Knowledge of the nervous system parts and functions is a key to understanding diseases of the system. It also helps in the recognition of related signs and symptoms. The relevant medical terminology is a part of that knowledge.
The nervous system is an integrated arrangement of cells, tissues, organs and other elements that regulate several functions and actions of organisms. In vertebrates, it along with the endocrine system and other systems enables the body to respond appropriately and make the necessary adjustments to internal and external stimuli. Many of its functions are the result of neurons’ release of neurotransmitters which act through synapses to cause electrical events in target cells.
The nervous system is comprised of a number of parts ranging from the most complex, the brain, down to the individual neurons, functional substances, and supporting cells. Functional parts include the cerebrum, brainstem, nuclei, basal ganglia, cranial nerves, spinal cord, ganglia, spinal nerve roots, and peripheral nerves. Supporting parts include neuroglia, cerebrospinal fluid, ventricles, and meninges.
Divisions of the Nervous System
The main anatomical divisions are the central nervous system (CNS) and the peripheral nervous system. The cerebral cortex consists of a left hemisphere and a right hemisphere. The right hemisphere governs creative thinking. The left hemisphere oversees logical thinking. Each hemisphere is subdivided into four parts. They are the frontal lobe, parietal lobe, temporal lobe and occipital lobe. Each region has specialized functions.
The subdivided functional areas of the cerebral cortex are the motor areas which control muscular movement, the sensory areas which receive sensory input from various parts of the body, the visual area which regulates vision, the auditory area which regulates hearing and Broca’s area, the area responsible for translating thoughts into speech. Each of the aforementioned functional areas has an association area which mediates more complex functions.
The spinal cord consists of 31 segments named based on the parts of the body they provide function for. Cervical segments provide movement and sensation of the neck and many portions of the upper extremities. Included is a segment which mediates movement of the diaphragm during breathing. Thoracic segments provide movement and sensation of portions of the hands, chest wall and abdominal wall. There is also a segment which effects ejaculation. Lumbar segments provide movement and sensation of the lower extremities. Sacral segments participate in penile erection and function of the bladder and bowels.
The functional divisions of the nervous system as a whole are the somatic nervous system and the autonomic nervous system. The autonomic nervous system is subdivided. Those subdivisions are the sympathetic nervous system and the parasympathetic nervous system.
Afferent nerve fibers – They are nerve fibers that convey impulses from outside of the central nervous system to the CNS. They regulate sensation. The main sensations they govern are pain, touch, temperature, position and vibration.
Afferent nerve – It is a nerve that sends sensory impulses from outside the central nervous system toward the CNS.
Astrocytes –They are star-shaped cells throughout the brain and spinal cord. Projections from their cell surfaces cause their star-shaped appearance. They perform many functions pertaining to the maintenance of neurons. They also promote the tight junctions between the lining cells of the blood vessels within the brain, the basis of the blood brain barrier.
Autonomic nervous system – It is the part of the peripheral nervous system which regulates the activity of heart muscle, smooth muscle and glands. Smooth muscle is that involved in involuntary actions or functions primarily of organs in the cavities of the chest, abdomen and pelvis. In contrast to the somatic nervous system, the autonomic nervous system is not under voluntary control.
Axon – The long projection from the body of the nerve cell (neuron). Together with its covering or sheath, it forms the nerve fiber. It is the channel through which impulses travel from one nerve cell to another nerve cell or from a nerve cell to a different type of cell during communication.
Basal ganglia – They are a specific group of interconnected nuclei deep in the gray matter of the brain cortex and upper portion of the brainstem. Some of their functions are a mystery. But they seem to be in some way involved with voluntary movements, posture, speaking, and motivation. Their dysfunction seems to be involved in some diseases. Parkinson’s disease, a disorder in which the substantia nigra is deficient in his production of dopamine, is a case in point.
Blood brain barrier (BBB) – It is an apparent physiologic blockade between the penetrating blood vessels of the brain and the various physical elements of the brain. It acts as a filter, preventing many substances from entering the brain tissue from the blood stream. This is especially true for some medications.
