Central Nervous System – Anatomy, Types, Functions

Central nervous system is the brain and spinal cord, while the peripheral nervous system consists of everything else. The central nervous system’s responsibilities include receiving, processing, and responding to sensory information.

The brain is an organ of nervous tissue that is responsible for responses, sensation, movement, emotions, communication, thought processing, and memory. Protection for the human brain comes from the skull, meninges, and cerebrospinal fluids. The nervous tissue is extremely delicate and can suffer damage by the smallest amount of force. In addition, it has a blood-brain barrier preventing the brain from any harmful substance that could be floating in the blood.

The spinal cord is a vital aspect of the CNS found within the vertebral column. The purpose of the spinal cord is to send motor commands from the brain to the peripheral body as well as to relay sensory information from the sensory organs to the brain. Spinal cord protection is by bone, meninges, and cerebrospinal fluids.

Structure and Function

The brain is broken up into two hemispheres, the left, and the right. While they are in constant communication, the left and right hemisphere are responsible for different behaviors, known as brain lateralization. The left hemisphere is more dominant with language, logic, and math abilities. The right hemisphere is more creative, being dominant in artistic and musical situations, and intuition.

Cerebral cortex

The cerebral cortex is the outermost layer that surrounds the brain. It is composed of gray matter and filled with billions of neurons used to conduct high-level executive functions. The cortex divides into four lobes; frontal, parietal, occipital, and temporal by different sulci.[rx] The frontal lobe, located anteriorly to the central sulcus, is responsible for voluntary motor function, problem-solving, attention, memory, and language. Located in the frontal lobe are the motor cortex and the Broca area. The motor cortex allows for the precise voluntary movements of our skeletal muscles, while the Broca area controls motor functions responsible for producing language. The parietal lobe is separated from the occipital lobe by the parieto-occipital sulcus and is behind the central sulcus. It is responsible for processing sensory information and contains the somatosensory cortex. Neurons in the parietal lobe receive information from sensory and proprioceptors throughout the body, process the can, and form an understanding about what is being touched based on previous knowledge. The occipital lobe, known as the visual processing center, contains the visual cortex. Similar to the parietal lobe, the occipital lobe receives information from the retina and then uses past visual experiences to interpret and recognize the stimuli. Lastly, the temporal lobe processes auditory stimuli through the auditory cortex. Mechanoreceptors located in the hair cells lining the cochlea are activated by sound energy, which in turn sends impulses to the auditory cortex. The impulse is processed and stored based on previous experiences. The Wernicke area is in the temporal lobe and functions in speech comprehension.

Basal nuclei

The basal nuclei, also known as basal ganglia, is located deep within the cerebral white matter and is composed of the caudate nucleus, putamen, and globus pallidus. These structures form the pallidum and striatum. The basal ganglia are responsible for muscle movements and coordination.[rx]

Thalamus

The thalamus is the relay center of the brain. It receives afferent impulses from sensory receptors located throughout the body and processes the information for distribution to the appropriate cortical area. It is also responsible for regulating consciousness and sleep.

Hypothalamus

While the hypothalamus is one of the smallest parts of the brain, it is vital to maintaining homeostasis. The hypothalamus connects the central nervous system to the endocrine system. It is responsible for heart rate, blood pressure, appetite, thirst, temperature, and the release of various hormones. The hypothalamus also communicates with the pituitary gland to release or inhibit antidiuretic hormone, corticotropin-releasing hormone, gonadotropin-releasing hormone, growth hormone-releasing hormone, prolactin inhibiting hormone, thyroid releasing hormone, and oxytocin.[rx]

Pons

Found in the brainstem, the pons connects the medulla oblongata and the thalamus. It is composed of tracts responsible for relaying impulses from the motor cortex to the cerebellum, medulla, and thalamus.

