Brainstem – Anatomy, Types, Tract, Functions

Brainstem – Anatomy, Types, Tract, Functions

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.

Key Points

Invertebrate anatomy, the brainstem is the posterior part of the brain adjoining, and structurally continuous with, the spinal cord.

Though small, the brainstem is an extremely important part of the brain, as the nerve connections from the motor and sensory systems of the cortex pass through it to communicate with the peripheral nervous system.

The brainstem also plays an important role in the regulation of cardiac and respiratory function, consciousness, and the sleep cycle.

The brainstem consists of the medulla oblongata, pons, and midbrain.

Key Terms

pons: Contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that deal primarily with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture.

midbrain: Associated with vision, hearing, motor control, sleep and wake cycles, alertness, and temperature regulation.

medulla: The lower half of the brainstem contains the cardiac, respiratory, vomiting, and vasomotor centers and regulates autonomic, involuntary functions such as breathing, heart rate, and blood pressure.

Diseases of the brainstem can result in abnormalities in cranial nerve function, leading to visual and hearing disturbances, changes in sensation, muscle weakness, vertigo, coordination problems, swallowing and speech difficulty, and voice changes.

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.

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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.

This diagram labels the cranial nerves, including olfactory, oculomotor, trochlear, abducens, hypoglossal, vestibulocochlear, accessory, vagus, facial, glossopharangeal, facial, trigeminal, and optic.

Human Brain with Cranial Nerves: Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves. Only the first and the second pair emerge from the cerebrum; the remaining ten pairs emerge from the brainstem.

Medulla Oblongata

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

Key Points

The medulla oblongata is the lower half of the brainstem. It controls autonomic functions and connects the higher levels of the brain to the spinal cord.

The medulla oblongata is responsible for regulating several basic functions of the autonomic nervous system, including respiration, cardiac function, vasodilation, and reflexes like vomiting, coughing, sneezing, and swallowing.

Key Terms

tuberculum cinereum: A raised area between the rootlets of the accessory nerve and posterolateral sulcus that overlies the spinal tract of the trigeminal nerve.

cerebellar peduncle: The structure that connects the medulla to the cerebellum.

sympathetic system: The division of the autonomic nervous system responsible for stimulating the body’s fight-or-flight response.

olivary body: Either of a pair of prominent oval structures in the medulla oblongata containing the olivary nuclei. These structures are involved in cerebellar motor learning and the perception of sound.

parasympathetic system: The division of the autonomic nervous system responsible for the relaxation or inhibition of various body functions.

EXAMPLES

A stroke can injure the pyramidal tract, medial lemniscus, and hypoglossal nucleus. This causes a syndrome called a medial medullary syndrome, a type of alternating hemiplegia characterized by recurrent episodes of paralysis on one side of the body.

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.

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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.

Key Points

The pons is a structure located on the brainstem, named after the Latin word for “bridge.”

This white matter includes tracts that conduct signals from the cerebrum down to the cerebellum and medulla, as well as tracts that carry the sensory signals up into the thalamus.

The pons contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that deal primarily with 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 plays a role in generating dreams.

The functions of these four nerves include sensory roles in hearing, equilibrium, taste, and in 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.

Key Terms

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.

pneumotaxic center: A network of neurons in the rostral dorsal lateral pons that regulates the respiratory rate; also known as the pontine respiratory group (PRG).

Basal plate: The region of the neural tube ventral to the sulcus limitans and containing primarily motor neurons.

alar plate: Also called the alar lamina, it is a neural structure in the embryonic nervous system; the caudal part later becomes the sensory axon aspect of the spinal cord.

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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.

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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.

Key Points

The midbrain or mesencephalon 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, the midbrain comprises the tectum (or corpora quadrigemina), tegmentum, ventricular mesocoelia (or “iter”), and cerebral peduncles, as well as several nuclei and fasciculi.

During embryonic development, the midbrain arises from the second vesicle, (mesencephalon) of the neural tube.

The mesencephalon is considered part of the brainstem.

Key Terms

mesencephalon: A part of the brain located rostral to the pons and caudal to the thalamus and the basal ganglia, composed of the tectum (dorsal portion) and the tegmentum (ventral portion).

substantia nigra: Brain structure located in the midbrain that plays an important role in reward and movement.

tectum: The dorsal part of the midbrain, responsible for auditory and visual reflexes.

tegmentum: The ventral portion of the midbrain, a multisynaptic network of neurons involved in many unconscious homeostatic and reflexive pathways.

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

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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.

Key Points

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.

