinated neurons conduct pain whereas faster, thicker, myelinated neurons conduct casual touch.

Ascending Pathways

In the spinal cord, the somatosensory system includes ascending pathways from the body to the brain. One major target within the brain is the postcentral gyrus in the cerebral cortex. This is the target for neurons of the dorsal column–medial lemniscal pathway and the ventral spinothalamic pathway.

Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex. Other ascending pathways, particularly those involved with control of posture, are projected to the cerebellum, including the ventral and dorsal spinocerebellar tracts.

Another important target for afferent somatosensory neurons that enter the spinal cord are those neurons involved with local segmental reflexes.

Spinal nerve: The formation of the spinal nerve from the dorsal and ventral roots.

Parietal Love: Primary Somatosensory Area

The primary somatosensory area in the human cortex is located in the postcentral gyrus of the parietal lobe. This is the main sensory receptive area for the sense of touch.

Like other sensory areas, there is a map of sensory space called a homunculus at this location. Areas of this part of the human brain map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area.

For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area. Somatosensory information involved with proprioception and posture also target an entirely different part of the brain, the cerebellum.

Cortical Homunculus

This is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex; namely, the portion of the human brain directly responsible for the movement and exchange of sensory and motor information of the body.

Homunculus: Image representing the cortical sensory homunculus.

Thalamus

The thalamus is a midline symmetrical structure within the brain of vertebrates including humans; it is situated between the cerebral cortex and midbrain, and surrounds the third ventricle.

Its function includes relaying sensory and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep, and alertness.

Thalamic nuclei: The ventral posterolateral nucleus receives sensory information from the body.

Sensory receptors are dendrites of sensory neurons specialized for receiving specific kinds of stimuli. Sensory receptors are classified by three methods Classification by receptor complexity: Free nerve endings are dendrites whose terminal ends have little or no physical specialization.

A sensory receptor, or sense organ, is the part that responds to a stimulus in the internal or external environment of an organism. It is the input to the sensory system.^[rx] In response to stimuli, the sensory receptor fires off a nerve fiber that goes to the central nervous system.

Classification by receptor complexity

• Free nerve endings are dendrites whose terminal ends have little or no physical specialization.
• Encapsulated nerve endings are dendrites whose terminal ends are enclosed in a capsule of connective tissue.
• Sense organs (such as the eyes and ears) consist of sensory neurons with receptors for the special senses (vision, hearing, smell, taste, and equilibrium) together with connective, epithelial, or other tissues.

Classification by location

• Exteroceptors occur at or near the surface of the skin and are sensitive to stimuli occurring outside or on the surface of the body. These receptors include those for tactile sensations, such as touch, pain, and temperature, as well as those for vision, hearing, smell, and taste.
• Interoceptors (visceroceptors) respond to stimuli occurring in the body from visceral organs and blood vessels. These receptors are the sensory neurons associated with the autonomic nervous system.
• Proprioceptors respond to stimuli occurring in skeletal muscles, tendons, ligaments, and joints. These receptors collect information concerning body position and the physical conditions of these locations.

Classification by type of stimulus detected

• Mechanoreceptors respond to physical force such as pressure (touch or blood pressure) and stretch.
• Photoreceptors respond to light.
• Thermoreceptors respond to temperature changes.
• Chemoreceptors respond to dissolved chemicals during sensations of taste and smell and to changes in internal body chemistry such as variations of O ₂, CO ₂, or H ⁺ in the blood.
• Nociceptors respond to a variety of stimuli associated with tissue damage. The brain interprets the pain.

A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to stimuli. Although traditionally around five human senses were known (namely sight, smell, touch, taste, and hearing), it is now recognized that there are many more. Senses used by other non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli (such as a sound or smell) for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism’s cognition, behavior and thought.

Overview of Sensation

Sensation refers to our ability to detect or sense the physical qualities of our environment.

Our senses can be broadly grouped into exteroception, for the detection of stimuli that occur outside of our body, and interoception, for stimuli occurring inside of our bodies. However, what constitutes a sense is a matter of great debate, leading to difficulties in precisely defining what it is. Traditionally, human beings are considered to have five main senses: sight, hearing, taste, smell, and touch.

The peripheral nervous system (PNS) consists of sensory receptors that extend from the central nervous system (CNS) to communicate with other parts of the body. These receptors respond to changes and stimuli in the environment. Sense organs (made up of sensory receptors and other cells ) operate the senses of vision, hearing, equilibrium, smell, and taste.

Sight

Sight or vision (ophthalmoception) is the ability of the eye(s) to focus and detect images of visible light on photoreceptors in the retina that generate electrical nerve impulses for varying colors, hues, and brightness. There are two types of photoreceptors: rods and cones. Rods are very sensitive to light but do not distinguish colors. Cones distinguish colors but are less sensitive to dim light. The inability to see is called blindness.

Hearing

Hearing or audition (audio option) is the sense of sound perception. Mechanoreceptors in the inner ear turn vibration motion into electrical nerve pulses. The vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear that detect the mechanical motion of the fibers.

Sound can also be detected as vibrations conducted through the body by a tactician. The inability to hear is called deafness or hearing impairment.

Taste6

Taste (gustaoception) refers to the ability to detect substances such as food, certain minerals, poisons, etc. The sense of taste is often confused with the concept of flavor, which is a combination of taste and smell perception. The flavor depends on odor, texture, and temperature as well as on taste.

Humans receive tastes through sensory organs called taste buds, or gustatory calyculi, concentrated on the upper surface of the tongue. Five basic tastes exist: sweet, bitter, sour, salty, and umami. The inability to taste is called ageusia.

Smell

The olfactory system is the sensory system used for the sense of smell (olfaction). This sense is mediated by specialized sensory cells of the nasal cavity.  In humans, olfaction occurs when odorant molecules bind to specific sites on the olfactory receptors in the nasal cavity. These receptors are used to detect the presence of smell. They come together at a structure (the glomerulus) that transmits signals to the olfactory bulb in the brain. The inability to smell is called anosmia.

Touch

Touch or somatosensation (tactioception, tactician, or mechanoreception), is a perception resulting from the activation of neural receptors in the skin, including hair follicles, tongue, throat, and mucosa. A variety of pressure receptors respond to variations in pressure (firm, brushing, sustained, etc.).

The touch sense of itching is caused by insect bites or allergies that involve special itch-specific neurons in the skin and spinal cord. The loss or impairment of the ability to feel anything touched is called tactile anesthesia.

Paresthesia is a sensation of tingling, pricking, or numbness of the skin that may result from nerve damage and may be permanent or temporary.

Types of Sensation

A third sensory modality requires cortical analysis to provide a more complex interpretation of primary sensory information. All three types of sensation should be evaluated in every patient examined.

Exteroceptive sensation (also termed superficial sensation): receptors in the skin and mucous membranes

Tactile or touch sensation (thigmesthesia):

• Anesthesia: absence of touch appreciation
• Hypoesthesia: decrease of touch appreciation
• Hyperesthesia: exaggeration of touch sensation, which is often unpleasant
• (Terms above are unfortunately used indiscriminately to apply to losses of all types of sensation. They are not specific for loss of tactile sensation.)

Pain sensation (algesia):

• Analgesia: absence of pain appreciation
• Hypoalgesia: decrease of pain appreciation
• Hyperalgesia: exaggeration of pain appreciation, which is often unpleasant

Temperature sensation, both hot and cold (thermesthesia):

• Thermanalgesia: absence of temperature appreciation
• Thermhypesthesia: decrease of temperature appreciation
• Thermhyperesthesia: exaggeration of temperature sensation, which is often unpleasant

Sensory perversions :

• Paresthesia: abnormal sensations perceived without specific stimulation. They may be tactile, thermal, or painful; episodic or constant.
• Dysesthesia: painful sensations elicited by a nonpainful cutaneous stimulus such as a light touch or gentle stroking over affected areas of the body. Sometimes referred to as hyperpathia or hyperalgesia. Often perceived as intense burning, dysesthesias may outlast the stimulus by several seconds.

Proprioceptive sensation (also termed deep sensation): receptors located in muscles, tendons, ligaments, and joints

• Joint position sense (arthresthesia): Absence is described as such
• Vibratory sense (pallesthesia): Absence is described as such
• Kinesthesia: perception of muscular motion. Usually not measured in routine clinical evaluation.

Cortical sensory functions: interpretative sensory functions that require analysis of individual sensory modalities by the parietal lobes to provide discrimination. Individual sensory modalities must be intact to measure cortical sensation.

• Stereognosis: the ability to recognize and identify objects by feeling them. The absence of this ability is termed astereognosis.
• Graphesthesia: ability to recognize symbols written on the skin. The absence of this ability is termed graphanesthesia.
• Two-point discrimination: ability to recognize simultaneous stimulation by two blunt points. Measured by the distance between the points required for recognition. Absence is described as such.
• Touch localization (topognosis): ability to localize stimuli to parts of the body. Topagnosia is the absence of this ability.
• Double simultaneous stimulation: ability to perceive a sensory stimulus when corresponding areas on the opposite side of the body are stimulated simultaneously. Loss of this ability is termed sensory extinction.

Additional Senses

Many scientists and philosophers argue that humans have additional senses including:

• Pain or nociception (physiological pain): Signals nerve and other tissue damage.
• Balance or equilibrioception: Allows the sensing of body movement, direction, and acceleration, and to attain and maintain postural equilibrium and balance.
• Body awareness or proprioception: Provides the parietal cortex of the brain with information on the relative positions of the parts of the body.
• Sense of time or conception: This refers to how the passage of time is perceived and experienced but is not associated with a specific sensory system. According to psychologists and neuroscientists, however, human brains have a system governing the perception of time.
• Temperature sensing or thermoception: The sensation of heat and the absence of heat (cold).

If interoceptive senses are also considered, the sensation can be expanded to include stretch (as in muscles or organs like the lungs), oxygen and carbon dioxide sensing, pH sensing, and more.

While the exact definition of sensation is still controversial, most scientists agree that all senses rely on four fundamental sensory capacities:

• Chemical detection (chemoreception).
• Light detection (photoreception).
• Force detection (mechanoreception).
• Temperature detection (chemoreception).

Our nervous system has sensory systems and organs that mediate each sense and these systems rely on chemoreceptors, photoreceptors, mechanoreceptors, or thermoreceptors to detect the state of the internal or external environment.

Other internal sensations and perceptions

An internal sensation and perception also known as interoception^[rx] is “any sense that is normally stimulated from within the body”.^[rx] These involve numerous sensory receptors in internal organs. Interoception is thought to be atypical in clinical conditions such as alexithymia.^[rx] Some examples of specific receptors are:

• Hunger is governed by a set of brain structures (e.g., the hypothalamus) that are responsible for energy homeostasis.^[rx]
• Pulmonary stretch receptors are found in the lungs and control the respiratory rate.
• Peripheral chemoreceptors in the brain monitor the carbon dioxide and oxygen levels in the brain to give a perception of suffocation if carbon dioxide levels get too high.^[52]
• The chemoreceptor trigger zone is an area of the medulla in the brain that receives inputs from blood-borne drugs or hormones and communicates with the vomiting center.
• Chemoreceptors in the circulatory system also measure salt levels and prompt thirst if they get too high; they can also respond to high blood sugar levels in diabetics.
• Cutaneous receptors in the skin not only respond to touch, pressure, temperature and vibration, but also respond to vasodilation in the skin such as blushing.
• Stretch receptors in the gastrointestinal tract sense gas distension that may result in colic pain.
• Stimulation of sensory receptors in the esophagus results in sensations felt in the throat when swallowing, vomiting, or acid reflux.
• Sensory receptors in pharynx mucosa, similar to touch receptors in the skin, sense foreign objects such as mucous and food that may result in a gag reflex and corresponding gagging sensation.
• Stimulation of sensory receptors in the urinary bladder and rectum may result in perceptions of fullness.
• Stimulation of stretch sensors that sense dilation of various blood vessels may result in pain, for example headache caused by vasodilation of brain arteries.
• Cardioversion refers to the perception of the activity of the heart.^{[rx][rx][rx][rx]}
• Opsins and direct DNA damage in melanocytes and keratinocytes can sense ultraviolet radiation, which plays a role in pigmentation and sunburn.
• Baroreceptors relay blood pressure information to the brain and maintain proper homeostatic blood pressure.

The perception of time is also sometimes called a sense, though not tied to a specific receptor.

Sensation to Perception

The goal of sensation is detection, while the goal of perception is to create useful information about our environment.

Sensation and perception are two distinct stages of processing during human sensing. The sensation is a function of the low level, biochemical, and neurological mechanisms that allow the receptor cells of a sensory organ to detect an environmental stimulus.

Stimuli from the environment (distal stimuli) are transformed into neural signals, which are then interpreted by the brain through a process called transduction. Transduction can be likened to a bridge connecting sensation to perception. This raw pattern of neural activity is called the proximal stimulus. Perception refers to the mental processes that are reflected in statements like “I see a blue wall” that represent the understanding of the real-world causes of sensory input. In other words, the goal of sensation is detection, while the goal of perception is to create useful information about the environment.

The neural signals are transmitted to the brain and processed. The resulting mental recreation of the distal stimulus is the percept. The sound stimulating a person’s auditory receptors is the proximal stimulus, and the brain’s interpretation of this as the ringing of a telephone is the percept.

All perception involves signals in the nervous system that result from physical stimulation of the sense organs. For example, vision involves light striking the retinas of the eyes, the smell is mediated by odor molecules, and hearing involves sound waves.

However, perception is not the passive receipt of these signals but is a process of organization, identification, and interpretation. Although the senses were traditionally viewed as passive receptors, the study of illusions and ambiguous images has demonstrated that the brain’s perceptual systems actively influence sensory systems in an attempt to construct useful representations of our environment.

Perception is particularly important to our ability to understand speech. The sound of a word can vary widely according to the words around it and the tempo of the speech, as well as the physical characteristics, accent, and mood of the speaker. Listeners manage to perceive words across this wide range of different conditions. Another variation is that reverberation can make a large difference in sounds, such as hearing a word spoken from the far side of a room and the same word spoken up close. The process of perceiving speech begins at the level of the sound within the auditory signal and the process of audition. After processing the initial auditory signal, speech sounds are further processed to extract acoustic cues and phonetic information. This speech information can then be used for higher-level language processes, such as word recognition.

Sensory Modalities

A sensory modality (also called a stimulus modality) is an aspect of a stimulus or what is perceived after a stimulus.

Sensing

Senses are transducers from the physical world to the realm of the mind. Another broadly acceptable definition of a sense is a system that consists of a group of sensory cell types, responding to a specific physical phenomenon, and corresponding to a particular group of regions within the brain where the signals are received and interpreted.

Disputes about the number of senses typically arise around the classification of the various cell types and their mapping to regions of the brain.

Sensory Modalities

A sensory modality (also called a stimulus modality) is an aspect of a stimulus or what is perceived after a stimulus. The term sensory modality is often used interchangeably with sense. The basic sensory modalities include light, sound, taste, temperature, pressure, and smell.

Light Modality

The sensory modality for vision is light. To perceive a light stimulus, the eye must first refract the light so that it directly hits the retina. The transduction of light into neural activity occurs via the photoreceptors in the retina.

When a particle of light hits the photoreceptors of the eye, the photopigment of the photoreceptor undergoes a chemical change leading to a chain of chemical reactions occur. A message is sent to a neuron called the bipolar cell through the use of a nerve impulse. Finally, a message is sent to the ganglion cell and then, finally, the brain.

Sound Modality

The sensory modality for audition is sound. Sound is created through air pressure. A vibrating object compresses the surrounding molecules of air as it moves towards a given point, and expands the molecules as it moves away from the point.

The eardrum is stimulated by vibrations in the air. It collects and sends these vibrations to receptor cells. The ossicles (three tiny bones in the middle ear) pass the vibrations to the fluid-filled cochlea (a spiral, shell-shaped auditory organ of the inner ear ). The vibrations move through the liquid in the cochlea where the receptive organ is able to sense it.

Taste Modality

Taste stimuli are encountered by receptor cells located in taste buds on the tongue and pharynx. Receptor cells disseminate onto different neurons and convey the message of a particular taste in a single medullar nucleus.

Taste perception is created by combining multiple sensory inputs. Different modalities help determine the perception of taste.

Temperature Modality

Temperature modality excites or elicits a symptom through cold or hot temperature. The cutaneous somatosensory system detects changes in temperature.

Thermal stimuli from a homeostatic set point excite temperature-specific sensory nerves in the skin. Specific thermosensory fibers respond to warmth and too cold.

Pressure Modality

Tactile stimulation can be direct, such as through bodily contact, or indirect, such as through the use of a tool or probe. Tactual perception gives information regarding cutaneous stimuli (pressure, vibration, and temperature), kinesthetic stimuli (limb movement), and proprioceptive stimuli (position of the body).

Smell Modality

The sense of smell is called olfaction. Materials constantly shed molecules, which float into the nose or are taken in through breathing. Inside the nasal chambers is the neuroepithelium lining.

It contains the receptors responsible for detecting molecules that are small enough to smell. These receptor neurons then synapse at the olfactory cranial nerve, which sends the information to the olfactory bulbs in the brain for initial processing.

Multimodal Perception

Multimodal perception is the ability of the mammalian nervous system to combine all of the different inputs of the sensory system to result in an enhanced detection or identification of a particular stimulus.

Integration of all sensory modalities occurs when multimodal neurons receive sensory information that overlaps with different modalities. Multimodal perception comes into effect when a unimodal stimulus fails to produce a response.

Additional Senses

Balance (or equilibrioception) is the sense that allows an organism to sense body movement, direction, and acceleration, and also attain and maintain postural equilibrium and balance. The organ of equilibrioception is the vestibular labyrinthine system found in both of the inner ears.

In technical terms, this organ is responsible for two senses: angular momentum and acceleration (known together as equilibrioception). The vestibular nerve conducts information from sensory receptors in three ampullae, each of which senses fluid motion in three semicircular canals caused by a three-dimensional rotation of the head.

The vestibular nerve also conducts information from the utricle and the saccule; these contain hair-like sensory receptors that bend under the weight of otoliths (small crystals of calcium carbonate) that provide the inertia needed to detect head rotation, linear acceleration, and the direction of gravitational force.

Thermoception is the sense of heat or absence of heat (cold) by the skin and internal skin passages. Perceiving changes in temperature is referred to as heat flux (the rate of heat flow) in these areas.

There are specialized receptors for cold (declining temperature) and heat. The cold receptors infer wind direction, an important part of the animal’s sense of smell. The heat receptors are sensitive to infrared radiation and can occur in specialized organs, for instance in pit vipers.

The thermoreceptors in the skin are quite different from the homeostatic thermoreceptors in the brain (hypothalamus), which provides feedback on internal body temperature.