Medulla oblongata

The medulla oblongata is at the bottom of the brain stem, where the spinal cord meets the foramen magnum of the skull. It is responsible for autonomic functions, some of which are crucial for survival. The medulla oblongata monitors the bodies respiratory system using chemoreceptors. These receptors are able to detect changes in blood chemistry. For example, if the blood is too acidic, the medulla oblongata will increase the respiratory rate allowing for more oxygen to reach the blood.[rx] It is also a cardiovascular and vasomotor center. The medulla oblongata can regulate the body’s blood pressure, pulse, and cardiac contractions based on the body’s needs. Lastly, it controls reflexes like vomiting, swallowing, coughing, and sneezing.

Cerebellum

The cerebellum, also known as the little brain, is responsible for smooth, coordinated voluntary movements. It subdivides into three lobes: the anterior, posterior, and flocculonodular lobes. The cerebellum contains a cerebellar circuit, using Purkinje cells and cerebellar peduncles to communicate to other parts of the brain. The superior cerebellar peduncle is composed of white matter that connects the cerebellum to the midbrain and allows for coordination in the arms and legs. The inferior cerebellar peduncle connects the medulla and cerebellum using proprioceptors to maintain balance and posture. Lastly, the middle cerebellar peduncle is used as a one-way communication method from the pons to the cerebellum. It is mostly composed of afferent fibers that alert the cerebellum about voluntary motor actions. The cerebellum is in constant communication with the cerebral cortex, taking higher-level instructions about the brain’s intentions, processing them through the cerebellar cortex, then sending messages to the cerebral motor cortex to make voluntary muscle contractions. These contractions are calculated to determine the force, direction, and momentum necessary to ensure each contraction is smooth and coordinated.

Limbic System

The limbic system is composed of the piriform cortex, hippocampus, septal nuclei, amygdala, nucleus accumbens, hypothalamus, and anterior nuclei of the thalamus.[rx] The fornix and fiber tracts connect the limbic system parts allowing them to control emotion, memory, and motivation. The piriform cortex is part of the olfactory system and is in the cortical area of the limbic system. The hypothalamus receives most of the limbic output, which explains psychosomatic illnesses, where emotional stressors cause somatic symptoms. For example, a patient who is currently having financial struggles might present to his primary care physician with hypertension and tachycardia. The septal nuclei, amygdala, and nucleus accumbens are found in the subcortical areas and are responsible for pleasure, emotional processing, and addiction, respectively.

Reticular formation

Reticular formation is an extensive network of pathways containing neurons that begins in the brainstem and travels from the top of the midbrain to the medulla oblongata. These pathways have projecting reticular neurons that affect the cerebral cortex, cerebellum, thalamus, hypothalamus, and spinal cord. The reticular formation controls the body’s level of consciousness through the reticular activation system, also known as RAS. Sensory axons, found in visual, auditory, and sensory impulses, activate RAS neurons in the brain stem. These neurons then relay information to the thalamus and cerebrum. Continuous stimulation of the RAS neurons causes the cerebrum to stay in an aroused state; this gives the feeling of alertness. However, RAS can filter out repetitive, weak stimuli; this prevents the brain from responding to unimportant information, as well as being sensory overloaded.

Spinal cord

The spinal cord proper extends from the foramen magnum of the skull to the first or second lumbar vertebrae. It creates a two-way pathway between the brain and the body and divides into four regions –  cervical, thoracic, lumbar, and sacral. These regions are then broken down into 31 segments with 31 pairs of spinal nerves. There are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. Each nerve exits the vertebral column passing through the intervertebral foramina and to its designated location in the body.

Due to cervical and lumbar enlargements, the spinal cord differs in width throughout its structure. The cervical enlargement occurs at C3 to T1, and the lumbar enlargement is at L1 to S2. The white matter is present on the outside of the spinal cord, with gray matter located in its core and cerebrospinal fluid in the central canal. The gray commissure, the dorsal, lateral, and ventral horns are all composed of gray matter. The gray commissure surrounds the central canal. The dorsal horns are made of interneurons, while the ventral horns are somatic motor neurons. Afferent neurons in the dorsal roots carry impulses from the body’s sensory receptors to the spinal cord, where the information begins to be processed. The ventral horns contain efferent motor neurons, which control the body’s periphery. The axons of motor neurons are found in the body’s skeletal and smooth muscle to regulate both involuntary and voluntary reflexes.