The reticular formation consists of more than 100 small neural networks with varied functions including motor control, cardiovascular control, pain modulation, sleep, and habituation.

Bilateral damage to the reticular formation of the midbrain may lead to coma or death.

Traditionally, the nuclei of the reticular formation are divided into three columns: the median column or the Raphe nuclei, the medial column or the magnocellular nuclei, and the lateral column or parvocellular nuclei.

Key Terms

magnocellular nuclei: Nuclei within the reticular formation involved in motor coordination.

parvocellular nuclei: Nuclei within the reticular formation that are involved in the regulation of expiration during breathing and other motor functions.

raphe nuclei: Located in the pons of the brainstem, the principal site of the synthesis of the neurotransmitter serotonin. Serotonin plays an important role in mood regulation, particularly when stress is associated with depression and anxiety.

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.

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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.

This cross-section of the pons indicates the Cochlear Nucleus, Medial and Lateral Vestibular Nucleus, Inferior Cerebellar Peduncle, Spinal Nucleus of the Trigeminal Nerve, Spinal Nucleus of the Trigeminal Nerve, Middle Cerebellar Peduncle, Facial Nucleus, Lateral Lemniscus, Superior Olivary Nucleus, Central Tegmental Tract, Medial Lemniscus, Corticospinal Tract, Pontine Nuclei, Basilar Sulcus of the Pons, Pontocerebellar Fibers, Abducens Nucleus, Pontine Reticular Formation, Root of CN VI, Median Sulcus of 4th Ventricle, Raphe Nucleus, Medial Longitudinal Fasciculus, and Nucleus Prepositus Hypoglossi.

Cross Section of the Pons: A cross-section of the lower part of the pons showing the pontine reticular formation labeled as #9.

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. 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. It also plays a role in wakefulness and sleep.

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.
  • 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.

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.
  • 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.
  • 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.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.

Blood Supply and Lymphatics

The blood supply to the brainstem is mostly from the vertebrobasilar system. The blood supply can be divided into a group of arteries supplying each region:

Midbrain

  • Anteromedial: supplied by the posterior cerebral artery.
  • Anterolateral: supplied by the posterior cerebral artery and branches of the anterior choroidal artery.
  • Lateral: supplied by the posterior cerebellar artery, the choroidal artery, and the collicular artery.
  • Posterior: supplied by the superior cerebellar artery, the posteromedial choroidal artery.

Pons

  • Anteromedial: supplied by the pontine perforating arteries, branches of the basilar artery.
  • Anterolateral: supplied by the anterior inferior cerebellar artery.
  • Lateral: supplied by the lateral pontine perforating arteries, branches of the basilar artery, anterior inferior cerebellar artery, or the superior cerebellar artery.

Medulla oblongata

  • Anteromedial: supplied by the anterior spinal artery and vertebral artery.
  • Anterolateral: supplied by the anterior spinal artery and vertebral artery.
  • Lateral: supplied by the posterior inferior cerebellar artery.
  • Posterior: supplied by the posterior spinal artery.

Brainstem infarction is an area of tissue death resulting from a lack of oxygen supply to any part of the brainstem. The knowledge of anatomy, vascular supply, and physical examination can be life-saving in the setting of an acute infarct and provide precise diagnosis and management. Time becomes an essential factor in management. Early intervention has shown to dramatically reduced morbidity and mortality.

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The significant conduits of blood to the CNS are the internal carotid arteries and the vertebral arteries; they give rise to the many arterial branches that perfuse the CNS.  The most inferior portion of the brainstem is the medulla oblongata. Caudally it receives the majority of its blood supply from the anterior spinal artery anterior-medially and the posterior spinal artery posterior laterally. Superiorly it receives the majority of its blood supply from the vertebral artery laterally, basilar artery branches anteriorly, and the posterior inferior cerebellar artery posteriorly.

Moving superiorly, the pons is the next brainstem structure encountered.  Most of its blood supply comes from branches of the basilar artery. Superiorly, it is also perfused on its posterior lateral portion by branches of the superior cerebellar artery and branches of the anterior inferior cerebellar artery.

The next brainstem structure superior to the pons is the midbrain. It is perfused anterior medially by branches of the basilar artery, anterior laterally by branches of the posterior choroidal arteries and quadrigeminal artery originating from the posterior cerebral artery, and posteriorly by the quadrigeminal and superior cerebellar arteries.

The most superior brainstem structure, the diencephalon, is supplied anteriorly by branches of the anterior cerebral artery. Its posterior portions receive supply from branches of the posterior cerebral artery such as thalamogeniculate branches, thalamo perforating branches, and branches of the posterior communicating artery.