Proprioception, the kinesthetic sense, provides the parietal cortex of the brain with information on the relative positions of the parts of the body. Neurologists test this sense by telling patients to close their eyes and touch their own nose with the tip of a finger. Assuming proper proprioceptive function, at no time will the person lose awareness of where their hand actually is, even though it is not being detected by any of the other senses. Proprioception and touch are related in subtle ways, and their impairment results in deep and surprising deficits in perception and action.

Nociception (physiological pain) signals nerve or other tissue damage. The three types of pain receptors are cutaneous (skin), somatic (joints and bones), and visceral (body organs).

It was previously believed that pain was simply the overloading of pressure receptors, but research in the first half of the 20th century showed that pain is a distinct phenomenon that intertwines with all of the other senses, including touch.

Conception refers to how the passage of time is perceived and experienced. Although the sense of time is not associated with a specific sensory system, psychological and neuroscientific research indicates that human brains do have a system governing the perception of time.

It is composed of a highly distributed system involving the cerebral cortex, cerebellum, and basal ganglia. One particular component, the suprachiasmatic nucleus, is responsible for the circadian (daily) rhythm, while other cell clusters appear capable of shorter-range (ultradian) timekeeping.

Cellular

Receptors

Sensory receptors become activated by stimuli in the environment by receiving signals. The transmission of any message in the neurons of our body requires it to be in the form of an action potential; the sensation must undergo conversion into electrical signals. The structures which convert mechanical signals into electrical signals are receptors.

Receptors get classified based on the type of stimulus activating them. The following are common types of receptors

Mechanoreceptors

• Activated by changes in pressure
• Common mechanoreceptors include
• Pacinian corpuscles in the subcutaneous tissue
• Meissner corpuscles in non-hairy skin
• Baroreceptors in the carotid sinus
• Hair cells on the organ of Corti and in the semicircular canals

Photoreceptors

• Activated by light

• Rods and cones (located in the retina)

  • Rods are sensitive to low-intensity light and function better in a dark environment
  • Cones have a better threshold for light, therefore function well in daylight. They also participate in color vision.

Chemoreceptors

• Activated by chemicals
• They serve for olfaction and taste

Thermoreceptors

• Located in the skin and include cold and warm receptors. These are present in the skin.

Nociceptors

• These become activated by extreme pressure, temperature, or noxious chemicals. These are also in the skin.

There are different types of receptors present into the skin or muscles for all modalities of senses.[rx]

1) Touch

Human skin divides into hairy and nonhairy or glabrous skin. This classification is associated with different touch receptors inside the skin. There are four types of touch receptors. Those which are slowly adapting called Merkel cells (slowly adapting type 1) and Ruffini corpuscles (slowly adapting type2). Those with Pacinian afferents called Pacinian corpuscles and rapidly adaptive ones called Meissner corpuscles.[rx]

• Merkel’s disc

Structurally simplest among all are Merkel cells which are present in the basal layer of the epidermis. These are located in non-hairy skin. Present in a high amount at the fingertips, they are sensitive selectively to the particular component of stress or strain, and because of that they are more selective in detecting corners, edges and curvatures; this is useful for reading Braille, etc.

• Ruffini nerve endings

These types of receptors are less densely placed and therefore have less sensitivity. These receptors are located in hairy skin. They are more sensitive to stretch, so become stimulated during stretching of the skin. An example is stretching during motion and for the direction of force detection, along with muscle spindle for hand shape and finger position perception, etc.

• Pacinian corpuscle

Distributed throughout the palm and finger, the Pacinian corpuscle is the most sensitive receptor type. These are large, onion-like layered structures enclosing a single nerve ending. Pacinian corpuscles function as a mechanical filter to protect from very large, low-frequency stress during manual labor.

• Meissner corpuscle      

They are most sensitive to dynamic changes in the skin, and they are relatively insensitive for static changes of the skin.

One distinguishing characteristic of each receptor is the degree of adaptation.

• “Very rapidly adapting” such as Pacinian corpuscle
•  “Rapidly adapting” such as Meissner corpuscle and hair follicles
• “Slowly adapting” includes Ruffini corpuscle, Merkel receptors, and tactile discs

 2) Thermal receptors

These are slowly adapting receptors that can detect changes in skin temperature. These may be cold or warm. These receptors have some baseline firing rates. Cold receptors are sensitive between 10 and 32 degrees C. Firing at a baseline rate during 30 to 35 degrees C, the firing rate increases in cold while it decreases when the temperature increases. In the same way, warm receptors have baseline firing around 38 degrees C and increase with an increase in temperature.[rx]

3) Vibration

Vibration sensations are useful in the performance of balancing tasks, along with proprioception. Vibrational sense perception is by Pacinian corpuscles and Meissner corpuscles because both of these receptors are sensitive to low-frequency vibrations.[rx]

4) Pain

Noxious stimulation is converted into an electric signal by unencapsulated nerve endings that terminate in the dermis and epidermis. Noxious stimuli such as intense hot or cold, or long-standing pressure cause activation of free nerve endings; this requires threshold stimulation to activate the endings, but once activated, signals are transmitted continuously.[rx]

5) Proprioception

The Golgi tendon organ senses the position of joints and joint movement near attached muscle tendons attached, and by muscle, spindles present inside the extrafusal muscle fibers. They fire signals when stretched.

References

A sensation is a physical feeling. Floating can be a very pleasant sensation. A sensation of burning or tingling may be experienced in the hands. [ + of] Synonyms are feeling, sense, impression, perception More Synonyms of sensation.

A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to stimuli. Although traditionally around five human senses were known (namely sight, smell, touch, taste, and hearing), it is now recognized that there are many more. Senses used by other non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli (such as a sound or smell) for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism’s cognition, behavior and thought.

Overview of Sensation

Sensation refers to our ability to detect or sense the physical qualities of our environment.

Our senses can be broadly grouped into exteroception, for the detection of stimuli that occur outside of our body, and interoception, for stimuli occurring inside of our bodies. However, what constitutes a sense is a matter of great debate, leading to difficulties in precisely defining what it is. Traditionally, human beings are considered to have five main senses: sight, hearing, taste, smell, and touch.

The peripheral nervous system (PNS) consists of sensory receptors that extend from the central nervous system (CNS) to communicate with other parts of the body. These receptors respond to changes and stimuli in the environment. Sense organs (made up of sensory receptors and other cells ) operate the senses of vision, hearing, equilibrium, smell, and taste.

Sight

Sight or vision (ophthalmoception) is the ability of the eye(s) to focus and detect images of visible light on photoreceptors in the retina that generate electrical nerve impulses for varying colors, hues, and brightness. There are two types of photoreceptors: rods and cones. Rods are very sensitive to light but do not distinguish colors. Cones distinguish colors but are less sensitive to dim light. The inability to see is called blindness.

Hearing

Hearing or audition (audio option) is the sense of sound perception. Mechanoreceptors in the inner ear turn vibration motion into electrical nerve pulses. The vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear that detect the mechanical motion of the fibers.

Sound can also be detected as vibrations conducted through the body by a tactician. The inability to hear is called deafness or hearing impairment.

Taste6

Taste (gustaoception) refers to the ability to detect substances such as food, certain minerals, poisons, etc. The sense of taste is often confused with the concept of flavor, which is a combination of taste and smell perception. The flavor depends on odor, texture, and temperature as well as on taste.

Humans receive tastes through sensory organs called taste buds, or gustatory calyculi, concentrated on the upper surface of the tongue. Five basic tastes exist: sweet, bitter, sour, salty, and umami. The inability to taste is called ageusia.

Smell

The olfactory system is the sensory system used for the sense of smell (olfaction). This sense is mediated by specialized sensory cells of the nasal cavity.  In humans, olfaction occurs when odorant molecules bind to specific sites on the olfactory receptors in the nasal cavity. These receptors are used to detect the presence of smell. They come together at a structure (the glomerulus) that transmits signals to the olfactory bulb in the brain. The inability to smell is called anosmia.

Touch

Touch or somatosensation (tactioception, tactician, or mechanoreception), is a perception resulting from the activation of neural receptors in the skin, including hair follicles, tongue, throat, and mucosa. A variety of pressure receptors respond to variations in pressure (firm, brushing, sustained, etc.).

The touch sense of itching is caused by insect bites or allergies that involve special itch-specific neurons in the skin and spinal cord. The loss or impairment of the ability to feel anything touched is called tactile anesthesia.

Paresthesia is a sensation of tingling, pricking, or numbness of the skin that may result from nerve damage and may be permanent or temporary.

Types of Sensation

A third sensory modality requires cortical analysis to provide a more complex interpretation of primary sensory information. All three types of sensation should be evaluated in every patient examined.

Exteroceptive sensation (also termed superficial sensation): receptors in the skin and mucous membranes

Tactile or touch sensation (thigmesthesia):

• Anesthesia: absence of touch appreciation
• Hypoesthesia: decrease of touch appreciation
• Hyperesthesia: exaggeration of touch sensation, which is often unpleasant
• (Terms above are unfortunately used indiscriminately to apply to losses of all types of sensation. They are not specific for loss of tactile sensation.)

Pain sensation (algesia):

• Analgesia: absence of pain appreciation
• Hypoalgesia: decrease of pain appreciation
• Hyperalgesia: exaggeration of pain appreciation, which is often unpleasant

Temperature sensation, both hot and cold (thermesthesia):

• Thermanalgesia: absence of temperature appreciation
• Thermhypesthesia: decrease of temperature appreciation
• Thermhyperesthesia: exaggeration of temperature sensation, which is often unpleasant

Sensory perversions :

• Paresthesia: abnormal sensations perceived without specific stimulation. They may be tactile, thermal, or painful; episodic or constant.
• Dysesthesia: painful sensations elicited by a nonpainful cutaneous stimulus such as a light touch or gentle stroking over affected areas of the body. Sometimes referred to as hyperpathia or hyperalgesia. Often perceived as intense burning, dysesthesias may outlast the stimulus by several seconds.

Proprioceptive sensation (also termed deep sensation): receptors located in muscles, tendons, ligaments, and joints

• Joint position sense (arthresthesia): Absence is described as such
• Vibratory sense (pallesthesia): Absence is described as such
• Kinesthesia: perception of muscular motion. Usually not measured in routine clinical evaluation.

Cortical sensory functions: interpretative sensory functions that require analysis of individual sensory modalities by the parietal lobes to provide discrimination. Individual sensory modalities must be intact to measure cortical sensation.

• Stereognosis: the ability to recognize and identify objects by feeling them. The absence of this ability is termed astereognosis.
• Graphesthesia: ability to recognize symbols written on the skin. The absence of this ability is termed graphanesthesia.
• Two-point discrimination: ability to recognize simultaneous stimulation by two blunt points. Measured by the distance between the points required for recognition. Absence is described as such.
• Touch localization (topognosis): ability to localize stimuli to parts of the body. Topagnosia is the absence of this ability.
• Double simultaneous stimulation: ability to perceive a sensory stimulus when corresponding areas on the opposite side of the body are stimulated simultaneously. Loss of this ability is termed sensory extinction.

Additional Senses

Many scientists and philosophers argue that humans have additional senses including:

• Pain or nociception (physiological pain): Signals nerve and other tissue damage.
• Balance or equilibrioception: Allows the sensing of body movement, direction, and acceleration, and to attain and maintain postural equilibrium and balance.
• Body awareness or proprioception: Provides the parietal cortex of the brain with information on the relative positions of the parts of the body.
• Sense of time or conception: This refers to how the passage of time is perceived and experienced but is not associated with a specific sensory system. According to psychologists and neuroscientists, however, human brains have a system governing the perception of time.
• Temperature sensing or thermoception: The sensation of heat and the absence of heat (cold).

If interoceptive senses are also considered, the sensation can be expanded to include stretch (as in muscles or organs like the lungs), oxygen and carbon dioxide sensing, pH sensing, and more.

While the exact definition of sensation is still controversial, most scientists agree that all senses rely on four fundamental sensory capacities:

• Chemical detection (chemoreception).
• Light detection (photoreception).
• Force detection (mechanoreception).
• Temperature detection (chemoreception).

Our nervous system has sensory systems and organs that mediate each sense and these systems rely on chemoreceptors, photoreceptors, mechanoreceptors, or thermoreceptors to detect the state of the internal or external environment.

Other internal sensations and perceptions

An internal sensation and perception also known as interoception^[rx] is “any sense that is normally stimulated from within the body”.^[rx] These involve numerous sensory receptors in internal organs. Interoception is thought to be atypical in clinical conditions such as alexithymia.^[rx] Some examples of specific receptors are:

• Hunger is governed by a set of brain structures (e.g., the hypothalamus) that are responsible for energy homeostasis.^[rx]
• Pulmonary stretch receptors are found in the lungs and control the respiratory rate.
• Peripheral chemoreceptors in the brain monitor the carbon dioxide and oxygen levels in the brain to give a perception of suffocation if carbon dioxide levels get too high.^[52]
• The chemoreceptor trigger zone is an area of the medulla in the brain that receives inputs from blood-borne drugs or hormones and communicates with the vomiting center.
• Chemoreceptors in the circulatory system also measure salt levels and prompt thirst if they get too high; they can also respond to high blood sugar levels in diabetics.
• Cutaneous receptors in the skin not only respond to touch, pressure, temperature and vibration, but also respond to vasodilation in the skin such as blushing.
• Stretch receptors in the gastrointestinal tract sense gas distension that may result in colic pain.
• Stimulation of sensory receptors in the esophagus results in sensations felt in the throat when swallowing, vomiting, or acid reflux.
• Sensory receptors in pharynx mucosa, similar to touch receptors in the skin, sense foreign objects such as mucous and food that may result in a gag reflex and corresponding gagging sensation.
• Stimulation of sensory receptors in the urinary bladder and rectum may result in perceptions of fullness.
• Stimulation of stretch sensors that sense dilation of various blood vessels may result in pain, for example headache caused by vasodilation of brain arteries.
• Cardioversion refers to the perception of the activity of the heart.^{[rx][rx][rx][rx]}
• Opsins and direct DNA damage in melanocytes and keratinocytes can sense ultraviolet radiation, which plays a role in pigmentation and sunburn.
• Baroreceptors relay blood pressure information to the brain and maintain proper homeostatic blood pressure.

The perception of time is also sometimes called a sense, though not tied to a specific receptor.

The five senses: Photographic depiction of the five senses.

Sensation to Perception

The goal of sensation is detection, while the goal of perception is to create useful information about our environment.

Sensation and perception are two distinct stages of processing during human sensing. The sensation is a function of the low level, biochemical, and neurological mechanisms that allow the receptor cells of a sensory organ to detect an environmental stimulus.

Stimuli from the environment (distal stimuli) are transformed into neural signals, which are then interpreted by the brain through a process called transduction. Transduction can be likened to a bridge connecting sensation to perception. This raw pattern of neural activity is called the proximal stimulus. Perception refers to the mental processes that are reflected in statements like “I see a blue wall” that represent the understanding of the real-world causes of sensory input. In other words, the goal of sensation is detection, while the goal of perception is to create useful information about the environment.

The neural signals are transmitted to the brain and processed. The resulting mental recreation of the distal stimulus is the percept. The sound stimulating a person’s auditory receptors is the proximal stimulus, and the brain’s interpretation of this as the ringing of a telephone is the percept.

All perception involves signals in the nervous system that result from physical stimulation of the sense organs. For example, vision involves light striking the retinas of the eyes, the smell is mediated by odor molecules, and hearing involves sound waves.

However, perception is not the passive receipt of these signals but is a process of organization, identification, and interpretation. Although the senses were traditionally viewed as passive receptors, the study of illusions and ambiguous images has demonstrated that the brain’s perceptual systems actively influence sensory systems in an attempt to construct useful representations of our environment.

Perception is particularly important to our ability to understand speech. The sound of a word can vary widely according to the words around it and the tempo of the speech, as well as the physical characteristics, accent, and mood of the speaker. Listeners manage to perceive words across this wide range of different conditions. Another variation is that reverberation can make a large difference in sounds, such as hearing a word spoken from the far side of a room and the same word spoken up close. The process of perceiving speech begins at the level of the sound within the auditory signal and the process of audition. After processing the initial auditory signal, speech sounds are further processed to extract acoustic cues and phonetic information. This speech information can then be used for higher-level language processes, such as word recognition.

Sensory Modalities

A sensory modality (also called a stimulus modality) is an aspect of a stimulus or what is perceived after a stimulus.

Sensing

Senses are transducers from the physical world to the realm of the mind. Another broadly acceptable definition of a sense is a system that consists of a group of sensory cell types, responding to a specific physical phenomenon, and corresponding to a particular group of regions within the brain where the signals are received and interpreted.

Disputes about the number of senses typically arise around the classification of the various cell types and their mapping to regions of the brain.

Sensory Modalities

A sensory modality (also called a stimulus modality) is an aspect of a stimulus or what is perceived after a stimulus. The term sensory modality is often used interchangeably with sense. The basic sensory modalities include light, sound, taste, temperature, pressure, and smell.

Light Modality

The sensory modality for vision is light. To perceive a light stimulus, the eye must first refract the light so that it directly hits the retina. The transduction of light into neural activity occurs via the photoreceptors in the retina.

When a particle of light hits the photoreceptors of the eye, the photopigment of the photoreceptor undergoes a chemical change leading to a chain of chemical reactions occur. A message is sent to a neuron called the bipolar cell through the use of a nerve impulse. Finally, a message is sent to the ganglion cell and then, finally, the brain.

Sound Modality

The sensory modality for audition is sound. Sound is created through air pressure. A vibrating object compresses the surrounding molecules of air as it moves towards a given point, and expands the molecules as it moves away from the point.

The eardrum is stimulated by vibrations in the air. It collects and sends these vibrations to receptor cells. The ossicles (three tiny bones in the middle ear) pass the vibrations to the fluid-filled cochlea (a spiral, shell-shaped auditory organ of the inner ear ). The vibrations move through the liquid in the cochlea where the receptive organ is able to sense it.

Taste Modality

Taste stimuli are encountered by receptor cells located in taste buds on the tongue and pharynx. Receptor cells disseminate onto different neurons and convey the message of a particular taste in a single medullar nucleus.

Taste perception is created by combining multiple sensory inputs. Different modalities help determine the perception of taste.

Temperature Modality

Temperature modality excites or elicits a symptom through cold or hot temperature. The cutaneous somatosensory system detects changes in temperature.

Thermal stimuli from a homeostatic set point excite temperature-specific sensory nerves in the skin. Specific thermosensory fibers respond to warmth and too cold.

Pressure Modality

Tactile stimulation can be direct, such as through bodily contact, or indirect, such as through the use of a tool or probe. Tactual perception gives information regarding cutaneous stimuli (pressure, vibration, and temperature), kinesthetic stimuli (limb movement), and proprioceptive stimuli (position of the body).

Smell Modality

The sense of smell is called olfaction. Materials constantly shed molecules, which float into the nose or are taken in through breathing. Inside the nasal chambers is the neuroepithelium lining.

It contains the receptors responsible for detecting molecules that are small enough to smell. These receptor neurons then synapse at the olfactory cranial nerve, which sends the information to the olfactory bulbs in the brain for initial processing.

