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The basic structural and functional unit of the
nervous system is the
nerve cell or
neuron.
All neurons have a cell body
(soma) which contains
cellular organelles that are typical of most of the cells in the body and includes a
nucleus, a
nucleolus and lots of rough
endoplasmic reticulum. Most nerve cells have processes called
dendrites, which act like antennae for the cell, in that they receive input to the cell. Most neurons also have a single long process called an
axon, which is capable of transmitting a pulse of electricity
(nerve impulse or
action potential) from the cell body to some distant target in the brain or the periphery. These axons may be quite long (up to a
metre or more in the case of a nerve cell in the spinal cord and has an axon which contacts a muscle in the foot). Axons usually break up into smaller branches (terminal branches) near their target. These terminal branches end in swellings which make a specialized contact with the target cell.
If the target cell is another neuron, the swelling is called a
bouton, and the specialized contact is called a
synapse. If the target is a muscle fiber, the
bouton is often called a
motor end plate and the synapse is often referred to as a
neuromuscular junction. There is usually a gap between the terminal swelling and the target cell
(postsynaptic). The electrical impulse does not cross this gap, but rather causes a chemical
(neurotransmitter) to be released from the axon terminals. The
neurotransmitter diffuses across the gap and causes electrical changes to occur in the postsynaptic cell. Neurons come in all sizes and shapes, but the basic functions of all neurons are more or less similar: they receive (and integrate) inputs, and relay their output, in the form of an
action potential, to some other target cell. The cell body is mainly responsible for meeting the metabolic needs of the cell and its position with respect to the axon and dendrites is somewhat variable.
Over part of its length, the sensory axon is actually conducting nerve impulses
toward the cell body. Over the rest of its length the axon is conducting nerve impulses
away from the cell body. The best functional definition of an axon is that it is a nerve process which is capable of transmitting a nerve impulse
(action potential) over some distance. The nervous system also contains cells which are not neurons and which do not
directly participate in the task of sending and receiving electrical signals. These supporting cells are called
glial cells. There are several types of
glial cell, but only two types need to be considered here: those that form
myelin sheaths around axons in the central and
peripheral nervous system (PNS). Generally, axons are not naked, but they are often wrapped in an insulating material referred to as
myelin. Myelin is formed by glial cells that wrap themselves around axons. The presence of a myelin sheath around an axon increases the velocity at which an axon will conduct a nerve impulse down its length because the nerve impulse effectively jumps from one space to another between insulating cells. Nerve impulses therefore travel faster in
myelinated axons than in
unmyelinated axons. The myelin sheath is formed by flattened out cells that wrap themselves around the axon. In the central nervous system the cells that form the myelin sheath are called
oligodendroglia or
oligodendrocytes. In the
peripheral nervous system (PNS) they are called axon sheath cells.
The sheath itself is essentially flattened cell membrane, with all of the cytoplasm squeezed out except in the outermost layer. The major component of a cell membrane is the
phospholipid bilayer and where many layers of membrane are stacked on top of one another it will have a fatty appearance due to the presence of this
phospholipid. Unembalmed lipid (such as fat found on meats) has a glistening white appearance. Myelinated axons therefore will have a glistening white appearance in the central and
peripheral nervous system (PNS), and are referred to as
white matter. Areas containing mainly cell bodies tend to lack myelin and are referred to as
grey matter.
In the periphery, cell bodies are not usually found in isolation, but rather exist in clusters known as
ganglia. If a bundle of axons travelling together in the periphery contains sensory axons only, it is called a
sensory nerve and if it contains
only motor axons (going to muscles), it is called a
motor nerve. Virtually all nerves in the body contain both sensory and motor axons and are therefore called
mixed nerves. Many of the axons in any nerve will be
myelinated and gives nerves their glistening white appearance.
Individual axons are enveloped in a connective tissue wrapping called
endoneurium. Bundles
(fascicles) of axons are wrapped in a connective tissue covering called
perineurium. The nerve as a whole is enveloped in a connective tissue sheath called the
epineurium. These connective tissue sheaths help to give peripheral nerves a certain toughness and resistance to tearing.
Major subdivisions of the nervous systemThe nervous system is a network of millions of cells which communicate with one another by means of nerve impulses transmitted along
axons of nerve cells. The nervous system allows a
sense to be appreciated of the internal or external environment, to
process that information and
react to it in some way. This usually involves
movement and therefore muscle contraction.
The nervous system consists of two major subdivisions: the
central nervous system (CNS) and the
peripheral nervous system (PNS). The
CNS consists of the
brain and
spinal cord. This is where almost all the important
processing in the nervous system takes place. There appears to be no communication between neurons receiving sensory information and neurons encoding motor output outside the
CNS. Therefore, for even the simplest reflex activity to take place, the sensory information evoking the reflex must be relayed to the
CNS, and the motor output must leave the
CNS and go to the muscles. Here there is an anatomical problem: the
CNS is housed entirely within the
dorsal cavity of the body, which is made up of the
cranial cavity, housing the brain, and the
vertebral canal, housing the spinal cord. The receptors which relay sensory input to the
CNS and the muscles which are to be controlled, lie almost entirely
outside the
dorsal cavity. Therefore, there needs to be some way of relaying sensory input and motor outflow between the periphery and the
CNS. This is the function of the
peripheral nervous system.