The anatomical lymphatic drainage of the central nervous system was described over 100 years ago, but until the last decade, the widely held consensus was that CNS lymphatics do not exist. The lymphatics of the CNS are small channels within the meninges and differ from the basic structure of the peripheral lymphatics. The CNS lymphatic system is still poorly understood but is thought to participate in immune cell transport, cerebrospinal fluid drainage, and interstitial fluid drainage. The lymphatics of the central nervous system continue to be a topic of investigation.

Nerves –  twelve cranial nerves emerge from the brainstem

The entire brainstem is composed of neural tissue. Ten of the twelve cranial nerves also emerge from the brainstem.

Midbrain

  • The oculomotor nerve (cranial nerve III) – Arises from the oculomotor sulcus on the medial portion of the crus cerebri. It is a motor nerve that receives inputs from two nuclei. The first nucleus is the oculomotor nucleus; it serves as its main motor nucleus and is in the anterior midline of the periaqueductal grey at the level of the superior colliculus. The second nucleus is the Edinger-Westphal nucleus, which provides parasympathetic motor inputs. The somatic motor fibers from the oculomotor nucleus provide innervation to all the extraocular muscles, with the exceptions of the superior oblique and lateral rectus muscles. The parasympathetic motor fibers of the Edinger-Westphal nucleus provide innervation to the ciliary muscles and constrictor pupillae after passing through the ciliary ganglion.
  • The Trochlear nerve (cranial nerve IV) – Exits from the posterior surface of the midbrain and is the only cranial nerve to exit posteriorly. It is a motor nerve with its nucleus located in the midline of the brainstem, also in the anterior portion of the periaqueductal grey, but inferior to the oculomotor nucleus. The nerve innervates the superior oblique ocular muscle, which is responsible for moving the eye downward and laterally. A unique feature of the trochlear nerve among the cranial nerves is that it is the only cranial nerve decussating peripherally. The nerve decussates at the superior medullary velum after leaving the brainstem, which causes cranial nerve nuclei deficits to appear as loss of function of the contralateral superior oblique muscle. Injuries that happen to the nerve distal to the decussation result in ipsilateral deficits to the superior oblique muscle.

Pons

  • The Trigeminal Nerve (cranial nerve V) – Arises from the superior anterior lateral pons as the largest cranial nerve. It contains both motor and sensory fibers. It arises as a smaller motor nerve and a larger sensory nerve. The sensory fibers provide innervation to the face and head. The motor fibers provide innervation to the muscles of mastication, mylohyoid, anterior belly of the digastric, tensor tympani, and tensor veli palatini. The trigeminal motor nucleus situates in the superior posterior lateral pons. The motor nucleus also receives corticobulbar fibers from both hemispheres as well as fibers from the reticular formation, medial longitudinal fasciculus, and red nucleus. The nerve has contributions from three sensory nuclei: The principal sensory nucleus of cranial nerve V, the mesencephalic nucleus, and the spinal trigeminal nucleus. The principal sensory nucleus of cranial nerve V lies directly lateral to the trigeminal motor nucleus and receives inputs from nerves that convey touch and pressure. The mesencephalic nucleus is located on the lateral aspect of the periaqueductal grey, anterior lateral to the fourth ventricle, and ascends to the height of the inferior colliculus.  The mesencephalic nucleus conveys proprioceptive input from the teeth, hard palate, temporomandibular joint, and muscles of mastication. The spinal trigeminal nucleus is located in the inferior posterior lateral pons and extends inferiorly through the medulla into the superior spinal cord. The spinal trigeminal nucleus receives pain and temperature input for the sensory distribution of the trigeminal nerve.
  • The Abducens nerve (cranial nerve VI) – Is a motor nerve that emerges anteriorly and medially from the junction of the pons and medulla.  The abducens nucleus is in the midline of the inferior tegmentum of the pons just ventral to the fourth ventricular floor. It provides innervation to the lateral rectus muscle, which is responsible for the abduction of the eye.
  • The facial nerve (cranial nerve VII) – Emerges from the junction of the pons and the medulla lateral to the abducens nerve at the cerebellopontine angle. It is both a motor and a sensory nerve and emerges as two separate roots; these include a medial motor root and a lateral sensory root. The facial motor nucleus situates in the anterior lateral inferior pons just anterior and medial to the spinal trigeminal nucleus. The muscles of facial expression derive innervation from the motor nucleus of the facial nerve. The upper face receives corticobulbar fibers that partially decussate from both hemispheres, which allow sparing of deficits with lesions at the level of the cranial nerve nuclei while the lower muscles corticobulbar fibers fully decussate. The sensory nucleus is the upper portion of the solitary nucleus, which is located posterior and lateral to the facial nerve motor nucleus. It receives afferent fibers for taste from the anterior two-thirds of the tongue and sensation for the skin near the auricle of the ear. Its parasympathetic nucleus is the superior salivatory nucleus and is located laterally to the abducens nucleus but posterior to the facial motor nucleus. It innervates the submandibular and submental salivary glands.
  • The Vestibulocochlear nerve (cranial nerve VIII) – Arises from the brainstem directly lateral to the sensory root of the facial nerve. Cranial nerve VIII has two distinct portions, the vestibular, responsible for balance, and the cochlear, which is responsible for hearing. Cranial nerve VIII is purely a sensory nerve and both of its portions course together until they reach their nuclei within the brainstem.  The vestibular portion of the nerve provides input to the vestibular nuclei located along the lateral portion of the fourth ventricle in the inferior pons. The vestibular nuclei are composed of four different nuclei (superior, inferior, lateral, and medial.) These nuclei send tracts to three separate areas: the cerebellum via the vestibulocerebellar tract, the spinal cord through the vestibulospinal tract, and the nuclei of cranial nerves III, IV, and VI by the medial longitudinal fasciculus. The cochlear portion provides input to the dorsal and ventral cochlear nuclei. These nuclei are in the anterior lateral portion of the inferior pons. The posterior cochlear nucleus processes high-frequency sounds while the anterior cochlear nucleus processes low-frequency sound. The anterior cochlear nucleus projects fibers to the ipsilateral superior olive and then to the lateral lemniscus. The posterior cochlear nucleus projects to the contralateral lateral lemniscus.