Multimodal Perception

Multimodal perception is the ability of the mammalian nervous system to combine all of the different inputs of the sensory system to result in an enhanced detection or identification of a particular stimulus.

Integration of all sensory modalities occurs when multimodal neurons receive sensory information that overlaps with different modalities. Multimodal perception comes into effect when a unimodal stimulus fails to produce a response.

Additional Senses

Balance (or equilibrioception) is the sense that allows an organism to sense body movement, direction, and acceleration, and also attain and maintain postural equilibrium and balance. The organ of equilibrioception is the vestibular labyrinthine system found in both of the inner ears.

In technical terms, this organ is responsible for two senses: angular momentum and acceleration (known together as equilibrioception). The vestibular nerve conducts information from sensory receptors in three ampullae, each of which senses fluid motion in three semicircular canals caused by a three-dimensional rotation of the head.

The vestibular nerve also conducts information from the utricle and the saccule; these contain hair-like sensory receptors that bend under the weight of otoliths (small crystals of calcium carbonate) that provide the inertia needed to detect head rotation, linear acceleration, and the direction of gravitational force.

Thermoception is the sense of heat or absence of heat (cold) by the skin and internal skin passages. Perceiving changes in temperature is referred to as heat flux (the rate of heat flow) in these areas.

There are specialized receptors for cold (declining temperature) and heat. The cold receptors infer wind direction, an important part of the animal’s sense of smell. The heat receptors are sensitive to infrared radiation and can occur in specialized organs, for instance in pit vipers.

The thermoreceptors in the skin are quite different from the homeostatic thermoreceptors in the brain (hypothalamus), which provides feedback on internal body temperature.

Proprioception, the kinesthetic sense, provides the parietal cortex of the brain with information on the relative positions of the parts of the body. Neurologists test this sense by telling patients to close their eyes and touch their own nose with the tip of a finger. Assuming proper proprioceptive function, at no time will the person lose awareness of where their hand actually is, even though it is not being detected by any of the other senses. Proprioception and touch are related in subtle ways, and their impairment results in deep and surprising deficits in perception and action.

Nociception (physiological pain) signals nerve or other tissue damage. The three types of pain receptors are cutaneous (skin), somatic (joints and bones), and visceral (body organs).

It was previously believed that pain was simply the overloading of pressure receptors, but research in the first half of the 20th century showed that pain is a distinct phenomenon that intertwines with all of the other senses, including touch.

Conception refers to how the passage of time is perceived and experienced. Although the sense of time is not associated with a specific sensory system, psychological and neuroscientific research indicates that human brains do have a system governing the perception of time.

It is composed of a highly distributed system involving the cerebral cortex, cerebellum, and basal ganglia. One particular component, the suprachiasmatic nucleus, is responsible for the circadian (daily) rhythm, while other cell clusters appear capable of shorter-range (ultradian) timekeeping.

Cellular

Receptors

Sensory receptors become activated by stimuli in the environment by receiving signals. The transmission of any message in the neurons of our body requires it to be in the form of an action potential; the sensation must undergo conversion into electrical signals. The structures which convert mechanical signals into electrical signals are receptors.

Receptors get classified based on the type of stimulus activating them. The following are common types of receptors

Mechanoreceptors

• Activated by changes in pressure
• Common mechanoreceptors include
• Pacinian corpuscles in the subcutaneous tissue
• Meissner corpuscles in non-hairy skin
• Baroreceptors in the carotid sinus
• Hair cells on the organ of Corti and in the semicircular canals

Photoreceptors

• Activated by light
• Rods and cones (located in the retina)

  • Rods are sensitive to low-intensity light and function better in a dark environment
  • Cones have a better threshold for light, therefore function well in daylight. They also participate in color vision.

Chemoreceptors

• Activated by chemicals
• They serve for olfaction and taste

Thermoreceptors

• Located in the skin and include cold and warm receptors. These are present in the skin.

Nociceptors

• These become activated by extreme pressure, temperature, or noxious chemicals. These are also in the skin.

There are different types of receptors present into the skin or muscles for all modalities of senses.[rx]

1) Touch

Human skin divides into hairy and nonhairy or glabrous skin. This classification is associated with different touch receptors inside the skin. There are four types of touch receptors. Those which are slowly adapting called Merkel cells (slowly adapting type 1) and Ruffini corpuscles (slowly adapting type2). Those with Pacinian afferents called Pacinian corpuscles and rapidly adaptive ones called Meissner corpuscles.[rx]

• Merkel’s disc

Structurally simplest among all are Merkel cells which are present in the basal layer of the epidermis. These are located in non-hairy skin. Present in a high amount at the fingertips, they are sensitive selectively to the particular component of stress or strain, and because of that they are more selective in detecting corners, edges and curvatures; this is useful for reading Braille, etc.

• Ruffini nerve endings

These types of receptors are less densely placed and therefore have less sensitivity. These receptors are located in hairy skin. They are more sensitive to stretch, so become stimulated during stretching of the skin. An example is stretching during motion and for the direction of force detection, along with muscle spindle for hand shape and finger position perception, etc.

• Pacinian corpuscle

Distributed throughout the palm and finger, the Pacinian corpuscle is the most sensitive receptor type. These are large, onion-like layered structures enclosing a single nerve ending. Pacinian corpuscles function as a mechanical filter to protect from very large, low-frequency stress during manual labor.

• Meissner corpuscle      

They are most sensitive to dynamic changes in the skin, and they are relatively insensitive for static changes of the skin.

One distinguishing characteristic of each receptor is the degree of adaptation.

• “Very rapidly adapting” such as Pacinian corpuscle
•  “Rapidly adapting” such as Meissner corpuscle and hair follicles
• “Slowly adapting” includes Ruffini corpuscle, Merkel receptors, and tactile discs

 2) Thermal receptors

These are slowly adapting receptors that can detect changes in skin temperature. These may be cold or warm. These receptors have some baseline firing rates. Cold receptors are sensitive between 10 and 32 degrees C. Firing at a baseline rate during 30 to 35 degrees C, the firing rate increases in cold while it decreases when the temperature increases. In the same way, warm receptors have baseline firing around 38 degrees C and increase with an increase in temperature.[4]

3) Vibration

Vibration sensations are useful in the performance of balancing tasks, along with proprioception. Vibrational sense perception is by Pacinian corpuscles and Meissner corpuscles because both of these receptors are sensitive to low-frequency vibrations.[rx]

4) Pain

Noxious stimulation is converted into an electric signal by unencapsulated nerve endings that terminate in the dermis and epidermis. Noxious stimuli such as intense hot or cold, or long-standing pressure cause activation of free nerve endings; this requires threshold stimulation to activate the endings, but once activated, signals are transmitted continuously.[rx]

5) Proprioception

The Golgi tendon organ senses the position of joints and joint movement near attached muscle tendons attached, and by muscle, spindles present inside the extrafusal muscle fibers. They fire signals when stretched.

References

The cerebral cortex is composed of a complex association of tightly packed neurons covering the outermost portion of the brain. It is the gray matter of the brain. Lying right under the meninges, the cerebral cortex divides into four lobes: frontal, temporal, parietal, and occipital lobes, each with a multitude of functions. It is characteristically known for its bulges of brain tissue known as gyri, alternating with deep fissures known as sulci. The enfolding of the brain is an adaptation to the dramatic growth in brain size during evolution. The various folding of brain tissue allowed large brains to fit in relatively small cranial vaults that had to remain small to accommodate the birth process. Notable sulci include the Sylvian fissure which divides the temporal lobe from the frontal and parietal lobe, the central sulcus which separates the frontal and parietal lobes, the parieto-occipital sulcus which divides the parietal and occipital lobes, and the calcarine sulcus which divides the cuneus from the lingual gyrus.

Sensory Areas

Sensory areas of the brain receive and process sensory information, including sight, touch, taste, smell, and hearing.

Sensory areas are the areas of the brain that receive and process sensory information. The cerebral cortex is connected to various subcortical structures such as the thalamus and the basal ganglia. Most sensory information is routed to the cerebral cortex via the thalamus. Olfactory information, however, passes through the olfactory bulb to the olfactory cortex, bypassing the thalamus. The cortex is commonly described as composed of three parts: sensory, motor, and association areas. Parts of the cortex that receive sensory inputs from the thalamus are called primary sensory areas. Each of the five senses relates to specific groups of brain cells that categorize and integrate sensory information.

The Five Sensory Modalities

The five commonly recognized sensory modalities, including sight, hearing, taste, touch, and smell, are processed as follows:

Somatosensory System

The primary somatosensory cortex, located across the central sulcus and behind the primary motor cortex, is configured to generally correspond with the arrangement of nearby motor cells related to specific body parts.

Taste

The primary gustatory area is near the face representation within the postcentral gyrus.

Olfaction

The olfactory cortex is located in the uncus, found along the ventral surface of the temporal lobe. Olfaction is the only sensory system that is not routed through the thalamus.

Vision

The visual area is located on the calcarine sulcus deep within the inside folds of the occipital lobe.

Hearing

The primary auditory cortex is located on the transverse gyri that lie on the back of the superior temporal convolution of the temporal lobes.

Organization of Sensory Maps

In general, each brain hemisphere receives information from the opposite side of the body. For example, the right primary somatosensory cortex receives information from the left limbs, and the right visual cortex receives information from the left eye. The organization of sensory maps in the cortex reflects that of the corresponding sensing organ, in what is known as a topographic map. Neighboring points in the primary visual cortex, for example, correspond to neighboring points in the retina. This topographic map is called a retinotopic map.

Similarly, there is a tonotopic map in the primary auditory cortex and a somatotopic map in the primary sensory cortex. This somatotopic map has commonly been illustrated as a deformed human representation, the somatosensory homunculus, in which the size of different body parts reflects the relative density of their innervation.

A cortical homunculus is a physical representation of the human body located within the brain. This neurological map of the anatomical divisions of the body depicts the portion of the human brain directly associated with the activity of a particular body part. Simply put, it is the view of the body from the brain’s perspective. Areas with lots of sensory innervation, such as the fingertips and the lips, require more cortical area to process finer sensation.

Motor Areas

The motor areas, arranged like a pair of headphones across both cortex hemispheres, are involved in the control of voluntary movements.

The motor areas of the brain are located in both hemispheres of the cortex. They are positioned like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine movements performed by the hand. The right half of the motor area controls the left side of the body, and the left half of the motor area controls the right side of the body.

Motor Cortex Divisions

Motor Cortex: Topography of the human motor cortex, including the premotor cortex, SMA, primary motor cortex, primary somatosensory cortex, and posterior parietal cortex.

The motor cortex is divided into three areas:

• Primary motor cortex: Main contributor to the generation of neural impulses that control the execution of movement.
• Premotor cortex: Located anterior to the primary motor cortex and responsible for some aspects of motor control.
• Supplementary motor area (SMA): Functions include internally generated planning of movement, planning of sequences of movement, and the coordination of the two sides of the body. It is located on the midline surface of the hemisphere anterior to the primary motor cortex.

Motor Cortex Functions

Motor functions are also controlled by these additional structures:

• Posterior parietal cortex: Guides planned movements, spatial reasoning, and attention.
• Dorsolateral prefrontal cortex: Important for executive functions, including working memory, cognitive flexibility, and abstract reasoning.

Buried deep in the white matter of the cerebral cortex are interconnected subcortical masses of cerebral gray matter called basal nuclei (or basal ganglia) that are involved in motor control. The basal nuclei receive input from the substantia nigra of the midbrain and motor areas of the cerebral cortex and send signals back to both of these locations.

Motor Cortex Map

The majority of neurons in the motor cortex project to the spinal cord synapse on interneuron circuitry in the spinal cord. The view that each point in the motor cortex controls a muscle or a limited set of related muscles has been debated. Various experiments examining the motor cortex map showed that each point in motor cortex influences a range of muscles and joints, indicating significant overlapping in the map.

Association Areas

Associative areas of the cortex integrate current states with past states to predict proper responses based on sets of stimuli.

Association areas produce a meaningful perceptual experience of the world, enable us to interact effectively, and support abstract thinking and language. The parietal, temporal, and occipital lobes, all located in the posterior part of the cortex, organize sensory information into a coherent perceptual model of our environment centered on our body image. The frontal lobe or prefrontal association complex is involved in planning actions and movement, as well as abstract thought.

Language abilities are localized in the left hemisphere in Broca’s area for language expression and Wernicke’s area for language reception. The association areas are organized as distributed networks, and each network connects areas distributed across widely spaced regions of the cortex. Distinct networks are positioned adjacent to one another, yielding a complex series of interwoven networks. In humans, association networks are particularly important to language function.

The processes of language expression and reception occur in areas other than just the perisylvian structures such as the prefrontal lobe, basal ganglia, cerebellum, pons, caudate nucleus, and others. The association areas integrate information from different receptors or sensory areas and relate the information to past experiences. Then the brain makes a decision and sends nerve impulses to the motor areas to elicit responses.

Methods of Brain Function Analysis

Behavioral and neuroscientific methods are used to get a better understanding of how our brain influences the way we think, feel and act. Many different methods help us analyze the brain and give an overview of the relationship between the brain and behavior. This promotes understanding of the ways in which associations are made by multiple brain regions, allowing the appropriate responses to occur in a given situation. Well-known techniques are EEG (electroencephalography), which records the brain’s electrical activity, and fMRI (functional magnetic resonance imaging), which tells us more about brain functions. Other methods, such as the lesion method, are not as well-known, but still very influential in modern neuroscientific research.

In the lesion method, patients with brain damage are examined to determine which brain structures were damaged and to what extent this influences the patient’s behavior. The concept of the lesion method is based on the idea of finding a correlation between a specific brain area and an occurring behavior. From experiences and research observations, it can be concluded that damage to part of the brain causes behavioral changes or interferes in performing a specific task.

For example, a patient with a lesion in the parietal-temporal-occipital association area has an agraphia, which means he is unable to write although he has no deficits in motor skills. Consequently, researchers deduce that if structure X is damaged and changes in behavior Y occur, X has a relation to Y.

Hemispheric Lateralization

The human brain is composed of a right and a left hemisphere, and each participates in different aspects of brain function.

A longitudinal fissure separates the human brain into two distinct cerebral hemispheres connected by the corpus callosum. The two sides resemble each other and each hemisphere’s structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each cortical hemisphere are distinct.

The hemispheres of the cerebral cortex: The human brain is divided into two hemispheres–left and right. Scientists continue to explore how some cognitive functions tend to be dominated by one side or the other; that is, how they are lateralized.

Broad generalizations are often made in popular psychology about one hemisphere having a broad label, such as “logical” for the left side or “creative” for the right. But although measurable lateral dominance occurs, most functions are present in both hemispheres. The extent of specialization by hemisphere remains under investigation. If a specific region of the brain or even an entire hemisphere is either injured or destroyed, its functions can sometimes be taken over by a neighboring region even in the opposite hemisphere, depending upon the area damaged and the patient’s age. When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.

While many functions are lateralized, this is only a tendency. The implementation of a specific brain function significantly varies by individual. The areas of exploration of this causal or effectual difference of a particular brain function include gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains, is still being studied. Short of having a hemispherectomy (removal of a cerebral hemisphere), no one is a “left-brain only” or “right-brain only” person.

Lateralization and Handedness

Brain function lateralization is evident in the phenomena of right- or left-handedness, but a person’s preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people have left-hemisphere dominance for language, 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of left-handed people have bilateral language functions. Even within various language functions (e.g., semantics, syntax, prosody), degree and even hemisphere of dominance may differ.

Language functions such as grammar, vocabulary, and literal meaning are typically lateralized to the left hemisphere, especially in right-handed individuals. While language production is left-lateralized in up to 90% of right-handed subjects, it is more bilateral or even right-lateralized in approximately 50% of left-handers. In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.

Further Lateral Distinctions

The processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally but may show right-hemisphere dominance. Numerical estimation, comparison, and online calculation depend on bilateral parietal regions. Exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing. Dyscalculia is a neurological syndrome associated with damage to the left temporoparietal junction. This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to understand or apply mathematical concepts.

Lateralization and Evolution

Specialization of the two hemispheres is general in vertebrates including fish, frogs, reptiles, birds, and mammals, with the left hemisphere specialized to categorize information and control routine behavior. The right hemisphere is responsible for responses to novel events and behavior in emergencies, including the expression of intense emotions. Feeding is an example of routine left-hemisphere behavior, while escape from predators is an example of a right-hemisphere behavior. This suggests that the evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain.

Split-Brain Phenomenon

Patients with split-brain are individuals who have undergone corpus callosotomy, a severing of a large part of the corpus callosum (usually as a treatment for severe epilepsy). The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are cut, the two halves of the brain have a reduced capacity to communicate with each other.

The widespread lateralization of many vertebrate animals indicates an evolutionary advantage associated with the specialization of each hemisphere. The evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain. In a 2011 study published in the journal of Brain Behavioral Research, lateralization of a few specific functions, as opposed to overall brain lateralization, was correlated with parallel task efficiency.

Blood Supply and Lymphatics

The brain weighs 2% of total body weight. It receives about 15% of the cardiac output.

Anterior Circulation

The anterior portion of the brain is supplied mainly by branches of the paired internal carotid artery. It accounts for 80% of the blood supply of the brain.

Internal Carotid Arteries (ICA) – The internal carotid artery runs upward through the neck and enters the skull through the carotid canal, located in the petrous portion of the temporal bone just superior to the jugular fossa. The internal carotid branches into the anterior cerebral artery and continues to form the middle cerebral artery. The ICA provides the anterior supply to the circle of Willis.

Anterior Cerebral Arteries (ACA) – The anterior cerebral arteries are branches of the ICA and supply the frontal and superior medial parietal lobes; this includes part of the motor cortex that controls the movement of the contralateral lower limb, the sensory cortex that controls sensation in the contralateral lower limb, Broca’s area, and the prefrontal cortex. Both ACAs connect to each other via the anterior communicating artery. Although ACA infarcts are rare due to the collateral circulation provided by the anterior communicating arteries, one would experience contralateral motor and sensory deficits in the lower limbs.

Middle Cerebral Arteries (MCA) – The middle cerebral artery is the most common site for a stroke, accounting for up to 80% of ischemic strokes that occur in the brain.[rx] It arises from the ICA and courses laterally through the sphenoid ridge to the Sylvian fissure. It is responsible for supplying the majority of the lateral hemispheres except for the superior portion of the parietal lobe (ACA) and the inferior portions of the temporal and occipital lobes (PCA). Lenticulostriate branches of the MCA supply the basal ganglia and internal capsule. Damage to the middle cerebral artery on the Left can cause deficits due to damage to Broca’s area, Wernicke’s area, and contralateral sensorimotor deficits in the upper extremities and head. Damage to the MCA on the right side would spare Wernicke’s and Broca’s area given the patients’ dominant hemisphere is on the left. It is important to note the resultant contralateral sensorimotor deficits in the upper extremities with an MCA stroke versus contralateral sensorimotor deficits in the lower extremities with an ACA stroke.