Brain – It is the largest organ of the central nervous system. It controls movement, sensation, coordination, speech, memory, emotions, and a number of integrated functions of the body. It is located within the skull.
Cauda equina – It is the collection of nerve fibers which continue to travel through the vertebral canal beyond the conus medullaris. It is the origin of the six sacral and single coccygeal spinal nerves. It resembles a horse’s tail.
Central canal of the spinal cord – It is the tiny channel that runs throughout the length of the center of the spinal cord. It opens into the fourth ventricle in the medulla oblongata. It contains cerebrospinal fluid.
Cerebellum – It is the portion of the brain that governs coordination. It is also one of the structures involved in balance. It is in the lower portion of the cranial cavity behind the brainstem.
Cerebrospinal fluid (CSF) – It is the clear fluid that circulates within the brain and spinal cord. It serves as a cushion for the brain. It is present in the brain ventricles and the subarachnoid space. It flows from the ventricles into the subarachnoid space through openings in the side of the fourth ventricle of the brainstem.
Cerebrum –It occupies the upper portion of the cranial cavity. It is the main portion of the brain consisting of 2 hemispheres. The dominant hemisphere is the one that controls skilled movement and dexterity. In right-handed individuals it is the left hemisphere. In left-handed individuals it is the right hemisphere. The corpus callosum connects the hemispheres. Each hemisphere has 4 lobes. They are the frontal lobes, parietal lobes, temporal lobes and occipital lobes.
Corpus callosum – It is the thick band of fibers which connects the left and right hemispheres of the brain. It transmits sensory, motor and cognitive information between the 2 sides of the brain, thus allowing them to communicate.
Corticospinal tract – It is a bundle of nerve fibers arising from the internal capsule. It carries motor commands from the brain to the spinal cord.
Cranial nerves – They are peripheral nerves that exit from the brain or brain stem. Individual nerves are involved in the following functions:
- constriction of the pupils
- facial sensation
- movement of facial muscles
- shoulder shrugging
- tongue movement
Efferent nerve fibers – They are nerve fibers that send impulses from the central nervous system to a location outside of it. The most obvious are those that convey motor impulses. Motor impulses are those that cause movement of skeletal muscles. The release of acetylcholine from terminal axons of nerve cells onto the neuromuscular plate of muscle fibers at the junction between the nerve and muscle fibers is what produces movement. Efferent nerve fibers also transmit autonomic impulses that cause endocrine glands and exocrine glands to act.
End organ – Is a specialized structure to which the peripheral end of a sensory or motor nerve connects. It is the source of stimulation of a sensory nerve. In the case of a motor nerve it is the object of the effect. Muscle movement is an example.
Epidural space – It is the space between the dura mater and the vertebral canal.
Frontal lobes – They are the parts of either hemisphere of the cerebral cortex just behind the forehead. Areas in them regulate voluntary muscle movement as well as higher intellectual functions including emotions, cognition and will.
Ganglion – It is a cluster of nerve cell bodies located outside of the central nervous system. Ganglia serve as relay points or intermediary connections between various structures of the nervous system. The 3 major groups are:
- Dorsal root ganglia – Those beside the spinal cord that contain the cell bodies of the afferent nerves.
- Cranial nerve ganglia – Those which make connections with fibers of the 3th, 5th, and 9th cranial nerves. They are in the vicinity of the face.
- Autonomic ganglia – Those of the autonomic nervous system whose cell bodies connect with axons from the central nervous system and whose axons connect with cells of the target tissues or organs they regulate. Those within the sympathetic trunk to either side of the vertebral column (spine) are sympathetic ganglia. Parasympathetic ganglia also exist. They are located near or within the walls of the organs they supply. The intestine is one example.
Glial cells – Are the supporting cells of the nervous system. Unlike neurons, they don’t transmit electrical impulses. Their main functions are to maintain homeostasis and the health of the system. There are 3 types of glial cells in the brain and spinal cord. They are the astrocytes, microglia and oligodendrocytes. In peripheral nerves they are the Schwann cells and satellite cells.
Gray matter – It is the grayish appearing tissue of the brain and spinal cord. It is composed of nerve cell bodies and dendrites. Most of the brain’s gray matter is in the outer layer or cortex. In the spinal cord it is in the central portion and is in the shape of a butterfly looking down on the cord in cross-section.