The spinal cord ends in a cone-shaped structure called conus medullaris and is supported to the end of the coccyx by the filum terminale. Ligaments are found throughout the spinal column, securing the spinal cord from top to bottom.

Ascending pathway to the brain

Sensory information travels from the body to the spinal cord before reaching the brain. This information ascends upwards using first, second, and third-order neurons. First-order neurons receive impulses from skin and proprioceptors and send them to the spinal cord. They then synapse with second-order neurons. Second-order neurons live in the dorsal horn and send impulses to the thalamus and cerebellum. Lastly, third-order neurons pick up these impulses in the thalamus and relay it to the somatosensory portion of the cerebrum. Somatosensory sensations are pressure, pain, temperature, and the body’s senses.

Descending pathway

Descending tracts send motor signals from the brain to lower motor neurons. These efferents neurons then produce muscle movement.[rx]

Blood Supply and Lymphatics

Due to the importance and delicate nature of the central nervous system, the body closely monitors the blood traveling to and from it. The cardiovascular system ensures continuous, oxygenated blood as a drop-in oxygenation level can be detrimental. The common carotid arteries branch off of the aorta, which carries oxygen-rich blood from the heart for distribution. The common carotid further branches into right and left internal and external carotid arteries, which supply the cranium with blood. Vertebral arteries begin in the neck and branch as they enter into the skull through the foramen magnum. They supply the anterior portion of the spinal cord. The vertebral arteries then merge into the basilar artery. The basilar artery is responsible for delivering blood to the brainstem and cerebellum. The circle of Willis ensures that blood will continue to circulate even if one of the arteries is not working appropriately. The internal carotid and vertebral arteries compose the circle of Willis.[rx] After being used in the CNS, blood then travels back to the lungs for oxygenation. Multiple dural venous sinuses do this:

• Superior sagittal sinus
• The confluence of sinuses
• Transverse sinuses
• Sigmoid sinuses
• Jugular veins
• Carotid arteries
• Superior vena cava
• Lungs

Function

The functions of the sympathetic nervous system are expansive and involve many organ systems and various types of adrenergic receptors.

The effects in which SNS acts in direct contrast to the PNS function include the following:

• In the eye, sympathetic activation causes the radial muscle of the iris (alpha-1) to contract, which leads to mydriasis, allowing more light to enter. Furthermore, the ciliary muscle (beta-2) relaxes, allowing for far vision to improve.
• In the heart (beta-1, beta-2), sympathetic activation causes an increased heart rate, the force of contraction, and rate of conduction, allowing for increased cardiac output to supply the body with oxygenated blood.
• In the lungs, bronchodilation (beta-2) and decreased pulmonary secretions (alpha-1, beta-2) occur to allow more airflow through the lungs.
• In the stomach and intestines, decreased motility (alpha-1, beta-2) and sphincter contraction (alpha-1), as well as contraction of the gallbladder (beta-2), occur to slow down digestion to divert energy to other parts of the body.
• The exocrine and endocrine pancreas (alpha-1, alpha-2) decreases both enzyme and insulin secretion.
• In the urinary bladder, there is relaxation of the detrusor muscle and contraction of the urethral sphincter (beta-22) to help stop urine output during sympathetic activation.
• The kidney (beta-1) increases renin secretion to increase intravascular volume.
• The salivary glands (alpha-1, beta-2) work through small volume potassium and water secretion.