Posterior Circulation

The posterior cerebral circulation supplies the occipital lobes, cerebellum, and brainstem via branches of the vertebral arteries. It accounts for 20% of cerebral blood flow.

Basilar Artery – As the vertebral arteries course superiorly into the skull through the foramen magnum, they fuse to form the basilar artery. Often referred to as the vertebrobasilar system, the combination of the vertebral arteries with the basilar arteries provides the posterior supply to the circle of Willis. The basilar artery runs cranially in the central groove of the pons within the pontine cistern. It travels adjacent to CNVI to the upper pontine border and the appearance of CNIII where it terminates. The basilar artery gives off various branches including the anterior inferior cerebellar artery, labyrinth arteries, pontine arteries, superior cerebellar artery, and then finally bifurcates and terminates as the posterior cerebral arteries. Basilar artery occlusion represents up to 4% of all ischemic strokes. Clinical features localizing to the cerebellum or brainstem such as hearing loss, truncal ataxia, extraocular movement abnormalities, and nystagmus, may help to differentiate ischemia in the posterior circulation from other clinical diagnoses.[rx] One of the most disabling locations for a basilar artery occlusion is a mid-basilar occlusion with bilateral pontine ischemia. Patients with this condition appear comatose but can be fully conscious and paralyzed with only limited vertical eye movements. This phenomenon termed “locked-in syndrome” has a high mortality rate of approximately 75% in the acute phase.[rx] Another basilar artery occlusion can occur at the distal “top of the basilar syndrome” where the superior cerebellar artery and posterior cerebral artery terminate. It may result in cortical blindness. Physical examination findings may include vertical gaze and convergence disorders, slowed smooth pursuit movements, skew deviation, and convergence-retraction nystagmus, and light-near dissociation. The top of the basilar syndrome can have further clinical findings if there is an involvement of the superior cerebellar or posterior cerebral arteries.

Posterior Cerebral Arteries (PCA) – The posterior cerebral arteries are the terminal branch of the basilar artery and supply the overwhelming majority of the occipital lobe. It is joined with the MCA in the circle of Willis via the posterior communicating artery. As the posterior cerebral arteries branch off from the basilar artery, they travel around the midbrain, through the quadrigeminal cistern, and with the calcarine artery in the calcarine sulcus. The posterior cerebral arteries have various branches including the posterior communicating artery, the thalamoperforating branches, and the posterior choroidal arteries. The most significant manifestation of a PCA stroke is contralateral hemianopia with macular sparing. The macula is spared due to the dual collateral circulation provided by the MCA. If the PCA stroke involves the dominant hemisphere (usually left) patients may exhibit alexia without agraphia (patients can write but cannot read). Larger infarcts involving the internal capsule and thalamus may cause contralateral hemiparesis and hemisensory loss.

Function

The Frontal Lobe

It is the largest lobe, located in front of the cerebral hemispheres, and has significant functions for our body, and these are:

• Prospective memory a type of memory that involves remembering the plans made, from a simple daily plan to future lifelong plans.[rx]
• Speech and language

The frontal lobe has an area called Broca’s area located in the posterior inferior frontal gyrus involved in speech production. A recent study shows that the exact function of Broca’s area is to mediate sensory representations that originate in the temporal cortex and going to the motor cortex.[rx]

• Personality – During the past centuries, several researchers have described that there are personality changes that occurred after frontal lobe injuries. One of the most important cases was about Phineas Gage, who was a gentle, polite sociable young, man until a large iron rod went through his eye-damaging his prefrontal cortex. This injury made him emotionally insensitive, perform socially inappropriate behaviors, and was unable to make a rational judgment. A recent study suggests that when there is damage to the prefrontal cortex, there are five sub-types of personality changes that occur, and these include:

• Executive disturbances
• Disturbed social behavior
• Emotional Dysregulation
• Hypo-emotionality/de-energization
• Distress[rx]
• Decision making

The ability to decide on something involves reasoning, learning, and creativity. A study conducted in 2012 proposed a new model to understand how the decision-making process occurs in the frontal lobe, specifically how the brain creates a new strategy to a new-recurrent situation or an open-ended environment; they called it the PROBE model.

There are typically three possible ways to adapt to a situation:

Selecting a previously learned strategy that applies precisely to the current situationAdjusting an already learned approach Developing a creative behavioral method

The PROBE model illustrates that the brain can compare three to four behavioral methods at most, then choose the best strategy for the situation.[rx]

• Movement control – The frontal lobe has the motor cortex divided into two regions: the primary motor area located posterior to the precentral sulcus and non-primary motor areas, including the premotor cortex, supplementary motor area, and cingulate motor areas. The exact function of each structure and its role in the movement is still an active research area.[6]

The Parietal Lobe

It is located posterior to the frontal lobe and superior to the temporal lobe and classified into two functional regions.

The anterior parietal lobe contains the primary sensory cortex (SI), located in the postcentral gyrus (Broadman area BA 3, 1, 2). SI receives the majority of the sensory inputs coming from the thalamus, and it’s responsible for interpreting the simple somatosensory signals like (touch, position, vibration, pressure, pain, temperature).[7]

The posterior parietal lobe has two regions: the superior parietal lobule and the inferior parietal lobule.

• The superior parietal lobule contains the somatosensory association (BA 5, 7) cortex which is involved in higher-order functions like motor planning action.
• The inferior parietal lobule (supramarginal gyrus BA 40, angular gyrus BA 39) has the  Secondary somatosensory cortex (SII), which receives the somatosensory inputs from the thalamus and the contralateral SII, and they integrate those inputs with other major modalities (examples: visual inputs, auditory inputs) to form higher-order complex functions like:

  • Sensorimotor planning
  • Learning
  • Language
  • Spatial recognition
  • Stereognosis: the ability to differentiate between objects regarding their size, shape, weight, and any other differences.[rx]

The Temporal Lobe

The second most substantial portion occupies the middle cranial fossa and lies posterior to the frontal lobe and inferior to the parietal lobe. There are two surfaces, the lateral surface and the medial surface.[rx]

The lateral surface is classified by the superior temporal sulcus and the lateral temporal sulcus into three gyri; the superior temporal gyrus and the middle temporal gyrus, and the inferior temporal gyrus.

• The superior temporal gyrus (STG) is further sub-divided into two surfaces, the dorsal surface (superior temporal plane STP) and the lateral surface of the STG.

The STP is located deep in the Sylvain fissure. The most significant anatomical landmark in STP is the Heschl gyrus (HG) which contains the primary auditory cortex. It is responsible for translating and processing all sounds and tones, and it is minimally affected by task requirements. Task requirement: a test where the examiner pronounces some words and asks the participant to categorize them acoustically, or phonemically, or semantically.[rx]The STP has another important area next to the HG called Wernicke’s area. In the past, this area was thought to have a significant role in speech perception and comprehension, but recent evidence shows that this area is not involved in this process. Researchers found that this process is not a simple task, but moreover, it is a complex task that is distributed all over the brain. The primary function of this area is the phonological representation, a process where the pronounced word is interpreted based on their tones and sound and trying to link it to a previously learned sound.[rx]

The lateral surface of the STG is thought to be the secondary auditory cortex that also functions in interpreting sounds, but mostly in the activities that involve task requirements.[rx]

• The middle temporal gyrus (MTG) has four sub-regions, the anterior, middle, posterior, and sulcus MTG.[rx]

The Anterior MTG is primarily involved in:

The default mode network has a specific activity that exists naturally in the brain at rest. So if one is studying or engaging in a game or doing any other activity that demands staying focused or setting a particular goal this mode will be deactivated.

• Sound recognition helps the other areas that we talked about before.
• Semantic retrieval a process that assigns meaning to the words or sounds by trying to retrieve the previously learned concepts if they existed.

The Middle MTG has two functions:

• Semantic memory a type of memory involved in remembering the thoughts or objectives that are common knowledge (for example, where the bathroom is located).
• A semantic control network a system of connections between different areas of the brain, including the middle MTG, to assign meaning to words, sounds that require both stored knowledge and mechanisms of semantic retrieval.

The Posterior MTG is thought to be part of the classical sensory language area.

The Sulcus MTG is involved in decoding gaze directions and in speech.

• The inferior temporal gyrus (IT) is involved in visual perception and facial perception by containing the ventral visual pathway, the pathway that carries the information from the primary visual cortex to the temporal lobe, to determine the content of the vision.[rx]

The medial surface of the temporal lobe (mesial temporal lobe) includes important structures (Hippocampus, Entorhinal, Perirhinal, Parahippocampal cortex) that are anatomically related and are mandatory for declarative memory. Declarative memory is a type of long-term memory that involves remembering the concepts or ideas and the events that happened or learned throughout life. It is further divided into three types of memory:

• Semantic memory was discussed previously (see middle MTG).
• Recognition memory the memory involved in recognizing an object and all the other details that relate to this object. There are two forms: recollection and familiarity.
• Recollection means one can remember the object and almost every detail related to that object, such as time and place.
• Familiarity means one remembers encountering the object previously but doesn’t recall any specific detail about it. For example, when someone says to a person, “Your face is familiar, but I can’t remember where and when we met.”
• Episodic memory is the type of memory that specializes in recalling an event and its associated details; this is different from recognition memory, in which someone can consciously memorialize a specific event that happened throughout their life without being exposed to a similar situation.

The medial temporal lobe (memory system) is still an active research area; more precisely, the exact function of each structure in this lobe is currently being studied.[rx]

The Occipital Lobe

The occipital lobe is the smallest lobe in the cerebrum cortex. It is located in the most posterior region of the brain, posterior to the parietal lobe and temporal lobe. The role of this lobe is visual processing and interpretation. Typically based on the function and structure, the visual cortex is divided into five areas (v1-v5). The primary visual cortex (v1, BA 17) is the first area that receives the visual information from the thalamus, and its located around the calcarine sulcus. The visual cortex receive, process, interpret the visual information, then this processed information is sent to the other regions of the brain to be further analyzed (example: inferior temporal lobe). This visual information helps us to determine, recognize, and compare the objects to each other.[rx]

References

The cerebellum (which is Latin for “little brain”) is a major structure of the hindbrain that is located near the brainstem. This part of the brain is responsible for coordinating voluntary movements. It is also responsible for a number of functions including motor skills such as balance, coordination, and posture.

The cerebellum is a vital component in the human brain as it plays a role in motor movement regulation and balance control. The cerebellum coordinates gait and maintains posture, controls muscle tone and voluntary muscle activity but is unable to initiate muscle contraction. Damage to this area in humans results in a loss in the ability to control fine movements, maintain posture, and motor learning.[rx][rx][rx]

The cerebellum, meaning “little brain” in Latin, is primarily responsible for the coordination of movement, maintaining posture and balance, muscle tone, and motor learning. Recent research has shown that the cerebellum may also have cognitive functions, but the exact mechanism and pathways are still unclear. In this chapter, we present the gross and functional anatomy of the cerebellum and then focus on the structures that are involved in clinical presentations and syndromes.

References

The cerebrum is the uppermost part of the brain. It contains two hemispheres split by a central fissure. The cerebrum itself contains the major lobes of the brain and is responsible for receiving and giving meaning to information from the sense organs, as well as controlling the body.

The cerebrum, telencephalon, or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In the human brain, the cerebrum is the uppermost region of the central nervous system. The cerebrum develops prenatally from the forebrain (prosencephalon). In mammals, the dorsal telencephalon, or pallium, develops into the cerebral cortex, and the ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also divided into approximately symmetric left and right cerebral hemispheres.

Structure

The cerebrum is the largest part of the brain. Depending upon the position of the animal it lies either in front or on top of the brainstem. In humans, the cerebrum is the largest and best-developed of the five major divisions of the brain.

The cerebrum is made up of the two cerebral hemispheres and their cerebral cortex cortices (the outer layers of grey matter), and the underlying regions of white matter.^[rx] Its subcortical structures include the hippocampus, basal ganglia, and olfactory bulb. The cerebrum consists of two C-shaped cerebral hemispheres, separated from each other by a deep fissure called the longitudinal fissure.

Cerebral cortex

The cerebral cortex, the outer layer of the grey matter of the cerebrum, is found only in mammals. In larger mammals, including humans, the surface of the cerebral cortex folds to create gyrus gyri (ridges) and sulci (furrows) which increase the surface area.^[rx]

The cerebral cortex is generally classified into four lobes: the frontal, parietal, occipital, and temporal lobes. The lobes are classified based on their overlying neurocranial bones.^[rx]

Cerebral hemispheres

The cerebrum is divided by the medial longitudinal fissure into two cerebral hemispheres, the right and the left. The cerebrum is contralaterally organized, i.e., the right hemisphere controls and processes signals from the left side of the body, while the left hemisphere controls and processes signals from the right side of the body.^[rx] There is a strong but not complete bilateral symmetry between the hemispheres. The lateralization of brain function looks at the known and possible differences between the two.

Overview of the Cerebrum

With the assistance of the cerebellum, the cerebrum controls all voluntary actions in the body.

The cerebrum, which lies in front or on top of the brainstem, comprises a large portion of the brain. In humans, it is the largest and best-developed of the brain’s five major divisions. The cerebrum is the newest structure in the phylogenetic sense, with mammals having the largest and most developed among all species.

The cerebrum contains the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In larger mammals, the cerebral cortex is folded into many gyri and sulci, which allows it to expand in a surface area without taking up the much greater volume. With the assistance of the cerebellum, the cerebrum controls all voluntary actions in the body.

Cerebral Cortex

The cortex is composed of two hemispheres, right and left, separated by a large sulcus. A thick fiber bundle, the corpus callosum, connects the two hemispheres, allowing information to be passed from one side to the other. The right hemisphere controls and processes signals from the left side of the body, while the left hemisphere controls and processes signals from the right side of the body.

The Four Brain Lobes

Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital.

The frontal lobe is located at the front of the brain, over the eyes, and contains the olfactory bulb. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement.

Two of the parietal lobe’s main functions are processing somatosensation (touch sensations such as pressure, pain, heat, cold) and proprioception (the sense of how parts of the body are oriented in space).

The temporal lobe is located at the base of the brain by the ears. It is primarily involved in processing and interpreting sounds. It also contains the hippocampus, which processes memory formation.

The occipital lobe is located at the back of the brain. It is primarily involved in vision: seeing, recognizing, and identifying the visual world.

Cerebrum Function

The cerebrum directs the conscious or volitional motor functions of the body. These functions originate within the primary motor cortex and other frontal lobe motor areas where actions are planned. Upper motor neurons in the primary motor cortex send their axons to the brainstem and spinal cord to synapse on the lower motor neurons, which innervate the muscles. Damage to motor areas of the cortex can lead to certain types of motor neuron disease. This kind of damage results in loss of muscular power and precision rather than total paralysis.

The olfactory sensory system is unique in that neurons in the olfactory bulb send their axons directly to the olfactory cortex, rather than to the thalamus first. Damage to the olfactory bulb results in a loss of the sense of smell. The olfactory bulb also receives “top-down” information from such brain areas as the amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra. Its potential functions can be placed into four non-exclusive categories: discriminating among odors, enhancing the sensitivity of odor detection, filtering out background odors, and permitting higher brain areas involved in arousal and attention to modify the detection or the discrimination of odors.

Speech and language are mainly attributed to parts of the cerebral cortex. Motor portions of language are attributed to Broca’s area within the frontal lobe. Speech comprehension is attributed to Wernicke’s area, at the temporal-parietal lobe junction. Damage to the Broca’s area results in expressive aphasia (non-fluent aphasia) while damage to Wernicke’s area results in receptive aphasia.

The cerebrum itself houses the four major lobes, and each lobe as its own set of functions. So although the cerebrum as a whole controls numerous functions in the body, this is mainly due to the function of each individual lobe and the interplay between them.

In general, the cerebrum controls all voluntary actions. It is also the control center for:

• sensory processing
• emotional control
• motor control
• personality
• learning
• problem solving
• language and speech
• visual information
• spatial information
• cognition and higher thought
• imagination
• creativity
• music interpretation

Areas in the cerebrum are responsible for receiving and interpreting much of the physical world around the body.

The sections below will detail which lobe controls which processes.

Frontal lobe

• speech
• behavior and personality
• emotions
• body movement
• intelligence and self-awareness

Parietal lobe

• language and symbol use
• visual perception
• sense of touch, pressure, and pain
• giving meaning to signals from other sensory information

Temporal lobe

• memory
• hearing
• understanding language
• organization and patterns

Occipital lobe

• light
• color
• movement
• spatial orientation

Insular lobe

• homeostasis
• compassion and empathy
• self-awareness
• cognitive function
• social experience

Cerebral Lobes

The cortex is divided into four main lobes: frontal, parietal, occipital, temporal.

Brain lobes were originally a purely anatomical classification, but we now know they are also associated with specific brain functions. The telencephalon (cerebrum), the largest portion of the human brain, is divided into lobes like the cerebellum. If not specified, the expression “lobes of the brain” refers to the telencephalon. There are four uncontested lobes of the telencephalon:

The Frontal Lobe

The frontal lobe is an area in the mammalian brain located at the front of each cerebral hemisphere and positioned anterior to (in front of) the parietal lobe and superior and anterior to the temporal lobes. It is separated from the parietal lobe by a space between tissues called the central sulcus and from the temporal lobe by a deep fold called the lateral (Sylvian) sulcus. The precentral gyrus, forming the posterior border of the frontal lobe, contains the primary motor cortex, which controls voluntary movements of specific body parts.

The frontal lobe contains most of the dopamine sensitive neurons in the cerebral cortex. The dopamine system is associated with reward, attention, short-term memory tasks, planning, and motivation. Dopamine tends to limit and select sensory information that the thalamus sends to the forebrain. A report from the National Institute of Mental Health indicates that a gene variant that reduces dopamine activity in the prefrontal cortex is related to poorer performance in that region during memory tasks; this gene variant is also related to a slightly increased risk for schizophrenia.

The frontal lobe is considered to contribute to our most human qualities. Damage to the frontal lobe can result in changes in personality and difficulty planning. The frontal lobes are the most uniquely human of all the brain structures.

The Parietal Lobe

The parietal lobe is a part of the brain positioned above (superior to) the occipital lobe and behind (posterior to) the frontal lobe. The parietal lobe integrates sensory information from different modalities, particularly spatial sense and navigation. For example, it comprises the somatosensory cortex and the dorsal stream of the visual system. This enables regions of the parietal cortex to map objects perceived visually into body coordinate positions.

Several portions of the parietal lobe are also important in language processing. Also, this lobe integrates information from various senses and assists in the manipulation of objects. Portions of the parietal lobe are involved with visuospatial processing.