Medulla Oblongata

  • The glossopharyngeal nerve (cranial nerve IX) – Emerges from the postolivary groove and contains motor fibers, sensory fibers, and parasympathetic nerves. The nerve shares several cranial nerve nuclei with cranial nerve X (the vagus nerve), the nucleus ambiguous, and the nucleus solitarius. Cranial nerve IX also uses the inferior salivatory nucleus. The superior portion of the nucleus ambiguous is located posterior to the inferior olivary nucleus and contains second-order motor cell nuclei for the glossopharyngeal nerve. The motor fibers from the superior nucleus ambiguous innervate the stylopharyngeus muscle. The cranial nerve IX parasympathetic cranial nerve nucleus is the inferior salivary nucleus; its postganglionic parasympathetic, visceral efferent fibers provide innervation to the parotid gland. The inferior salivary nucleus is located posterior to the nucleus ambiguous and receives afferent input from the hypothalamus, olfactory system, and nucleus solitarius. The last nuclei of the glossopharyngeal nerve are the nucleus solitarius. It is a sensory nucleus that receives taste from the posterior one-third of the tongue from the glossopharyngeal nerve. It also receives afferent impulses from the carotid sinus.
  • The Vagus nerve (cranial nerve X) – Emerges from the postolivary groove and contains motor fibers, sensory fibers, and parasympathetic fibers. The inferior portion of the nucleus ambiguous provides motor output to the muscles of the pharynx and larynx. The parasympathetic nucleus of the vagus nerve is the dorsal motor nucleus. The dorsal motor nucleus is located lateral to the hypoglossal nucleus and receives afferent fibers from the upper gastrointestinal tract, liver, pancreas, heart, and bronchi. The sensory nucleus for the vagus nerve is also the nucleus solitarius similar to the glossopharyngeal nerve. It receives afferent inputs from the carotid sinus as well.
  • The accessory nerve (cranial nerve XI) – Arises from the medulla between the olive and inferior cerebellar peduncle and upper cervical spinal cord to C5. It forms from the combination of both cranial and spinal nerve roots. This nerve supplies both the trapezius and sternocleidomastoid muscles with motor innervation. It’s efferent motor fibers arise from the nucleus ambiguous.
  • The hypoglossal nerve (cranial nerve XII) – Is a motor nerve and arises anteriorly from the medulla, its nucleus sits in the midline of the brainstem anterior to the fourth ventricle. The hypoglossal nerve innervates the muscles of the tongue and the hyoglossus, genioglossus, and styloglossus muscles. It receives innervation via cortico-nuclear fibers from both hemispheres of the brain. However, the genioglossus muscle is innervated only by the contralateral side.

References

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