Actions of SNS which do not oppose those of the PNS include the following:

• There is strong constriction through the alpha-1 receptor in arterioles of the skin, abdominal viscera, and kidney, and weak constriction through the alpha-1 and beta-2 receptors in the skeletal muscle.
• In the liver, increased glycogenolysis and gluconeogenesis (alpha-1, beta-2) occur to allow for glucose to be available for energy throughout the body.
• In the spleen, there is a contraction (alpha-1).
• Sweat glands and arrector pili muscles (muscarinic) work to increase sweating and erection of hair to help cool down the body.
• Lastly, the adrenal medulla (nicotinic receptor) increases the release of epinephrine and norepinephrine to act elsewhere in the body.[rx]

Actions of the PNS are listed below:

• In the male reproductive tract, parasympathetic stimulation of M3 receptors causes smooth muscle relaxation in the helicine arteries of the penis, allowing blood to fill the corpora cavernosa and corpus spongiosum, causing an erection. The PNS also gives excitatory signals to the vas deferens, seminal vesicles, and prostate.[5]
• In the eye, parasympathetic stimulation of M3 receptors causes contraction of the sphincter muscle of the iris leading to constriction of the pupil (miosis). Additionally, it causes contraction of the ciliary muscle improving near vision.
• In the heart, parasympathetic stimulation of M2 receptors causes decreased heart rate and velocity of conduction through the AV node.
• In the vasculature, parasympathetic stimulation of M3 receptors leads to vasodilation.
• In the lungs, parasympathetic stimulation of M3 receptors leads to bronchoconstriction. It also increases bronchial secretions.
• In salivary glands, parasympathetic stimulation of M1 and M3 receptors leads to high volume secretion of potassium ions, water, and amylase.
• In the stomach and intestines, parasympathetic stimulation of M receptors leads to increased motility and relaxation of sphincters. Stimulation of M receptors also increases gastric secretions to aid in digestion.
• In the gallbladder, parasympathetic stimulation of M3 receptors stimulates contraction to release bile.
• In the pancreas, parasympathetic stimulation of M3 receptors leads to the release of digestive enzymes and insulin.
• In the kidneys and bladder, parasympathetic stimulation of M3 receptors stimulates peristalsis of ureters, contraction of the detrusor muscle, and relaxation of the internal urethral sphincter aiding in the flow and excretion of urine.

After being released from cholinergic neurons and acting on muscarinic receptors, acetylcholine is quickly inactivated or removed from the neuroeffector junction to allow new signals to come through. In cholinergic synapses, this action is primarily performed enzymatically by acetylcholinesterase. It hydrolyzes acetylcholine into choline and acetate in less than one millisecond, making it one of the fastest enzymes in the body.[1]

Clinical Significance

Wernicke aphasia: Wernicke aphasia occurs most commonly as a result of a hemorrhagic or ischemic stroke. Strokes that occur in the left middle cerebral artery prevent oxygenated blood from reaching the Wernicke area. In Wernicke aphasia, a person can speak clearly and produce speech. However, their speech has no meaning. They have difficulty understanding language.

Broca aphasia: Broca aphasia, also known as expressive aphasia, is caused by a stroke, brain tumor, or brain trauma. When a stroke occurs in the Broca area, oxygen is cut off to that part of the brain. The hypoxia causes irreversible damage. During Broca aphasia, the person has difficulty producing speech. They can comprehend and know what they want to say; however, they are unable to form the words to communicate the message.[rx]

Traumatic brain injuries: Traumatic brain injuries (TBI) occur when there is a disruption to normal brain activity, which can occur during a sports injury, a car accident, by a penetrating object, or even a blunt object. TBI symptoms can vary depending on the severity of the injury. For example, a concussion can cause temporary dizziness or loss of consciousness, while a contusion causes lasting neurological damage. Contusions to the brain stem resulting in a coma. TBI can cause subdural or subarachnoid hemorrhage and cerebral edema. When the brain sustains a trauma, the blood vessels in the brain break. The blood begins to pool, increasing the intracranial pressure, and compressing the brain tissue. As the brain pushes through the skull onto the spinal cord, autonomic nervous system functions are lost.

Cerebrovascular Accidents: Cerebrovascular accidents, also known as strokes, occur when the brain is not able to get oxygenated blood. The lack of oxygen causes hypoxia, and tissues in the brain start to die. Commonly, strokes are caused by a blood clot that has traveled from one location in the body to the cerebral artery in the brain. Dependent on where the clot lands, determine the symptoms of the stroke. For example, some people may experience left-sided paralysis, while others might have slurred speech. Transient ischemic attacks are considered small strokes as their symptoms are more temporary. In any CVA, time is crucial. If necessary, doctors can administer tissue plasminogen activator which breaks down the clot or can surgically remove it. The severity of symptoms directly correlates to how long the brain’s oxygen supply has been cut off.