The Occipital Lobe

The two occipital lobes are the smallest of the four paired lobes in the human cerebral cortex. Located in the rearmost portion of the skull, the occipital lobes are part of the forebrain. At the front edge of the occipital there are several lateral occipital gyri separated by lateral occipital sulci. The occipital lobe is involved in the sense of sight; lesions in this area can produce hallucinations.

The Temporal Lobe

The temporal lobe is a region of the cerebral cortex located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobes are involved in many functions, such as retaining visual memories, processing sensory input, comprehending language, storing new memories, feeling and expressing emotion, and deriving meaning. The temporal lobe contains the hippocampus and plays a key role in the formation of explicit long-term memory, modulated by the amygdala. It is involved in the senses of smell and sound as well as in the processing of complex stimuli.

Adjacent areas in the superior, posterior, and lateral parts of the temporal lobes are involved in high-level auditory processing. The temporal lobe is involved in primary auditory perception such as hearing and holds the primary auditory cortex. The superior temporal gyrus includes an area where auditory signals from the ear first reach the cerebral cortex and are processed by the primary auditory cortex in the left temporal lobe.

The areas associated with vision in the temporal lobe interpret the meaning of visual stimuli and establish object recognition. The central part of the temporal cortices appears to be involved in the high-level visual processing of complex stimuli such as faces (fusiform gyrus) and scenes (parahippocampal gyrus). Anterior parts of this ventral stream for visual processing are involved in object perception and recognition.

White Matter of the Cerebrum

White matter is composed of myelinated axons and glia and connects distinct areas of the cortex.

White matter is one of the two components of the central nervous system (CNS). It consists mostly of glial cells and myelinated axons and forms the bulk of the deep parts of the cerebrum and the superficial parts of the spinal cord. In a freshly cut brain, the tissue of white matter appears pinkish white to the naked eye because myelin is composed largely of lipid tissue containing capillaries. The axons of white matter transmit signals from various grey matter areas (the locations of nerve cell bodies) of the cerebrum to each other and carry nerve impulses between neurons. While grey matter is primarily associated with processing and cognition, white matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.

Tracts

There are three different kinds of tracts (bundles of axons) that connect one part of the brain to another within the white matter:

• Projection tracts extend vertically between higher and lower brain areas and spinal cord centers, and carry information between the cerebrum and the rest of the body. Other projection tracts carry signals upward to the cerebral cortex. Superior to the brainstem, such tracts form a broad, dense sheet called the internal capsule between the thalamus and basal nuclei, then radiate in a diverging, fanlike array to specific areas of the cortex.
• Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures. The great majority of commissural tracts pass through the large corpus callosum. A few tracts pass through the much smaller anterior and posterior commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other.
• Association tracts connect different regions within the same hemisphere of the brain. Long association fibers connect different lobes of a hemisphere to each other, whereas short association fibers connect different gyri within a single lobe. Among their roles, association tracts link perceptual and memory centers of the brain.

Corpus Callosum

The corpus callosum (Latin: “tough body”), also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication. It is the largest white matter structure in the brain, consisting of 200 to 250 million contralateral axonal projections.

The posterior portion of the corpus callosum is called the splenium, the anterior is called the genu (or “knee”), and the area between the two is the truncus or body of the corpus callosum. The part between the body and the splenium is often markedly thin and thus called the isthmus. The rostrum is the part of the corpus callosum that projects posteriorly and inferiorly from the anteriormost genu. The rostrum is so named for its resemblance to a bird’s beak.

Agenesis of the corpus callosum (ACC) is a rare congenital disorder in which the corpus callosum is partially or completely absent. It is usually diagnosed within the first two years of life and may manifest as a severe syndrome in infancy or childhood, as a milder condition in young adults, or as an asymptomatic incidental finding. Initial symptoms of ACC usually include seizures that may be followed by feeding problems and delays in holding the head erect, sitting, standing, and walking. Hydrocephaly may also occur.

Other possible symptoms include impairments in mental and physical development, hand-eye coordination, and visual and auditory memory. In mild cases, symptoms such as seizures, repetitive speech, or headaches may not appear for years.

Basal Ganglia

The basal ganglia is important for the control of movement and forming habits, and each of its components has a complex internal anatomical and neurochemical organization.

The basal ganglia (or basal nuclei) are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus, and other brain areas. The components of the basal ganglia include the striatum, pallidum, substantia nigra, and subthalamic nucleus. Each of these components has a complex internal anatomical and neurochemical organization.

Structure

The main components of the basal ganglia are:

• The striatum, or neostriatum: This component consists of 3 divisions: the caudate, putamen, and ventral striatum (includes the nucleus accumbens). The striatum receives input from many brain areas but sends output only to other components of the basal ganglia.
• Globus pallidus, or pallidum: This component is composed of the globus pallidus external (GPe) and globus pallidus internal (GPi). The pallidum receives its most important input from the striatum (either directly or indirectly), and sends inhibitory output to a number of motor-related areas, including the part of the thalamus that projects to the motor-related areas of the cortex.
• Substantia nigra: This component consists of the substantia nigra pars compacta (SNc) and substantia nigra pars reticulate (SNr). The SNr functions similarly to the pallidum, and the SNc cells contain neuromelanin and produce dopamine (a neurotransmitter) for input to the striatum.
• The subthalamic nucleus (STN): The STN receives input mainly from the striatum and cortex, and projects to a portion of the pallidum (internal portion or GPi). It is the only portion of the ganglia that produces an excitatory neurotransmitter, glutamate. The role of the subthalamic nucleus is to stimulate the SNr-GPi complex, and it receives inhibitory input from the GPe and sends excitatory signals to the GPi.

Function

The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or habits such as bruxism, eye movements, and cognitive, emotional functions. Currently, popular theories implicate the basal ganglia primarily in action selection, that is, the decision of which several possible behaviors to execute at a given time. Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems and that a release of this inhibition permits a motor system to become active. The behavior switching that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.

The basal ganglia play a central role in a number of neurological conditions, including several movement disorders. The most notable are Parkinson’s disease, which involves degeneration of the melanin-pigmented dopamine-producing cells in the substantia nigra pars compacta (SNc), and Huntington’s disease, which primarily involves damage to the striatum. Basal ganglia dysfunction is also implicated in some other disorders of behavior control such as Tourette’s syndrome, ballismus (particularly hemibalismus), obsessive-compulsive disorder, and Wilson’s disease (hepatolenticular degeneration). With the exception of Wilson’s disease and hemiballismus, the neuropathological mechanisms underlying diseases of ganglia such as Parkinson’s’ and Huntington’s are not very well understood or are at best still developing theories.

The basal ganglia have a limbic sector whose components are the nucleus accumbens, ventral pallidum, and ventral tegmental area (VTA). This limbic sector is thought to play a central role in reward learning, particularly a pathway from the VTA to the nucleus accumbens that uses the neurotransmitter dopamine. A number of highly addictive drugs, including cocaine, amphetamine, and nicotine, are thought to work by increasing the efficacy of this dopamine signal.

Limbic System

The limbic system makes up the inner border of the cortex and is vital for emotion, motivation, and memory.

The limbic system, or paleomammalian brain, is a set of brain structures in the cortex and subcortex of the brain. It includes the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex, and fornix, and supports a variety of functions including emotion, behavior, motivation, long-term memory, and olfaction. The term “limbic” comes from the Latin limbus, for “border” or “edge,” because the limbic system forms the inner border of the cortex.

Limbic System Anatomy

The limbic system consists of various structures that each support distinctive brain functions.

Hippocampus and Associated Structures

• Hippocampus: Required for the formation of long-term memories and implicated in the maintenance of cognitive maps for navigation.
• Amygdala: Involved in signaling the cortex of motivationally significant stimuli, such as those related to reward and fear, and in social functions, such as mating.
• Fornix: A white matter structure that carries signals from the hippocampus to the mammillary bodies and septal nuclei.
• Mammillary body: Important for the formation of memory.

Septal Nuclei

• These lie below the rostrum of the corpus callosum and anterior to the lamina terminalis. The septal nuclei receive reciprocal connections from the olfactory bulb, hippocampus, amygdala, hypothalamus, midbrain, habenula, cingulate gyrus, and thalamus.

Limbic Lobe

A phylogenetically old structure consisting of the following structures:

• Parahippocampal gyrus: Plays a role in the formation of spatial memory
• Cingulate gyrus: Conducts autonomic functions regulating heart rate, blood pressure, and cognitive and attentional processing
• Dentate gyrus: Thought to contribute to the formation of new memories

Additional Structures

• Entorhinal cortex: Important memory and associative components
• Piriform cortex: Processes olfactory information
• Fornicate gyrus: Region encompassing the cingulate and parahippocampal gyrus
• Nucleus accumbens: Involved in reward, pleasure, and addiction
• Orbitofrontal cortex: Involved in cognitive processing during decision-making

Limbic System Function

The limbic system operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens, the brain’s pleasure center, which plays a role in sexual arousal and the “high” derived from certain recreational drugs.

The structures of the limbic system are involved in motivation, emotion, learning, and memory.

The limbic system is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. To cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery called a prefrontal lobotomy. Patients who underwent this procedure often became passive and lacked motivation.

Vasculature

The blood supply to the cerebrum can be simply classified into 3 distinct paired arterial branches:

• Anterior Cerebral Arteries – branches of internal carotid arteries, supplying the anteromedial aspect of the cerebrum.
• Middle Cerebral Arteries – continuation of internal carotid arteries, supplying most of the lateral portions of the cerebrum.
• Posterior Cerebral Arteries – branches of the basilar arteries, supplying both the medial and lateral sides of the cerebrum posteriorly.

Venous drainage of the cerebrum is via a network of small cerebral veins. These vessels empty into the dural venous sinuses – endothelial lined spaces between the outer and inner layers of dura mater.

Anatomy: Brain

• brain
  • grey matter
  • white matter
  • cerebrum
    • cerebral hemisphere (telencephalon)
      • cerebral lobes and gyri
        • frontal lobe
          • frontal pole
          • frontopolar cortex
          • superior frontal gyrus
          • middle frontal gyrus
          • inferior frontal gyrus
            • pars orbitalis
            • pars triangularis
            • pars opercularis
          • precentral gyrus
          • medial frontal gyrus
            • supplementary motor area
          • paracentral lobule
          • cingulate gyrus
            • anterior cingulate cortex
          • orbital gyrus
          • gyrus rectus
          • rostral gyrus
          • septal area
        • parietal lobe
          • postcentral gyrus
          • superior parietal lobule
          • inferior parietal lobule
            • supramarginal gyrus
            • angular gyrus
          • precuneus
        • occipital lobe
          • occipital pole
          • lingual gyrus
          • fusiform gyrus (Brodmann area 37)
          • calcarine (visual) cortex
          • cuneus
        • temporal lobe
          • temporal pole
          • Heschl gyrus
          • superior temporal gyrus
          • middle temporal gyrus
          • inferior temporal gyrus
          • fusiform gyrus
          • mesial temporal lobe
            • amygdala
            • hippocampus
              • malrotation of the hippocampus
            • uncus
            • dentate gyrus
              • band of Giacomini
            • parahippocampal gyrus
            • medial occipitotemporal gyrus
            • subiculum
            • entorhinal cortex
          • lateral parietotemporal line
        • basal forebrain
          • anterior perforated substance
          • substantia innominata
            • basal nucleus of Meynert
        • limbic system
          • limbic lobe
          • parahippocampus
          • septal nuclei
          • nucleus accumbens
          • Papez circuit
        • insula
          • limen insulae
          • operculum
      • cerebral sulci and fissures (A-Z)
        • calcarine fissure
        • callosal sulcus
        • central (Rolandic) sulcus
        • cingulate sulcus
        • collateral sulcus
        • inferior frontal sulcus
        • inferior occipital sulcus
        • inferior temporal sulcus
        • interhemispheric fissure
        • intraparietal sulcus
        • lateral (Sylvian) sulcus
          • anterior ramus of the lateral sulcus
          • ascending ramus of the lateral sulcus
          • circular sulcus
        • lateral occipital sulcus
        • marginal sulcus
        • occipitotemporal sulcus
        • olfactory sulcus
        • paracentral sulcus
        • paraolfactory sulcus
        • parieto-occipital fissure
        • posterior parolfactory sulcus
        • precentral sulcus
        • preoccipital notch
        • postcentral sulcus
        • rhinal sulcus
        • rostral sulcus
        • subparietal sulcus
        • superior frontal sulcus
        • superior occipital sulcus
        • superior temporal sulcus
      • cortical histology
        • Betz cells
    • white matter tracts
      • commissures
        • corpus callosum
          • indusium griseum
          • callososeptal interface
          • Probst bundles
        • anterior commissure
        • hippocampal commissure
          • psalterium
        • habenular commissure
        • posterior commissure
        • supraoptic commissure
          • Gudden commissure
          • Meynert commissure
          • Gasner commissure
      • fornix
      • forceps major
      • forceps minor
      • internal capsule
      • external capsule
      • extreme capsule
      • corona radiata
      • centrum semiovale
      • corticobulbar
      • medial lemniscus
      • optic radiation
        • Meyer loop
        • superior geniculocalcarine tract
      • subcortical U-fibers
      • terminal zones of myelination
      • uncinate fasciculus
    • deep grey matter
      • basal ganglia claustrum
        • caudate nucleus
          • caudothalamic groove
        • corpus striatum
        • lentiform nucleus
          • globus pallidus
          • putamen
        • neostriatum
        • nucleus accumbens
    • pituitary gland
      • posterior pituitary and stalk (part of diencephalon)
        • ectopic posterior pituitary
      • anterior pituitary
      • inferior hypophyseal arterial circle
    • diencephalon
      • thalamencephalon
        • thalamus
          • interthalamic adhesion
          • lateral geniculate nucleus
          • medial geniculate nucleus
        • metathalamus
        • epithalamus
          • habenula
          • stria medullaris
          • pineal gland
      • subthalamus
        • subthalamic nuclei
      • hypothalamus
        • supraoptic nucleus
        • mammillary bodies
        • tuber cinereum
  • brainstem
    • midbrain (mesencephalon)
      • tectal plate
      • tegmentum
      • cerebral peduncles
      • corpora quadrigemina
      • posterior perforated substance
      • periaqueductal grey matter
    • pons (part of metencephalon)
      • facial colliculus
      • superior olivary nucleus
    • medulla oblongata (myelencephalon)
      • olive
      • inferior olivary nucleus
      • nucleus of the tractus solitarius
      • nucleus ambiguus
      • dorsal vagal motor nucleus
    • white matter
    • grey matter
      • ​non-cranial nerve
        • substantia nigra
        • red nucleus
        • superior colliculi
        • inferior colliculi
      • cranial nerve nuclei
        • oculomotor nucleus
        • Edinger-Westphal nucleus
        • trochlear nucleus
        • motor nucleus of CN V
        • ​mesencephalic nucleus of CN V
        • main sensory nucleus of CN V
        • spinal nucleus of CN V
        • abducent nucleus
        • facial nucleus
        • superior salivatory nucleus
        • cochlear nuclei
        • vestibular nuclei
        • inferior salivatory nucleus
        • solitary tract nucleus
        • ambiguus nucleus
        • dorsal vagal motor nucleus
        • hypoglossal nucleus
  • cerebellum (part of metencephalon)
    • vermis
    • cerebellar hemisphere
      • cerebellar tonsil
      • dentate nucleus
    • cerebellar peduncles
      • superior cerebellar peduncle
      • middle cerebellar peduncle
      • inferior cerebellar peduncle
  • cranial meninges (meninx primitiva)
    • dura mater (pachymeninx)
      • falx cerebri
      • tentorium cerebelli
      • falx cerebelli
      • petroclinoid ligaments
      • diaphragma sellae
      • carotid cave
    • leptomeninges
      • arachnoid mater
        • Liliequest membrane
        • arachnoid granulations
          • aberrant arachnoid granulations
      • pia mater
        • velum interpositum
          • cavum velum interpositum
    • extradural space
    • subdural space
    • subarachnoid space
    • blood supply of the meninges
    • innervation of the meninges
  • CSF spaces
    • ventricular system
      • lateral ventricles
        • septum pellucidum
          • cavum septum pellucidum
          • cavum vergae
      • interventricular foramen (of Monro)
        • choroidal fissure
      • third ventricle
        • lamina terminalis
        • supraoptic recess
        • infundibular recess
        • pineal recess
        • suprapineal recess
      • aqueduct of Sylvius
      • fourth ventricle
        • superior medullary velum
        • inferior medullary velum
        • obex
        • area postrema
        • rhomboid fossa
      • calcar avis
      • foramen of Magendie
      • foramina of Lushka
      • tela choroidea
      • cerebrospinal fluid
        • choroid plexus
          • Bochdalek’s flower basket
    • subarachnoid cisterns
      • suprasellar cistern
        • empty sella sign
      • interpeduncular cistern
      • ambient cistern
        • transverse fissure
      • quadrigeminal cistern
      • pericallosal cistern
      • prepontine cistern
      • cerebellopontine cistern
      • premedullary cistern
      • Sylvian cistern
      • cisterna magna
        • mega cisterna magna
      • Meckel cave
  • cranial nerves (mnemonic)
    • olfactory nerve (CN I)
    • optic nerve (CN II)
      • optic chiasm
      • optic tract
    • oculomotor nerve (CN III)
    • trochlear nerve (CN IV)
    • trigeminal nerve (CN V) (mnemonic)
      • trigeminal ganglion
    • abducens nerve (CN VI)
    • facial nerve (CN VII) (segments mnemonic | branches mnemonic)
      • geniculate ganglion
        • greater (superficial) petrosal nerve
          • Vidian nerve
        • external petrosal nerve
        • nerve to stapedius
      • nervus intermedius
    • vestibulocochlear nerve (CN VIII)
      • vestibular ganglion (Scarpa’s ganglion)
    • glossopharyngeal nerve (CN IX)
      • Jacobson nerve
      • lesser petrosal nerve
    • vagus nerve (CN X)
      • Arnold’s nerve
    • spinal accessory nerve (CN XI)
    • hypoglossal nerve (CN XII)
  • functional neuroanatomy
    • Brodmann areas
    • cortical motor system
      • extrapyramidal system
    • cortical sensory system
      • pain and temperature sensation
      • vibration and proprioception sensation
    • auditory/speech system
      • Broca’s area (Brodmann area 44)
      • Wernicke’s area (Brodmann area 22)
    • visual system
    • olfactory system
    • default mode network
  • CNS development
    • brain development
      • neural plate
        • neural tube
      • prosencephalon
        • telencephalon
          • paraphysis elements
          • Rathke pouch
        • diencephalon
      • mesencephalon
      • rhombencephalon
        • metencephalon
        • myelencephalon
  • cerebral vascular supply
    • arteries
      • vascular territories
        • anterior circulation
        • posterior circulation
      • circle of Willis
        • internal carotid artery (ICA) (segments)
          • inferolateral trunk
          • meningohypophyseal trunk
            • inferior hypophyseal artery
          • capsular arteries (of McConnell) (variable)
          • superior hypophyseal artery
          • anterior cerebral artery (ACA)
            • anterior communicating artery (ACOM)
            • medial lenticulostriate arteries
            • recurrent artery of Heubner
            • medial frontobasal artery
            • frontopolar artery
            • orbitofrontal artery
            • pericallosal artery
              • callosomarginal artery
              • pericallosal moustache
          • middle cerebral artery (MCA)
            • M1 branches
              • lenticulostriate arteries
                • medial lenticulostriate arteries
                • lateral lenticulostriate arteries
              • anterior temporal artery
              • temporopolar artery
            • M2 branches
          • posterior communicating artery (PCOM)
          • ophthalmic artery
            • lacrimal artery
            • supraorbital artery
            • posterior ethmoidal artery
            • anterior ethmoidal artery
            • internal palpebral artery
            • supratrochlear artery (frontal artery)
            • dorsal nasal artery
          • anterior choroidal artery
        • vertebral artery
          • posterior inferior cerebellar artery (PICA)
          • basilar artery
            • anterior inferior cerebellar artery (AICA)
              • labyrinthine artery
            • pontine arteries
            • superior cerebellar artery (SCA)
            • posterior cerebral artery (PCA)
              • calcarine artery
              • splenial artery
              • posterior choroidal artery
                • medial posterior choroidal artery
                • lateral posterior choroidal artery
        • normal variants
          • intracranial arterial fenestration
          • internal carotid artery (ICA)
            • absent ICA
            • aberrant ICA
          • anterior cerebral artery (ACA)
            • azygos ACA
          • middle cerebral artery (MCA)
            • accessory MCA
            • duplicated MCA
            • MCA fenestration
          • posterior cerebral artery (PCA)
            • fetal origin of PCA / fetal PCOM
            • artery of Percheron
          • basilar artery
            • basilar artery fenestration
            • basilar artery hypoplasia
          • persistent carotid-vertebrobasilar artery anastomoses (mnemonic)
            • persistent primitive trigeminal artery
            • persistent otic artery
            • persistent hypoglossal artery
            • persistent proatlantal artery
              • persistent proatlantal intersegmental artery
          • vertebral artery
            • vertebral artery hypoplasia
          • ophthalmic artery
            • meningo-ophthalmic artery
    • cerebral venous system
      • dural venous sinuses
        • basilar venous plexus
        • cavernous sinus (mnemonic)
        • clival diploic veins
        • inferior petro-occipital vein
        • inferior petrosal sinus
        • inferior sagittal sinus
        • intercavernous sinus
        • internal carotid artery venous plexus of Rektorzik
        • jugular bulb
        • marginal sinus
        • occipital sinus
        • sigmoid sinus
        • sphenoparietal sinus
        • straight sinus
        • superior petrosal sinus
          • superior petrosal vein
        • superior sagittal sinus
        • torcula herophili
        • transverse sinus
      • cerebral veins
        • superficial veins of the brain
          • superior cerebral veins (superficial cerebral veins)
          • inferior cerebral veins
          • superficial middle cerebral vein
          • superior anastomotic vein (of Trolard)
          • inferior anastomotic vein (of Labbe)
      • deep veins of the brain
        • vein of Galen (median prosencephalic vein)
          • basal vein of Rosenthal
          • internal cerebral vein
            • thalamostriate vein
            • septal cerebral vein
        • venous circle of Trolard
      • normal variants
        • persistent falcine sinus
  • glymphatic pathway