Hypothalmus – It is the part of the brain just below the thalamus. Its functions include sleep and temperature regulation, control of appetite and satiety, regulation of sexual development, and control of the pituitary gland.
Internal capsule – It is a fanlike mass of white matter that contains all the fibers that transfer information between the cerebral cortex and spinal cord, brainstem and subcortical structures (i.e. thalamus and basal ganglia). It has three main divisions. One gives rise to the corticospinal tract.
Medial longitudinal fasciculus (MLF) – Is one of the crossed pair of nerve bundles on each side of the brainstem. The axons that compose it arise from various sources and terminate in different areas. It connects the nuclei of cranial nerves III, IV and VI that govern eye-movement. It also integrates eye movement directed by the gaze centers of the brain and head movement. Disruption of it by conditions such as a stroke or multiple sclerosis results in internuclear ophthalmoplegia.
Medulla oblongata – It is the lower most portion of the brainstem which joins to the spinal cord. It is the site of the centers that control breathing, circulation, heart rate and some other bodily functions.
Midbrain – it is the part of the brainstem just below the cerebral cortex.
Meninges – They are the 3 membranes covering the brain and spinal cord. The outermost membrane is the dura mater. The innermost membrane in direct contact with the surface of the brain and the spinal cord is the pia mater. The membrane in between the dura mater and the pia mater is the arachnoid.
Thecal sac is a synonymous term for the three layers of membranes that cover the spinal cord and contain CSF.
Motor neurons – They are nerve cells that transmit signals from the brain or spinal cord that produce action. In the somatic nervous system that action is skeletal muscle contraction and bodily movement. Such cells in the brain are termed upper motor neurons. If the cells are in the spinal cord they are lower motor neurons. Motor neurons of the autonomic nervous system also cause action. Depending on their location the action can be smooth muscle contraction or gland secretion.
Muscle contraction – The shortening of muscle fibers due to nerve signals to them. It results in an action upon the surface(s) of its attached tendon(s).
Myelin – Is the fat rich protein that covers the axons of nerves. In the brain and spinal cord it is the extension of the cell membrane of oligodendrocytes which wraps itself around the nerve fibers layer upon layer. It serves not only as a protective covering of nerve fibers. It is necessary for proper conduction of electrical signals through nerve fibers.
The myelin sheath of fibers in the central nervous system is not continuous. The reason is one oligodendrocyte does not totally cover one nerve axon. Rather, a given axon obtains partial covering from several different cells. The result is several gaps in the sheath. These gaps in the myelin sheath of nerve fibers are termed nodes of Ranvier. The presumed function of Nodes of Ranvier is to speed up the conduction of electrical signals through a nerve. Destruction of myelin in the brain and spinal cord appears to be the earliest and main pathophysiology of multiple sclerosis.
The situation is somewhat different in the peripheral nervous system. The myelin of peripheral nerves forms from Schwan cells wrapping their membranes around the axons. The difference here though is that one cell provides the full wrapping for one axon.
Neuroglia –It is the supporting tissue and the cells that comprise it of the nervous system. In the central nervous system it is the astrocytes, microglia and oligodendrocytes. In the peripheral nervous system it is the Schwan cells and satellite cells. For the most part, it is synonymous with the term glial cells.
Neuron (nerve cell) – It is any of the cells of the nervous system which conduct electrical signals. It consists of the cell body, dendrites and axon. The neuron cell body is the largest and main part of a neuron. It contains all the general parts of the nerve cell including the nucleus. It is more or less the control center which produces all of the other parts of the nerve cell including the dendrites, axons, and neurotransmitters .
Bipolar neurons have two projections from the cell body. One serves as an axon sending signals. The other serves as a dendrite receiving signals. These types of nerve cells are present in the retina of the eye, inner ear and roof of the nasal cavity.
Multipolar neurons have three or more processes extending from the cell body. One serves as an axon. The others serve as dendrites. It is the most common type of motor neuron. Its other common function is as an association neuron – one that conducts impulses between nerve cells in the brain and spinal cord. Its function is thus one of integration.