Alzheimer’s disease: Alzheimer’s disease (AD) is a common type of dementia in which one’s brain cells and neural connections begin to degenerate and die. This condition presents with loss of memory and cognitive decline. Alzheimer’s is progressive, with symptoms worsening over time.[rx] Scientists have found aggregations of beta-amyloid plaques and neurofibrillary tangles made of tau within the neurons in AD patients. These plaques and tangles result in the death of brain cells and form because of the misfolding of proteins within them. AD patients have a decrease in neural activity in the parietal cortex, hippocampus, and basal forebrain.

Parkinson’s disease: Parkinson’s disease is a nervous system disorder that results in the deterioration of dopamine-releasing neurons in the substantia nigra.[rx] The drop-in dopamine levels create tremors, unsteady movements, and loss of balance. Parkinson’s disease is progressive as it usually starts as a tremor in one hand. Many patients exhibit a pill-rolling movement in their hand, bradykinesia, stiffness, and a mask life face as symptoms progress. A Parkinson’s disease diagnosis results from looking at the patient’s symptoms, medical history, and a neurological and physical exam. While no cure exists for the disease, the severity of the symptoms can be controlled. Levodopa can pass through the blood-brain and undergo conversion into dopamine for CNS use. Deep brain stimulation is a surgical option that can stop the abnormal brain activity and thus control the tremors. However, deep brain stimulation does not keep the disease from progressing.

Huntington disease: Huntington disease is a hereditary, progressive brain disorder that is caused by a mutation in the huntingtin gene, HTT. The CAG segment in the HTT gene normally repeats up to 35 times. However, in someone with Huntington’s disease, the CAG segment is repeated up to 120 times. This large CAG segment causes the huntingtin protein to accumulate in the brain cells, which eventually leads to cell death. Initially, Huntington disease causes chorea, involuntary jerking, and hand-flapping movements. As the disease progresses, cognitive decline occurs. Fatally follows within 15 years of diagnosis.

Spinal cord traumas: Symptoms of spinal cord injuries is dependent on where the injury occurs. If damage to the sensory tracts occurs, the sensation can be affected. However, if the ventral roots or ventral horns are damaged, paralysis occurs. Flaccid paralysis is when nerve impulses do not reach the intended muscles. Without stimulation, the muscles are unable to contract. Spastic paralysis is when the motor neurons undergo irregular stimulation, causing involuntary contraction. Paraplegia, paralysis of the lower limbs, occurs when the spinal cord gets cut between T1 and L1. Quadriplegia, paralysis of all limbs, is a result of an injury in the cervical region.

Poliomyelitis: Poliomyelitis is an inflammation of the spinal cord due to the virus, Polio. Poliovirus spreads from human to human or through infected food and water. It demolishes the neurons in the ventral horn of the spinal cord leading to paralysis. The infection of the poliovirus is preventable through the administration of the vaccine.[rx]

Amyotrophic Lateral Sclerosis: Amyotrophic lateral sclerosis, known also as ALS and Lou Gehrig disease, destroys motor neurons that control voluntary and involuntary movements like breathing, speaking, and swallowing. The cause of ALS is not known, and unfortunately, there is no cure. Scientists believe that cell death is related to the excess amount of extracellular glutamate in ALS patients. Riluzole, which can disrupt the formation of glutamate, is used to slow down the progression and reduce the painful symptoms of ALS.

Multiple sclerosis: Multiple sclerosis is an autoimmune disease, in which the body attacks the myelin proteins of the central nervous system, disrupting the communication between the brain and the body. MS has a high prevalence in young adults and presents as pain, weakness, vision loss, and loss in coordination. The severity of symptoms varies from patient to patient. Medication is used to suppress the body’s immune system and can help control the adverse effects of this disease.

References