References

The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain, the brainstem is composed of the midbrain, the pons, and the medulla oblongata. … Ten pairs of cranial nerves come from the brainstem.

The brainstem (brain stem) is the distal part of the brain that is made up of the midbrain, pons, and medulla oblongata. Each of the three components has its own unique structure and function. Together, they help to regulate breathing, heart rate, blood pressure, and several other important functions. All of these brainstem functions are enabled because of its unique anatomy; since the brainstem houses cranial nerve nuclei and is a passageway for many important neural pathways. This article will discuss brainstem anatomy in a student-friendly mode and help you ace your neuroanatomy exams.

Functions of the Brain Stem

The brainstem regulates vital cardiac and respiratory functions and acts as a vehicle for sensory information.

Location and Basic Physiology

Invertebrate anatomy, the brainstem is the most inferior portion of the brain, adjoining and structurally continuous with the brain and spinal cord. The brainstem gives rise to cranial nerves 3 through 12 and provides the main motor and sensory innervation to the face and neck via the cranial nerves. Though small, it is an extremely important part of the brain, as the nerve connections of the motor and sensory systems from the main part of the brain that communicate with the peripheral nervous system pass through the brainstem. This includes the corticospinal tract (motor), the posterior column-medial lemniscus pathway (fine touch, vibration sensation, and proprioception ), and the spinothalamic tract ( pain, temperature, itch, and crude touch). The brain stem also plays an important role in the regulation of cardiac and respiratory function. It regulates the central nervous system (CNS) and is pivotal in maintaining consciousness and regulating the sleep cycle.

Components of the Brainstem

The three components of the brainstem are the medulla oblongata, midbrain, and pons.

Brainstem Anatomy: Structures of the brainstem are depicted on these diagrams, including the midbrain, pons, medulla, basilar artery, and vertebral arteries.

The medulla oblongata (myelencephalon) is the lower half of the brainstem continuous with the spinal cord. Its upper part is continuous with the pons. The medulla contains the cardiac, respiratory, vomiting, and vasomotor centers regulating heart rate, breathing, and blood pressure.

The midbrain (mesencephalon) is associated with vision, hearing, motor control, sleep and wake cycles, alertness, and temperature regulation.

The pons (part of the metencephalon) lies between the medulla oblongata and the midbrain. It contains tracts that carry signals from the cerebrum to the medulla and to the cerebellum. It also has tracts that carry sensory signals to the thalamus.

Brainstem Function

The brainstem has many basic functions, including regulation of heart rate, breathing, sleeping, and eating. It also plays a role in conduction. All information relayed from the body to the cerebrum and cerebellum and vice versa must traverse the brainstem. The ascending pathways from the body to the brain are the sensory pathways, including the spinothalamic tract for pain and temperature sensation and the dorsal column, fasciculus gracilis, and cuneatus for touch, proprioception, and pressure sensation. The facial sensations have similar pathways and also travel in the spinothalamic tract and the medial lemniscus.

Descending tracts are upper motor neurons destined to synapse on lower motor neurons in the ventral horn and intermediate horn of the spinal cord. In addition, upper motor neurons originate in the brain stem’s vestibular, red, tectal, and reticular nuclei, which also descend and synapse in the spinal cord. The brainstem also has integrative functions, including cardiovascular system control, respiratory control, pain sensitivity control, alertness, awareness, and consciousness.

Medulla Oblongata

The medulla oblongata controls autonomic functions and connects the higher levels of the brain to the spinal cord.

The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla. The medulla contains the cardiac, respiratory, vomiting, and vasomotor centers and regulates autonomic, involuntary functions such as breathing, heart rate, and blood pressure.

The Brain Stem with Pituitary and Pineal Glands: Medulla oblongata labeled at bottom left, in relation to the pons, pituitary gland, spinal cord, pineal gland, and cerebellum.

The medulla is often divided into two parts:

• An open or superior part where the dorsal surface of the medulla is formed by the fourth ventricle.
• A closed or inferior part where the metacoel (caudal part of the fourth ventricle) lies within the medulla oblongata.

Structure of the Medulla Oblongata

The region between the anterior median and anterolateral sulci is occupied by an elevation on either side known as the pyramid of the medulla oblongata. This elevation is caused by the corticospinal tract. In the lower part of the medulla, some of these fibers cross each other, thus obliterating the anterior median fissure. This is known as the decussation of the pyramids. Other fibers that originate from the anterior median fissure above the decussation of the pyramids and run laterally across the surface of the pons are known as the external arcuate fibers.

The region between the anterolateral and posterolateral sulcus in the upper part of the medulla is marked by a swelling known as the olivary body, caused by a large mass of gray matter known as the inferior olivary nucleus.

The posterior part of the medulla between the posterior median and posterolateral sulci contains tracts that enter it from the posterior funiculus of the spinal cord. These are the fasciculus gracilis, lying medially next to the midline, and the fasciculus cuneatus, lying laterally.

The fasciculi end in rounded elevations known as the gracile and cuneate tubercles. They are caused by masses of gray matter known as the nucleus gracilis and the nucleus cuneatus. Just above the tubercles, the posterior aspect of the medulla is occupied by a triangular fossa, which forms the lower part of the floor of the fourth ventricle. The fossa is bounded on either side by the inferior cerebellar peduncle, which connects the medulla to the cerebellum.

The lower part of the medulla, immediately lateral to the fasciculus cuneatus, is marked by another longitudinal elevation known as the tuberculum cinereum. It is caused by an underlying collection of gray matter known as the spinal nucleus of the trigeminal nerve. The gray matter of this nucleus is covered by a layer of nerve fibers that form the spinal tract of the trigeminal nerve.

The base of the medulla is defined by the commissural fibers, crossing over from the ipsilateral side in the spinal cord to the contralateral side in the brain stem; below this is the spinal cord.

Embryonic Development

During development, the medulla oblongata forms from the myelencephalon. The final neuroblasts from the alar plate of the neural tube produce the sensory nuclei of the medulla. The basal plate neuroblasts give rise to the motor nuclei.

The function of the Medulla Oblongata

The medulla oblongata controls autonomic functions and connects the higher levels of the brain to the spinal cord. It is also responsible for regulating several basic functions of the autonomic nervous system, including:

• Respiration: chemoreceptors
• Cardiac center: sympathetic system, parasympathetic system
• Vasomotor center: baroreceptors
• Reflex centers of vomiting, coughing, sneezing, and swallowing

Pons

The pons is a relay station between the forebrain and cerebellum that passes sensory information from the periphery to the thalamus.

Pons/Brainstem: Structure of the brainstem showing the location of the pons in relation to the midbrain and medulla.

The pons is a structure located on the brainstem, named after the Latin word for “bridge.” It is above the medulla, below the midbrain, and anterior to the cerebellum. The white matter of the pons includes tracts that conduct signals from the cerebrum down to the cerebellum and medulla and tracts that carry the sensory signals up into the thalamus.

Structure

The pons measure about 2.5 cm in length in adults. Most of it appears as a broad anterior bulge rostral to the medulla. Posteriorly, it consists mainly of two pairs of thick stalks called cerebellar peduncles. These connect the cerebellum to the pons and midbrain.

The pons contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that regulate sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture. Within the pons is the pneumatic center, a nucleus that regulates the change from inspiration to expiration. The pons also contains the sleep paralysis center of the brain and also play a role in generating dreams.

Development

During embryonic development, the metencephalon develops from the rhombencephalon and gives rise to two structures: the pons and the cerebellum. The alar plate produces sensory neuroblasts, which will give rise to the solitary nucleus and its special visceral afferent column, the cochlear and vestibular nuclei (which form the special somatic afferent fibers of the vestibulocochlear nerve), the spinal and principal trigeminal nerve nuclei (which form the general somatic afferent column of the trigeminal nerve), and the pontine nuclei, which is involved in motor activity. Basal plate neuroblasts give rise to the abducens nucleus (forms the general somatic efferent fibers), the facial and motor trigeminal nuclei (form the special visceral efferent column), and the superior salivatory nucleus, which forms the general visceral efferent fibers of the facial nerve.

Cranial Nerves of the Pons

A number of cranial nerve nuclei are present in the pons:

• The chief or pontine nucleus of the trigeminal nerve sensory nucleus (V)- mid-pons
• The motor nucleus for the trigeminal nerve (V)-mid-pons
• Abducens nucleus (VI)-lower pons
• Facial nerve nucleus (VII)-lower pons
• Vestibulocochlear nuclei (VIII)-lower pons

Functional Characteristics

The functions of the four nerves of the pons include sensory roles in hearing, equilibrium, taste, and facial sensations such as touch and pain. They also have motor roles in eye movement, facial expressions, chewing, swallowing, urination, and the secretion of saliva and tears. Central pontine myelinosis is a demyelination disease that causes difficulty with sense of balance, walking, sense of touch, swallowing, and speaking. If it is not diagnosed and treated, it can lead to death or locked-in syndrome (a condition in which a person is conscious but cannot move or communicate).

Midbrain

Midbrain nuclei

The midbrain consists of:

• Periaqueductal gray: The area of gray matter around the cerebral aqueduct, which contains various neurons involved in the pain desensitization pathway. Neurons synapse here and, when stimulated, cause activation of neurons in the nucleus raphe Magnus, which then projects down into the posterior grey column of the spinal cord and prevent pain sensation transmission.
• Oculomotor nerve nucleus: This is the third cranial nerve nucleus.
• Trochlear nerve nucleus: This is the fourth cranial nerve.
• Red nucleus: This is a motor nucleus that sends a descending tract to the lower motor neurons.
• Substantia nigra pars compacta: This is a concentration of neurons in the ventral portion of the midbrain that uses dopamine as its neurotransmitter and is involved in both motor function and emotion. Its dysfunction is implicated in Parkinson’s disease.
• Reticular formation: This is a large area in the midbrain that is involved in various important functions of the midbrain. In particular, it contains lower motor neurons, is involved in the pain desensitization pathway, is involved in the arousal and consciousness systems, and contains the locus coeruleus, which is involved in intensive alertness modulation and in autonomic reflexes.
• Central tegmental tract: Directly anterior to the floor of the fourth ventricle, this is a pathway by which many tracts project up to the cortex and down to the spinal cord.
• Ventral tegmental area: A dopaminergic nucleus, known as group A10 cells^[rx] is located close to the midline on the floor of the midbrain.
• Rostromedial tegmental nucleus: A GABAergic nucleus located adjacent to the ventral tegmental area.

The midbrain plays a major role in both wakefulness and regulation of homeostasis.

The midbrain or mesencephalon (from the Greek mesos, middle, and enkephalos, brain ) is a portion of the central nervous system (CNS) associated with vision, hearing, motor control, sleep and wake cycles, arousal (alertness), and temperature regulation. Anatomically, it comprises the tectum (or corpora quadrigemina), tegmentum, ventricular mesocoelia (or “iter”), and the cerebral peduncles, as well as several nuclei and fasciculi. Caudally (posteriorly) the mesencephalon adjoins the pons (metencephalon), and rostrally it adjoins the diencephalon (eg., thalamus, hypothalamus). The midbrain is located below the cerebral cortex and above the hindbrain placing it near the center of the brain.

Primary Midbrain Components

Brainstem Anatomy: Brainstem anatomy showing the location of the midbrain in relation to the midbrain, pons, medulla, basilar artery, and vertebral arteries.

The tectum (Latin for “roof”) is formed by the superior and inferior colliculi and comprises the rear portion of the midbrain. The superior colliculus regulates preliminary visual processing and eye movement, while the inferior colliculus is involved in auditory processing. Collectively, the colliculi are referred to as the corpora quadrigemina.

The tegmentum is involved in many unconscious homeostatic and reflexive pathways and is the motor center that relays inhibitory signals to the thalamus and basal nuclei to prevent unwanted body movement. It extends from the substantia nigra to the cerebral aqueduct (also called the ventricular mesocolic). The nuclei of cranial nerves III and IV are located in the tegmentum portion of the midbrain.

The substantia nigra is closely associated with motor system pathways of the basal ganglia. The human mesencephalon is archipallian in origin, sharing its general architecture with the most ancient of vertebrates. Dopamine produced in the substantia nigra plays a role in the motivation and habituation of species from humans to the most elementary animals such as insects. The midbrain is the smallest region in the brain and helps to relay information for vision and hearing.

The cerebral peduncles are located on either side of the midbrain and are its most anterior part, acting as the connectors between the rest of the midbrain and the thalamic nuclei. The cerebral peduncles assist in motor movement refinement, motor skill learning, and converting proprioceptive information into balance and posture maintenance.

Embryonic Development

During embryonic development, the midbrain arises from the second vesicle, also known as the mesencephalon, of the neural tube. Unlike the other two vesicles (the prosencephalon and rhombencephalon), the mesencephalon remains undivided for the remainder of neural development. It does not split into other brain areas while the prosencephalon, for example, divides into the telencephalon and the diencephalon. Throughout embryonic development, the cells within the midbrain continually multiply and compress the still-forming aqueduct of Sylvius or cerebral aqueduct. Partial or total obstruction of the cerebral aqueduct during development can lead to congenital hydrocephalus.

Reticular Formation

The reticular formation assists in the regulation of the sleep cycle and detecting sensory salience.

The reticular formation is a region in the pons involved in regulating the sleep-wake cycle and filtering incoming stimuli to discriminate irrelevant background stimuli. It is essential for governing some of the basic functions of higher organisms and is one of the phylogenetically oldest portions of the brain.

Divisions of the Reticular Formation

Traditionally, the nuclei are divided into three columns:

• Raphe nuclei (medium column)
• Magnocellular red nucleus (medial zone)
• Parvocellular reticular nucleus (lateral zone)

Sagittal division reveals more morphological distinctions. The raphe nuclei form a ridge in the middle of the reticular formation, and directly to its periphery, there is a division called the medial reticular formation. The medial reticular formation is large, has long ascending and descending fibers, and is surrounded by the lateral reticular formation. The lateral reticular formation is close to the motor nuclei of the cranial nerves and mostly mediates their function. The raphe nuclei is the place of synthesis of the neurotransmitter serotonin, which plays an important role in mood regulation.

The medial reticular formation and lateral reticular formation are two columns of neuronal nuclei with ill-defined boundaries that send projections through the medulla and into the mesencephalon (midbrain). The nuclei can be differentiated by function, cell type, and projections of efferent or afferent nerves. The magnocellular red nucleus is involved in motor coordination, and the parvocellular nucleus regulates exhalation.

The original functional differentiation was a division of caudal and rostral, based on the observation that damage to the rostral reticular formation induces hypersomnia in the cat brain. In contrast, damage to the more caudal portion of the reticular formation produces insomnia in cats. This study led to the idea that the caudal portion inhibits the rostral portion of the reticular formation.

Functions

The reticular formation consists of more than 100 small neural networks, with varied functions including:

• Somatic motor control: Some motor neurons send their axons to the reticular formation nuclei, giving rise to the reticulospinal tracts of the spinal cord. These tracts play a large role in maintaining tone, balance, and posture, especially during movement. The reticular formation also relays eye and ear signals to the cerebellum so that visual, auditory, and vestibular stimuli can be integrated in motor coordination. Other motor nuclei include gaze centers, which enable the eyes to track and fixate objects, and central pattern generators, which produce rhythmic signals to the muscles of breathing and swallowing.
• Cardiovascular control: The reticular formation includes the cardiac and vasomotor centers of the medulla oblongata.
• Pain modulation: Reticular formation is one means by which pain signals from the lower body reach the cerebral cortex. It is also the origin of the descending analgesic pathways. The nerve fibers in these pathways act in the spinal cord to block the transmission of some pain signals to the brain.
• Sleep and consciousness: The reticular formation has projections to the thalamus and cerebral cortex that allow it to exert some control over which sensory signals reach the cerebrum and come to our conscious attention. It plays a central role in states of consciousness like alertness and sleep. Injury to the reticular formation can result in an irreversible coma.
• Habituation: This is a process in which the brain learns to ignore repetitive, meaningless stimuli while remaining sensitive to others. A good example of this is when a person can sleep through loud traffic in a large city but is awakened promptly by the sound of an alarm or crying baby. Reticular formation nuclei that modulate the activity of the cerebral cortex are part of the reticular activating system.