Unipolar neurons have one process attached to the cell body which makes a side connection with a process separate from the cell. That second process serves as a dendrite at one end and an axon at the other end. It is the most common type of sensory neuron.
Neuromuscular junction – The area of contact between the axon of a motor neuron and the muscle fiber it supplies. It is a special type of synapse in which the excitable portion of the muscle fiber is not a dendrite, but a motor endplate.
Neurotransmitter – It is a chemical substance released from the terminal axon of a nerve cell. It travels across the synapse and sends an electrical impulse or message to another nerve cell or other type of target cell. Chief neurotransmitters are acetylcholine, norepinephrine, epinephrine, dopamine and serotonin.
Nucleus (nuclei pl.) – Is a well-organized cluster of neurons in the brain. Technically speaking it consists of the nerve cell bodies ± dendrites. Some nuclei have adjoining axons, such as those which form the fibers of the cranial nerves.
Occipital lobes – They are the back portions of either hemisphere of the cerebral cortex. They are the main centers of processing visual signals from the retina. The specific name for the portion of the cortex which effects vision is the calcarine cortex.
Oligodendrocyte – Is a type of glial cell in the brain and spinal cord that forms myelin. It does so by extending its cell membrane and wrapping it around the axons of nerve cells layer upon layer. In contrast to the Schwann cell, a given oligodendrocyte does not form the myelin of just one axon. Instead, it wraps its cell membrane around different axons forming only part of the myelin of a given nerve fiber.
Optic chiasm – It is the point in the brain where the optic nerve fibers from the inner half of the retina of each eye cross over and join with the nerve fibers from the outer half of the retina of the opposite eye forming the optic tract. The right optic tract carries signals from the outer half of the retina of the right eye and signals of the inner half of the retina of the left eye to the right side of the brain. The left optic tract does the opposite. A lesion in this area causes loss of vision in the outer fields of both eyes. That defect is termed bitemporal hemianopsia.
Optic tract – It is the continuation of optic nerve fibers past the optic chiasm. It is a mixed bundle containing nerve fibers from the lateral retina of the ipsilateral eye and the media retina of the contralateral eye. The nerve fibers of the respective right and left bundle connect on the same side with nerve cells of the lateral geniculate nucleus – a relay center in the thalamus of the brain. The axons of neurons of the lateral geniculate nucleus form the optic radiation – a nerve bundle which concludes the pathway between the eye and the brain as it connects with nerve cells in the calcarine cortex.
Parasympathetic nervous system – Is that portion of the autonomic nervous system which calms the body and helps conserve energy. A mnemonic for the activities it regulates is “rest and digest” or “feed and breed.” Activities under its control include saliva production, peristalsis, stomach and pancreatic secretion of digestive juices, bladder contraction, and sexual arousal. It also slows down the heart rate and causes constriction of bronchi.
Parietal lobes – They are the portions of either hemisphere of the cerebral cortex between the frontal lobes and the occipital lobes. They have a number of functions depending on the side. The right side oversees spatial orientation and may have an effect on personality. The left side is involved in mathematical and writing skills as well as the production and understanding of language.
Peripheral nerves – They are the nerves throughout the body that transmit signals to and from the spinal cord. The periphery is any region outside of the spinal cord or brain. They can be afferent, efferent, or autonomic.
Peripheral nervous system – It is the portion of the nervous system outside of the central nervous system.
Plexus – It is a branching network of nerve fibers outside of the central nervous system. The main ones are in the neck, axillary (armpit), and lower back regions. Some plexuses also contain ganglia.
Pons – It is the part of the brainstem between the midbrain and the medulla oblongata.
REM sleep (Rapid eye movement sleep) – It is one of the 5 stages of sleep characterized by quick random eye movements and muscle paralysis. Most people experience it nightly. Generally, it comprises about-20%-25% of an adult human’s total time spent sleeping, and slightly more than half an infant’s. Special neurons in the brainstem are more active during this sleep stage. REM sleep might be involved in stabilization of some aspects of memory and creative thinking.
Somatic nervous system – it is that portion of the peripheral nervous system which governs voluntary movement of muscles and processing of input related to the 5 senses, touch, taste, sight, hearing and smell.
Spinal cord – It is the portion of the central nervous system that travels through the vertebral canal. It is a continuation of the medulla oblongata and extends from the large opening in the front lower part of the skull bone (foramen magnum) to the upper portion of the lumbar region of the spinal column. The inner portion of it is composed primarily of gray matter. The front or ventral portion of the gray matter is termed the anterior horn. The neurons in it are termed anterior horn cells. They govern the movement of skeletal muscles. The back or dorsal portion of the gray matter is termed the posterior horn. The neurons in it are termed posterior horn cells. They receive signals from afferent spinal nerves pertaining to sensation, such as touch, and transmit them to the brain through tracts within the spinal cord. The outer portion is primarily white matter consisting of tracts of nerve fibers covered with myelin sheets. Some of the tracts send messages from the brain down to neurons in the spinal cord. Others send messages from neurons in the spinal cord up to the brain. Some send messages in both directions. Much like the brain, the meninges form the covering of the spinal cord. The outermost portion is the epidural space.
Spinal nerve root – It is the bundle of nerve fibers that comes off the spinal cord. Those that are axons of posterior horn cells and come off the dorsal or back side of the spinal cord are a part of the afferent spinal nerves as they synapse with the body of the cells of the dorsal root ganglia whose axons form the sensory fibers of tissues and organs. Those that are axons of the anterior horn cells and come off the ventral or front side of the spinal cord form the efferent spinal nerves. Fibers of dorsal and ventral nerve roots merge to form the individual spinal nerves at each of the 31 different segments.
Spinal nerves – They are extensions of nerve roots and are composed of mixed afferent and efferent nerve fibers. They convey sensory and motor signals from and to their destinations points respectively. Depending on their location and function, some spinal nerves also contain autonomic fibers.
Subdural space – It is the potential space between the dura mater and the arachnoid.
Substantia nigra – It is the pigmented portion of the basal ganglia complex at the base of the brain which produces the neurotransmitter, dopamine. The death of its cells appears to be the cause of Parkinson’s disease.
Sympathetic nervous system – It is at portion of the autonomic nervous system which has to do with fight or flight reactions. Some of the bodily responses it causes are the following:
- Dilation of the pupils
- Increased heart rate
- Increased force of contraction of the heart
- Increase blood flow to skeletal muscles
- Constriction of blood vessels in the scan
- Elevated blood glucose
- Erection of hair
Sympathetic trunk – It is a chain of sympathetic ganglia alongside the vertebral column (spine) extending from the neck to the coccyx (tailbone). There is one to either side of the spine. It is part of the sympathetic nervous system and distributes nerve fibers connecting with other ganglia, plexuses, cranial nerves, individual organs, nerves accompanying arteries and to spinal nerves.
Synapse – It is the functional junction of neurons. It consists of a tiny gap between the end of a nerve axon and the dendrites on the body of another nerve cell it is communicating with. The terminal of the axon transmits the message by releasing a neurotransmitter from vesicles. The neurotransmitter then produces an electrical response in the cell it is communicating with.
Tract – It is a bundle of nerve fibers having a common origin, function and stopping point. Tracts carry impulses in either direction from varying points within the brain and spinal cord. The connection points of many tracts are nuclei.
Thalmus – It is the large oval-shaped area of gray matter buried under the cerebral cortex. It relays sensory nerve impulses from the basal ganglia to the cerebellum, both of which are involved in the control of muscle movement. It is involved in sensory perception, movement, sleep and states of consciousness.
Ventricle – It is one of 4 fluid-filled cavities in the brain and brainstem which are extensions of the central canal of the spinal cord. They all connect with each other and with the subarachnoid space. They contain cerebrospinal fluid produced by a structure within them, the choroid plexus.
Vertebral canal – It is the bony encasement of the spinal cord, formed by a series of openings in the bones of the spine. Another term for it is the spinal canal. It is not the central canal of the spinal cord.
White matter – It is the whitish appearing tissue of the brain and spinal cord. It is composed of nerve fibers.