Effects of Damage

• Mass lesions in the brainstem cause severe alterations in the level of consciousness (such as coma) because of their effects on reticular formation. Lesions in the reticular formation have been found in the brains of people who have post-polio syndrome. Some imaging studies have shown abnormal activity in this area in people with chronic fatigue syndrome, indicating a high likelihood that damage to the reticular formation is responsible for the fatigue associated with these syndromes.

Major Brainstem Tracts

• The Reticular Formation – The reticular formation is found in the anterior portion of the brainstem and is composed of multiple tracts that have a large number of connections. The reticular formation extends from the spinal cord through the brainstem to the diencephalon. It receives input from various tracts including, spinothalamic tracts, spinoreticular tracts, the dorsal column medial lemniscus pathway, visual pathways, auditory pathways, vestibular pathways, and cerebelloreticular pathways. The reticular formation sends efferent fibers to the thalamic nuclei, cerebellum, red nucleus, corpus striatum, substantia nigra, hypothalamus, and subthalamic nucleus.[rx] The vast connections of the reticular formation allow it to modulate many different functions; some of these include movement coordination, autonomic regulation of blood pressure, heart rate, and respiratory rate, postural reflexes, neuro-vegetative reflexes, and taste.[rx] It also plays a role in wakefulness and sleep.[rx]

The Motor Tracts

• Corticospinal Tracts – The majority of the upper motor neurons of the motor tracts originate in the precentral gyrus. The corticospinal fibers descend through the posterior limb of the internal capsule to the crus cerebri and then down the anterior pons to the pyramids of the medulla. At the pyramids, the majority of the corticospinal fibers decussate and descend the spinal cord as the lateral corticospinal tract and eventually continue to supply motor innervation to the limbs and digits. The majority of corticospinal fibers that do not cross over at the medullary pyramids become the medial corticospinal tracts, located anteriorly in the spinal cord, and provide innervation to the muscles of the trunk.[rx]
• Corticobulbar Tracts – The corticobulbar tracts descend through the genu of the internal capsule and down through a similar course as the corticospinal fibers; however, the corticobulbar fibers exit this course and synapse at the appropriate cranial nerve nuclei at their respective levels. The majority of corticospinal fibers decussate while only some of the corticobulbar fibers decussate as described in the nerves section of this article. The corticobulbar tracts also contain connections with many of the sensory nuclei of the brainstem.[rx]

The Sensory Tracts

• Spinothalamic Tract or Anterolateral System – The spinothalamic tract is responsible for conveying pain and temperature information from the body to the brain. Peripheral neurons carry sensory information to the posterior column of the spinal cord. After synapsing in the spinal cord, the axons ascend two to three levels before decussating. After decussating, the fibers ascend as the lateral and anterior spinothalamic tracts in the anterior and lateral portions of the spinal cord. When the tracts ascend through the medulla, they merge to form the spinothalamic tract and course along the lateral portion of the medulla. The tract continues up the lateral portion of the anterior pons and midbrain to the ventral posterior lateral thalamus where the axons synapse and continue up through the posterior limb of the internal capsule to enter the post-central gyrus of the cortex.[rx]
• Dorsal Column-Medial Lemniscus – The dorsal column-medial lemniscus tract is responsible for carrying afferent proprioception, fine touch, two-point discrimination, and vibration to the cortex from the body. Peripheral neurons carry sensory information to the posterior column of the spinal cord and ascend in the posterior portions of the spinal cord as the gracile fasciculus and cuneate fasciculus. The neurons in these fasciculi will synapse of the gracile nucleus and cuneate nucleus at the level of the inferior medulla respectively. The second-order neurons will decussate at the level of the medulla and become the medial lemniscus. The medial lemniscus maintains a medial position within the brainstem as it ascends to the ventral posterior lateral thalamus. After synapsing in the thalamus, the fibers continue through the posterior limb of the internal capsule to the post-central gyrus of the cortex.[rx]
• Trigeminal Lemniscus and Spinotrigeminal Tract – Pain and temperature sensory input from the face enters the brainstem via cranial nerve V. The fibers that carry this information enter the brainstem and descend parallel to the spinal trigeminal nucleus before synapsing in it. Their descent forms the spinotrigeminal tract. After these fibers synapse, they decussate to the contralateral side and ascend as a part of the trigeminal lemniscus.[rx] The trigeminal lemniscus carries sensory axons from the second-order neurons of the principal sensory nucleus of the trigeminal nerve, which contain discriminative touch and oral cavity proprioception. These neurons do not descend before synapsing after entering the brainstem. Most of these fibers decussate to the contralateral side on their course to the ventral posterior medial thalamus and then proceed to the post-central gyrus of the cortex. The fibers of the trigeminal lemniscus ascend the pons and midbrain posterior to the medial lemniscus.
• Lateral Lemniscus – The lateral lemniscus carries auditory information from the cochlear nuclei at the level of the inferior pons superiorly to the superior olivary complex, nuclei of the lateral lemniscus, inferior colliculi, and eventually to the medial geniculate body which sends the auditory information to the temporal lobes of the cerebral cortex. Some of the fibers of the lateral lemniscus decussate while others do not. The lateral lemniscus travels up the posterior lateral portion of the pons and is important for sensory input to the brain.[rx]

Midbrain

Blood-Brain Barrier and Cerebrospinal Fluid/The brain is protected by the bones of the skull and by a covering of three thin membranes called meninges. The brain is also cushioned and protected by cerebrospinal fluid. This watery fluid is produced by special cells in the four hollow spaces in the brain, called ventricles.

Blood-Brain Barrier

Tight junctions present in the blood-brain barrier separate circulating blood from cerebrospinal fluid, regulating diffusion into the brain.

The blood-brain barrier (BBB) is a separation of circulating blood from the brain extracellular fluid in the central nervous system (CNS). Bacteriologist Paul Ehrlich observed that chemical dye injected into an animal would stain all of its organs except for the brain. In a later experiment, his student Edwin Goldmann found that when dye is directly injected into the cerebrospinal fluid (CSF) of animals’ brains, the brains were dyed while the rest of the organs were unaffected. This clearly demonstrated the existence of some sort of compartmentalization between the brain and the rest of the body. The concept of the BBB (then termed hematoencephalic barrier) was proposed by Lewandowsky in 1900. It was not until the introduction of the scanning electron microscope that the actual membrane could be observed and proven to exist.

Blood-Brain Barrier Structure

Function and Importance of the Blood-Brain BarrierThe BBB results from the selectivity of the tight junctions between endothelial cells in CNS vessels that restrict the passage of solutes. At the interface between blood and the brain, endothelial cells are joined by these tight junctions, which are composed of smaller subunits, frequently biochemical dimers that are transmembrane proteins such as occludin, claudins, and junctional adhesion molecule. Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex. This barrier also includes a thick basement membrane and astrocyte cell projections called astrocytic feet (forming the thin barrier called the glia limitans) that surround the endothelial cells of the BBB, providing biochemical support to those cells.

The BBB endothelial cells restrict the passage of substances from the bloodstream to a greater extent than endothelial cells in capillaries elsewhere in the body. The diffusion of microscopic particles (e.g., bacteria), large molecules, and hydrophilic molecules into the CSF is restricted, while the diffusion of small hydrophobic molecules (O2, CO2, hormones ) is permitted. Also, BBB cells actively transport metabolic products such as glucose across the barrier.

Non-Protected Areas of the Brain

Several areas of the human brain are not protected by the BBB. These include the circumventricular organs such as the area postrema, the median eminence of the hypothalamus, pineal gland, and posterior pituitary. The posterior pituitary and pineal gland are not covered by the BBB because they secrete hormones into the circulation. The median eminence is not covered by BBB because the pituitary secretions collect in this area before releasing into the circulation. The area postrema detects noxious substances present in the blood and is therefore not covered by the BBB.

Role of Blood-Brain Barrier in Infectious Processes

The BBB effectively protects the brain from many common bacterial infections, so brain infections are very rare. However, since antibodies and antibiotics are too large to cross the BBB, brain infections that do occur are often very serious and difficult to treat. However, the BBB becomes more permeable during inflammation. This allows some antibiotics and phagocytes to move across the BBB but also allows bacteria and viruses to cross. Diseases caused by spirochetes are exceptions to this bacterial exclusion. These include Borrelia (the cause of Lyme disease), and Treponema pallidum, which causes syphilis. These harmful bacteria seem to breach the BBB by physically tunneling through the blood vessel walls. Some toxins are made up of large molecules that cannot pass through the BBB. Neurotoxins such as botulinum toxin in food might affect peripheral nerves, but the BBB can often prevent such toxins from reaching the CNS, where they could cause serious or fatal damage.

Cerebrospinal Fluid and Its Circulation

Cerebrospinal fluid is a clear fluid that acts as a cushion for the brain and maintains overall central nervous system homeostasis.

Cerebrospinal fluid (CSF) is a clear, colorless bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord. The CSF occupies the space between the arachnoid mater (the middle layer of the brain cover, the meninges ) and the pia mater (the layer of the meninges closest to the brain). It constitutes the content of all intracerebral ventricles, cisterns, and sulci (singular sulcus), as well as the central canal of the spinal cord. It acts as a cushion or buffer for the cortex, providing basic mechanical and immunological protection for the brain inside the skull and serving a vital function in the cerebral autoregulation of cerebral blood flow.

CSF Production

Between 50 to 70% of CSF is produced in the brain by modified ependymal cells in the choroid plexus, and the remainder is formed around blood vessels and along ventricular walls. It circulates from the lateral ventricles to the foramen of Monro (interventricular foramen), third ventricle, aqueduct of Sylvius (cerebral aqueduct), fourth ventricle, foramen of Magendie (median aperture), foramen of Luschka (lateral apertures), and the subarachnoid space over the brain and the spinal cord. CSF is reabsorbed into venous sinus blood via arachnoid granulations.

The Choroid Plexus: This diagram indicates the (1) posterior medullary velum (2) choroid plexus (3) cisterna cerebellomedullaris of subarachnoid cavity (4) central canal (5) corpora quadrigemina (6) cerebral peduncle (7) anterior medullary velum (8) ependymal lining of ventricle (9) cisterna pontis of subarachnoid cavity

CSF is produced at a rate of 500 ml/day. Since the subarachnoid space around the brain and spinal cord can contain only 135 to 150 ml, large amounts are drained into the blood through arachnoid granulations in the superior sagittal sinus. Thus, the CSF turns over about 3.7 times a day. This continuous flow into the venous system dilutes the concentration of larger, lipid-insoluble molecules penetrating the brain and CSF. The CSF contains approximately 0.3% plasma proteins, or approximately 15 to 40 mg/dL, depending on the sampling site.

Functions of CSF

The functions of CSF include:

• Buoyancy: The actual mass of the human brain is about 1400 grams; however, the net weight of the brain suspended in the CSF is equivalent to a mass of 25 grams. The brain therefore exists in neutral buoyancy, which allows the brain to maintain its density without being impaired by its own weight.
• Protection: CSF protects the brain tissue from injury when jolted or hit.
• Chemical stability: CSF flows throughout the inner ventricular system in the brain and is absorbed back into the bloodstream, rinsing the metabolic waste from the central nervous system (CNS) through the blood-brain barrier. This allows for homeostatic regulation of the distribution of neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system.
• Prevention of brain ischemia: Decreasing the amount of CSF in the limited space inside the skull decreases total intracranial pressure and facilitates blood perfusion.
• Clearing waste: CSF has been shown by the research group of Maiken Nedergaard to be critical in the brain’s glymphatic system, the functional waste clearance pathway for the vertebrate CNS.

CSF as a Diagnostic Tool

When CSF pressure is elevated, cerebral blood flow may be constricted. When disorders of CSF flow occur, they may therefore affect not only CSF movement but also craniospinal compliance and intracranial blood flow, with subsequent neuronal and glial vulnerabilities. The venous system is also important in this equation. CSF can be tested for the diagnosis of various neurological diseases, usually with a procedure called lumbar puncture. Lumbar puncture is performed in an attempt to count the cells in the fluid and to detect the levels of protein and glucose. These parameters alone may be extremely beneficial in the diagnosis of subarachnoid hemorrhage and CNS infections such as meningitis. Moreover, a CSF culture examination may yield the microorganism that has caused the infection.

Ventricles

The ventricular system is a set of hollow cavities in the brain filled with cerebrospinal fluid.

The ventricular system is a set of four interconnected cavities (ventricles) in the brain and the location of CSF production. This system is continuous with the central canal of the spinal cord. The system comprises four ventricles:

• right and left lateral ventricles (the first and second ventricles)
• third ventricle
• fourth ventricle

CSF Production

The cavities of the cerebral hemispheres are called lateral ventricles or first and second ventricles. These two ventricles open into the third ventricle by a common opening called the foramen of Monro. CSF is produced by modified ependymal cells of the choroid plexus found in all components of the ventricular system except for the cerebral aqueduct and the posterior and anterior horns of the lateral ventricles. The brain and spinal cord are covered by a series of tough membranes called meninges, which protect these organs from rubbing against the bones of the skull and spine. The CSF within the skull and spine is found between the pia mater and the arachnoid and provides further cushioning. EndFra

CSF Flow Within Ventricles

CSF flows from the lateral ventricles via the foramina of Monro into the third ventricle, and then into the fourth ventricle via the cerebral aqueduct in the brainstem. From there, it passes into the central canal of the spinal cord and into the cisterns of the subarachnoid space via three small foramina: the central foramen of Magendie and the two lateral foramina of Luschka. The fluid then flows around the superior sagittal sinus to be reabsorbed via the arachnoid villi into the venous system. CSF within the spinal cord can flow all the way down to the lumbar cistern at the end of the cord around the cauda equina.

Ventricular System Dysfunction and Disease

Diseases of the ventricular system include abnormal enlargement (hydrocephalus) and inflammation of the CSF spaces (meningitis, ventriculitis) caused by infection or introduction of blood following trauma or hemorrhage. The aqueduct between the third and fourth ventricles is very small, as are the foramina. This means they can be easily blocked, causing high pressure in the lateral ventricles. This is a common cause of hydrocephalus (known colloquially as “water on the brain”), an extremely serious condition due to the damage caused by the pressure as well as the nature of the block (e.g., a tumor or inflammatory swelling).

Embryonic Development of the Ventricles

The structures of the ventricular system are embryologically derived from the center of the neural tube (the neural canal). As the future brain stem aspect of the primitive neural tube develops, the neural canal expands dorsally and laterally, creating the fourth ventricle.  The cerebral aqueduct is formed from the part of the neural canal that does not expand and remains the same at the level of the midbrain superior to the fourth ventricle. The fourth ventricle narrows at the obex, where the fourth ventricle narrows to become the central canal in the caudal medulla.

Functions of Brain/The brain is one of the largest and most complex organs in our body; it is composed of billions of neurons that communicate together by forming numerous connections and synapses. The brain weight is different between men and women; The male brain weighs about 1336 grams, and the female comes in at about 1198 grams, but this difference in weight has shown no effect on the function or intelligence[rx]. There are three main divisions cerebrum, cerebellum, brain stem. The cerebrum consists of two cerebral hemispheres the outer layer called the cortex (gray matter) and the inner layer (white matter). There are four lobes in the cortex, the frontal lobe, parietal lobe, temporal lobe, occipital lobe.

The Brain

The brain is the neurological center of an organism.

The human brain is the center of the human nervous system. It has the same general structure as the brains of other mammals but is larger than expected on the basis of body size when compared to other primates. Estimates for the number of neurons ( nerve cells) in the human brain range from 80 to 120 billion. Most of the expansion comes from the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the human brain is susceptible to many types of damage and disease. This includes degenerative disorders such as Parkinson’s disease, multiple sclerosis, and Alzheimer’s disease. A number of psychiatric conditions, such as schizophrenia and depression, are thought to be associated with brain dysfunction, although the nature of such brain anomalies is not well understood.

Cerebral Cortex

The cerebral hemispheres form the largest part of the human brain and are situated above most other brain structures. They are covered with a cortical layer and have a convoluted topography. The cerebral cortex is essentially a sheet of neural tissue folded in a way that allows a large surface area to fit within the confines of the skull. Anatomists call each cortical fold a sulcus and the smooth area between folds a gyrus. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.

Cortex Divisions

Diagram of Brain Regions: Demonstration of brain regions, including the four lobes and internal structures.

The left and right hemispheres of the cerebral cortex are nearly symmetrical. The hemispheres are connected by the corpus callosum, the largest white-matter structure in the brain. Anatomists conventionally divide each hemisphere into four lobes: the frontal (control of specialized motor control, learning, planning, and speech), parietal (control of somatic sensory functions), occipital (control of vision), and temporal lobes (control of hearing and some speech). The division into lobes does not actually arise from the structure of the cortex itself. Instead, each section is named after the skull bone that covers it. The borders between lobes are placed beneath the sutures that link the skull bones together. The only exception is the border between the frontal and parietal lobes, which is shifted backward from the corresponding suture to the central sulcus. This deep fold marks the line where the primary somatosensory cortex (main sensory receptive area for the sense of touch) and primary motor cortex (one of the principal areas of the brain involved in motor function) come together. Functionally, the cortex is commonly described as comprising three parts: sensory, motor, and association areas.

Brain Stem and Cerebellum

The cerebrum is attached to a stalk-like structure called the brain stem, which consists of the midbrain, pons, and medulla. At the rear of the brain beneath the cerebrum and behind the brainstem is the cerebellum. This structure has a horizontally furrowed surface with an appearance that is distinct from all other brain areas. These same structures are present in other mammals, although the cerebellum is not so large relative to the rest of the brain in non-human mammals.

Hippocampus

Found deep in the temporal lobe, the seahorse-shaped hippocampus is responsible for memory. The amygdala is a small, almond-shaped structure deep inside the anteroinferior region of the temporal lobe. It plays an important role in the mediation and control of activities and feelings such as love, friendship, affection, and mood expression. The amygdala is the center for danger identification, a fundamental part of self-preservation.

Thalamus

The thalamus is perched on top of the brainstem near the center of the brain, with nerve fibers projecting out to the cerebral cortex in all directions. Its functions include relaying sensory and motor signals to the cerebral cortex and regulating consciousness, sleep, and alertness. It likely acts as a relay between a variety of subcortical areas and the cerebral cortex.

Hypothalamus

The hypothalamus is a small part of the brain located just below the thalamus on both sides of the third ventricle. The hypothalamus also plays a role in emotion. Specifically, the lateral parts seem to be involved with pleasure and rage, while the medial part is linked to aversion, displeasure, and a tendency toward uncontrollably loud laughing. When the physical symptoms of emotion appear, the threat they pose returns to the limbic centers via the hypothalamus, then to the prefrontal nuclei, increasing anxiety.

Function

The frontal lobe

It is the largest lobe, located in front of the cerebral hemispheres, and has significant functions for our body, and these are:

• Prospective memory – a type of memory that involves remembering the plans that you made, from a simple daily plan to future lifelong plans.[rx]
• Speech and language  – The frontal lobe has an area called Broca’s area located in the posterior inferior frontal gyrus that is involved in speech production. A recent study shows that the exact function of Broca’s area is to mediate sensory representations that originate in the temporal cortex and going to the motor cortex.[rx]
• Personality – During the past centuries, several researchers have described that there are personality changes that occurred after frontal lobe injuries. One of the most important cases was about Phineas Gage, who was a gentle, polite sociable young, man until a large iron rod, went through his eye-damaging his prefrontal cortex. This injury made him emotionally insensitive, perform socially inappropriate behaviors, and was unable to make a rational judgment. A recent study suggests that when there is damage to the prefrontal cortex, there are five sub-types of personality changes that occur, and these include:
• Executive disturbances
• Disturbed social behavior
• Emotional Dysregulation
• Hypo-emotionality/De-energization
• Distress [rx]
• Decision making:

The ability to decide on something involves reasoning, learning, and creativity. A study conducted in 2012 proposed a new model to understand how the decision-making process occurs in the frontal lobe, specifically how the brain creates a new strategy to a new-recurrent situation or an open-ended environment, they called it the PROBE model.

There are typically three possible ways to adapt to a situation

• Selecting a previously learned strategy that applies precisely to the current situation
• Adjusting an already learned approach
• Developing a creative behavioral method

The PROBE model illustrates that the brain can compare from three to four behavioral methods at most, then choose the best strategy for the situation.[rx]

• Movement control – The frontal lobe has the motor cortex divided into two regions: the primary motor area located posterior to the precentral sulcus and non-primary motor areas including the premotor cortex, supplementary motor area, and cingulate motor areas. The exact function of each structure and its role in the movement is still an active research area.[rx]
• The parietal lobe – It is located posterior to the frontal lobe and superior to the temporal lobe and classified into two functional regions. The anterior parietal lobe contains the primary sensory cortex (SI), located in the postcentral gyrus (Broadman area BA 3, 1, 2). SI receives the majority of the sensory inputs that are coming from the thalamus, and it’s responsible for interpreting the simple somatosensory signals like (touch, position, vibration, pressure, pain, temperature).[rx]

The posterior parietal lobe has two regions: the superior parietal lobule and the inferior parietal lobule.

• The Superior parietal lobule contains the somatosensory association (BA 5, 7) cortex which is involved in higher-order functions like motor planning action.
• The Inferior parietal lobule (supramarginal gyrus BA 40, angular gyrus BA 39) has the  Secondary somatosensory cortex (SII), which receives the somatosensory inputs from the thalamus and the contralateral SII, and they integrate those inputs with other major modalities (examples: visual inputs, auditory inputs) to form higher-order complex functions like:
• Sensorimotor planning
• Learning
• Language
• Spatial recognition
• Stereognosis: the ability to differentiate between objects regarding their size, shape, weight, and any other differences.[rx]

The temporal lobe

The second most substantial portion occupies the middle cranial fossa and lies posterior to the frontal lobe and inferior to the parietal lobe. There are two surfaces, the lateral surface, and the medial surface.[rx]

The lateral surface is classified by the superior temporal sulcus and the lateral temporal sulcus into three gyri; the superior temporal gyrus and the middle temporal gyrus and the inferior temporal gyrus.

• The superior temporal gyrus (STG) is further sub-divided into two surfaces, the dorsal surface (superior temporal plane STP) and the lateral surface of the STG.
• The STP is located deep in the Sylvain fissure. The most significant anatomical landmark in STP is the Heschl gyrus (HG) which contains the primary auditory cortex. It is responsible for translating and processing all sounds and tones, and it is minimally affected by task requirements. Task requirement: a test where the examiner pronounces some words and asks the participant to categorize them acoustically, or phonemically, or semantically.[rx]The STP has another important area next to the HG called Wernicke’s area. In the past, this area was thought to have a significant role in speech perception and comprehension, but recent evidence shows that this area is not involved in this process. Researchers found that this process is not a simple task, but moreover, it is a complex task that is distributed all over the brain. The primary function of this area is the phonological representation, a process where the pronounced word is interpreted based on their tones and sound and trying to link it to a previously learned sound.[rx]
• The lateral surface of the STG is thought to be the secondary auditory cortex that also functions in interpreting sounds, but mostly in the activities that involve task requirements.[rx]
• The middle temporal gyrus (MTG) has four sub-regions, the anterior, middle, posterior, and sulcus MTG.[rx]

The Anterior MTG is primarily involved in

The default mode network has a specific activity that exists naturally in the brain at rest. So if you are studying or engaging in a game or doing any other activity that demands you to stay focused or setting a particular goal this mode will be deactivated.

• Sound recognition helps the other areas that we talked about before.
• Semantic retrieval a process that assigns meaning to the words or sounds by trying to retrieve the previously learned concepts if they existed.

The Middle MTG has two functions

• Semantic memory a type of memory involved in remembering the thoughts or objectives that are common knowledge (for example, where the bathroom is located).
• Semantic control network a system of connections between different areas of the brain including the middle MTG, to assign meaning to words, sounds that require both stored knowledge and mechanisms of semantic retrieval.

The Posterior MTG is thought to be part of the classical sensory language area.

The Sulcus MTG is involved in decoding gaze directions and in speech.

• The inferior temporal gyrus (IT) is involved in visual perception and facial perception by containing the ventral visual pathway, the pathway that carries the information from the primary visual cortex to the temporal lobe, to determine the content of the vision.[rx]
• The medial surface of the temporal lobe (medial temporal lobe) includes important structures (Hippocampus, Entorhinal, Perirhinal, Parahippocampal cortex) that are anatomically related and are mandatory for declarative memory. Declarative memory is a type of long-term memory that involves remembering the concepts or ideas and the events that happened or learned throughout your life. It is further divided into three types of memory:
• Semantic memory we talked about it previously (see middle MTG).
• Recognition memory the memory involved in recognizing an object, and all the other details that relate to this object. There are two forms: recollection and familiarity.
• Recollection means you can remember the object and almost every single detail that is related to that object, such as time and place.
• Familiarity means you remember encountering the object previously, but you don’t recall any specific detail about it. For example, when you say to a person, Your face is familiar, but I can’t remember where and when we met.
• Episodic memory the memory that specializes in recalling an event and its associated details, and this is different from recognition memory in which you can consciously able to memorialize a specific event that happened throughout your life without being exposed to a similar situation.

The medial temporal lobe (memory system) is still an active research area, more precisely the exact function of each structure in this lobe is currently being studied.[rx]

The occipital lobe

The occipital lobe is the smallest lobe in the cerebrum cortex, and it is located in the most posterior region of the brain, posterior to the parietal lobe and temporal lobe. The role of this lobe is visual processing and interpretation. Typically based on the function and structure, the visual cortex is divided into five areas (v1-v5). The primary visual cortex (v1, BA 17) is the first area that receives the visual information from the thalamus, and its located around the calcarine sulcus. The visual cortex receive, process, interpret the visual information, then this processed information is sent to the other regions of the brain to be further analyzed (example: inferior temporal lobe). This visual information helps us to determine, recognize, and compare the objects to each other.[rx]

References

Anatomy of Brain/The brain is one of the largest and most complex organs in our body; it is composed of billions of neurons that communicate together by forming numerous connections and synapses. The brain weight is different between men and women; The male brain weighs about 1336 grams, and the female comes in at about 1198 grams, but this difference in weight has shown no effect on the function or intelligence[rx]. There are three main divisions cerebrum, cerebellum, brain stem. The cerebrum consists of two cerebral hemispheres the outer layer called the cortex (gray matter) and the inner layer (white matter). There are four lobes in the cortex, the frontal lobe, parietal lobe, temporal lobe, occipital lobe.

The Brain

The brain is the neurological center of an organism.

The human brain is the center of the human nervous system. It has the same general structure as the brains of other mammals but is larger than expected on the basis of body size when compared to other primates. Estimates for the number of neurons ( nerve cells) in the human brain range from 80 to 120 billion. Most of the expansion comes from the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the human brain is susceptible to many types of damage and disease. This includes degenerative disorders such as Parkinson’s disease, multiple sclerosis, and Alzheimer’s disease. A number of psychiatric conditions, such as schizophrenia and depression, are thought to be associated with brain dysfunction, although the nature of such brain anomalies is not well understood.

Cerebral Cortex

The cerebral hemispheres form the largest part of the human brain and are situated above most other brain structures. They are covered with a cortical layer and have a convoluted topography. The cerebral cortex is essentially a sheet of neural tissue folded in a way that allows a large surface area to fit within the confines of the skull. Anatomists call each cortical fold a sulcus and the smooth area between folds a gyrus. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.

Cortex Divisions

Diagram of Brain Regions: Demonstration of brain regions, including the four lobes and internal structures.

The left and right hemispheres of the cerebral cortex are nearly symmetrical. The hemispheres are connected by the corpus callosum, the largest white-matter structure in the brain. Anatomists conventionally divide each hemisphere into four lobes: the frontal (control of specialized motor control, learning, planning, and speech), parietal (control of somatic sensory functions), occipital (control of vision), and temporal lobes (control of hearing and some speech). The division into lobes does not actually arise from the structure of the cortex itself. Instead, each section is named after the skull bone that covers it. The borders between lobes are placed beneath the sutures that link the skull bones together. The only exception is the border between the frontal and parietal lobes, which is shifted backward from the corresponding suture to the central sulcus. This deep fold marks the line where the primary somatosensory cortex (main sensory receptive area for the sense of touch) and primary motor cortex (one of the principal areas of the brain involved in motor function) come together. Functionally, the cortex is commonly described as comprising three parts: sensory, motor, and association areas.

Brain Stem and Cerebellum

The cerebrum is attached to a stalk-like structure called the brain stem, which consists of the midbrain, pons, and medulla. At the rear of the brain beneath the cerebrum and behind the brainstem is the cerebellum. This structure has a horizontally furrowed surface with an appearance that is distinct from all other brain areas. These same structures are present in other mammals, although the cerebellum is not so large relative to the rest of the brain in non-human mammals.

Hippocampus

Found deep in the temporal lobe, the seahorse-shaped hippocampus is responsible for memory. The amygdala is a small, almond-shaped structure deep inside the anteroinferior region of the temporal lobe. It plays an important role in the mediation and control of activities and feelings such as love, friendship, affection, and mood expression. The amygdala is the center for danger identification, a fundamental part of self-preservation.

Thalamus

The thalamus is perched on top of the brainstem near the center of the brain, with nerve fibers projecting out to the cerebral cortex in all directions. Its functions include relaying sensory and motor signals to the cerebral cortex and regulating consciousness, sleep, and alertness. It likely acts as a relay between a variety of subcortical areas and the cerebral cortex.

Hypothalamus

The hypothalamus is a small part of the brain located just below the thalamus on both sides of the third ventricle. The hypothalamus also plays a role in emotion. Specifically, the lateral parts seem to be involved with pleasure and rage, while the medial part is linked to aversion, displeasure, and a tendency toward uncontrollably loud laughing. When the physical symptoms of emotion appear, the threat they pose returns to the limbic centers via the hypothalamus, then to the prefrontal nuclei, increasing anxiety.

Function

The frontal lobe

It is the largest lobe, located in front of the cerebral hemispheres, and has significant functions for our body, and these are:

• Prospective memory – a type of memory that involves remembering the plans that you made, from a simple daily plan to future lifelong plans.[rx]
• Speech and language  – The frontal lobe has an area called Broca’s area located in the posterior inferior frontal gyrus that is involved in speech production. A recent study shows that the exact function of Broca’s area is to mediate sensory representations that originate in the temporal cortex and going to the motor cortex.[rx]
• Personality – During the past centuries, several researchers have described that there are personality changes that occurred after frontal lobe injuries. One of the most important cases was about Phineas Gage, who was a gentle, polite sociable young, man until a large iron rod, went through his eye-damaging his prefrontal cortex. This injury made him emotionally insensitive, perform socially inappropriate behaviors, and was unable to make a rational judgment. A recent study suggests that when there is damage to the prefrontal cortex, there are five sub-types of personality changes that occur, and these include:
• Executive disturbances
• Disturbed social behavior
• Emotional Dysregulation
• Hypo-emotionality/De-energization
• Distress [rx]
• Decision making:

The ability to decide on something involves reasoning, learning, and creativity. A study conducted in 2012 proposed a new model to understand how the decision-making process occurs in the frontal lobe, specifically how the brain creates a new strategy to a new-recurrent situation or an open-ended environment, they called it the PROBE model.

There are typically three possible ways to adapt to a situation

• Selecting a previously learned strategy that applies precisely to the current situation
• Adjusting an already learned approach
• Developing a creative behavioral method

The PROBE model illustrates that the brain can compare from three to four behavioral methods at most, then choose the best strategy for the situation.[rx]

• Movement control – The frontal lobe has the motor cortex divided into two regions: the primary motor area located posterior to the precentral sulcus and non-primary motor areas including the premotor cortex, supplementary motor area, and cingulate motor areas. The exact function of each structure and its role in the movement is still an active research area.[rx]
• The parietal lobe – It is located posterior to the frontal lobe and superior to the temporal lobe and classified into two functional regions. The anterior parietal lobe contains the primary sensory cortex (SI), located in the postcentral gyrus (Broadman area BA 3, 1, 2). SI receives the majority of the sensory inputs that are coming from the thalamus, and it’s responsible for interpreting the simple somatosensory signals like (touch, position, vibration, pressure, pain, temperature).[rx]

The posterior parietal lobe has two regions: the superior parietal lobule and the inferior parietal lobule.

• The Superior parietal lobule contains the somatosensory association (BA 5, 7) cortex which is involved in higher-order functions like motor planning action.
• The Inferior parietal lobule (supramarginal gyrus BA 40, angular gyrus BA 39) has the  Secondary somatosensory cortex (SII), which receives the somatosensory inputs from the thalamus and the contralateral SII, and they integrate those inputs with other major modalities (examples: visual inputs, auditory inputs) to form higher-order complex functions like:
• Sensorimotor planning
• Learning
• Language
• Spatial recognition
• Stereognosis: the ability to differentiate between objects regarding their size, shape, weight, and any other differences.[rx]

The temporal lobe

The second most substantial portion occupies the middle cranial fossa and lies posterior to the frontal lobe and inferior to the parietal lobe. There are two surfaces, the lateral surface, and the medial surface.[rx]

The lateral surface is classified by the superior temporal sulcus and the lateral temporal sulcus into three gyri; the superior temporal gyrus and the middle temporal gyrus and the inferior temporal gyrus.

• The superior temporal gyrus (STG) is further sub-divided into two surfaces, the dorsal surface (superior temporal plane STP) and the lateral surface of the STG.
• The STP is located deep in the Sylvain fissure. The most significant anatomical landmark in STP is the Heschl gyrus (HG) which contains the primary auditory cortex. It is responsible for translating and processing all sounds and tones, and it is minimally affected by task requirements. Task requirement: a test where the examiner pronounces some words and asks the participant to categorize them acoustically, or phonemically, or semantically.[rx]The STP has another important area next to the HG called Wernicke’s area. In the past, this area was thought to have a significant role in speech perception and comprehension, but recent evidence shows that this area is not involved in this process. Researchers found that this process is not a simple task, but moreover, it is a complex task that is distributed all over the brain. The primary function of this area is the phonological representation, a process where the pronounced word is interpreted based on their tones and sound and trying to link it to a previously learned sound.[rx]
• The lateral surface of the STG is thought to be the secondary auditory cortex that also functions in interpreting sounds, but mostly in the activities that involve task requirements.[rx]
• The middle temporal gyrus (MTG) has four sub-regions, the anterior, middle, posterior, and sulcus MTG.[rx]

The Anterior MTG is primarily involved in

The default mode network has a specific activity that exists naturally in the brain at rest. So if you are studying or engaging in a game or doing any other activity that demands you to stay focused or setting a particular goal this mode will be deactivated.

• Sound recognition helps the other areas that we talked about before.
• Semantic retrieval a process that assigns meaning to the words or sounds by trying to retrieve the previously learned concepts if they existed.

The Middle MTG has two functions

• Semantic memory a type of memory involved in remembering the thoughts or objectives that are common knowledge (for example, where the bathroom is located).
• Semantic control network a system of connections between different areas of the brain including the middle MTG, to assign meaning to words, sounds that require both stored knowledge and mechanisms of semantic retrieval.

The Posterior MTG is thought to be part of the classical sensory language area.

The Sulcus MTG is involved in decoding gaze directions and in speech.

• The inferior temporal gyrus (IT) is involved in visual perception and facial perception by containing the ventral visual pathway, the pathway that carries the information from the primary visual cortex to the temporal lobe, to determine the content of the vision.[rx]
• The medial surface of the temporal lobe (medial temporal lobe) includes important structures (Hippocampus, Entorhinal, Perirhinal, Parahippocampal cortex) that are anatomically related and are mandatory for declarative memory. Declarative memory is a type of long-term memory that involves remembering the concepts or ideas and the events that happened or learned throughout your life. It is further divided into three types of memory:
• Semantic memory we talked about it previously (see middle MTG).
• Recognition memory the memory involved in recognizing an object, and all the other details that relate to this object. There are two forms: recollection and familiarity.
• Recollection means you can remember the object and almost every single detail that is related to that object, such as time and place.
• Familiarity means you remember encountering the object previously, but you don’t recall any specific detail about it. For example, when you say to a person, Your face is familiar, but I can’t remember where and when we met.
• Episodic memory the memory that specializes in recalling an event and its associated details, and this is different from recognition memory in which you can consciously able to memorialize a specific event that happened throughout your life without being exposed to a similar situation.

The medial temporal lobe (memory system) is still an active research area, more precisely the exact function of each structure in this lobe is currently being studied.[rx]

The occipital lobe

The occipital lobe is the smallest lobe in the cerebrum cortex, and it is located in the most posterior region of the brain, posterior to the parietal lobe and temporal lobe. The role of this lobe is visual processing and interpretation. Typically based on the function and structure, the visual cortex is divided into five areas (v1-v5). The primary visual cortex (v1, BA 17) is the first area that receives the visual information from the thalamus, and its located around the calcarine sulcus. The visual cortex receive, process, interpret the visual information, then this processed information is sent to the other regions of the brain to be further analyzed (example: inferior temporal lobe). This visual information helps us to determine, recognize, and compare the objects to each other.[rx]

References