The peripheral nervous systemThe
PNS is a collection of neurons and their processes which relay information from the periphery to the
CNS, in which case they are
afferent or
sensory or from the
CNS to the periphery, in which case they are
efferent or
motor. The parts of the
PNS that actually connect the
CNS and the periphery are the
nerves. Those which connect the brain with the periphery (mainly in the head and neck) are called
cranial nerves. There are
12 pairs of
cranial nerves which leave the brain on its underside, then exit the
cranial cavity by a series of holes in the base of the skull, called
foramina. Some
cranial nerves are almost purely
sensory, such as those which mediate smell, vision, and hearing. Others are almost purely motor, such as those which move the eyes and tongue. Others are mixed.
The nerves linking the spinal cord and the periphery, which are responsible for sensory and motor innervation of the body outside of the head and neck, are called
spinal nerves. The spinal nerves exit the
vertebral canal by way of spaces between adjacent vertebrae, known as
intervertebral foramina.
There are
31 pairs of
spinal nerves, which are named according to the
intervertebral foramina through which they pass. There are
8 pairs of
cervical nerves,
12 thoracic nerves,
5 lumbar nerves,
5 sacral nerves and
1 coccygeal nerve. The
cervical nerves all
exit above the vertebra for which they are named, except for
C8, which
exits between C7 and
T1. There are only
7 cervical vertebrae. All other
spinal nerves exit below the vertebra for which they are named. All
spinal nerves are
mixed (
sensory and
motor), with the possible exception of the first
cervical and
coccygeal nerves, which often lack a
sensory component.
Spinal nerves: somatic componentsFor the somatic portion of the nervous system (the portion dealing with body parts other than viscera, vascular smooth muscle, and glands) there are two major types of nerve cells which connect the spinal cord to the periphery. The
primary sensory neurons (or
afferent neurons), which relay input from the periphery to the
spinal cord, and
spinal cord motor neuron (or
efferent neuron) which convey motor outflow from the
spinal cord to the periphery.
Primary sensory neurons have peripheral processes in the skin or muscles which are usually in contact with some kind of a specialised receptor (such as those for touch or
muscle stretch). Their central processes enter the
spinal cord where they make synapses with other neurons using a variety of
neurotransmitters. Axons of
spinal cord motor neurons pass to the periphery to innervate
striated muscle.
Acetylcholine is used as the
neurotransmitter at the
neuromuscular junction.
The simplest circuit is the two neuron
reflex arc or a circuit for a
monosynaptic reflex. The sole elements of this circuit would be the
primary sensory neuron relaying information from the periphery, the
spinal cord motor neuron relaying motor output to the muscles, and the synapse between them.This is the kind of curcuit that can mediate simple reflex activity, such as the stretch reflex, the classic example of which is the
(patellar) knee jerk reflex. Tapping the
patellar tendon it stretches this tendon. The stretch is relayed to the spinal cord by a
primary sensory neuron, which synapses directly on a spinal cord motor neuron which in turn sends a nerve impulse in its axon out to the
quadriceps femoris muscle, causing it to contract, jerking the lower leg forward. Aside from the intrinsic interest that the
knee jerk reflex holds for the clinician, this reflex has functional importance in everyday life in terms of being an
"anti-gravity" reflex. The most common stimulus that would put a stretch on the
patellar tendon would be if the knees were to start to buckle. In this case, the shortening of the
quadriceps femoris muscle would serve to straighten the leg at the knee to help prevent falling. The fact that there is only one synapse in this circuit makes it an especially fast circuit, which is important to its role as an anti-gravity reflex.
Every synapse in a circuit introduces a delay of about 1/2 msec
in response time, so the fewer synapses, the faster the response
Most circuits are not quite this simple and even for most other reflexes, such as the withdrawal reflex (when you burn your finger), there is at least one other neuron (an
interneuron) interposed in the circuit and for more complex behaviours (feeling a pen that is put into your hand and deciding how to hold it) the circuit will involve connections that ascend to the brain and descend back to the spinal cord.
When the spinal cord is cut in cross section the white matter is on the outside and is made up mainly of axons that are ascending or descending in the spinal cord. The gray matter (cell bodies) is centrally located and has a butterfly shape. There is some functional segregation in the spinal cord, so that neurons in the
dorsal horn are mainly sensory, while those in the
ventral horn are mainly motor. Spinal cord
motor neurons sit in the
ventral horn of the spinal cord. Their axons pass
ventrally and
laterally, forming what are known as
ventral roots, up to the point where they pass through an
intervertebral foremen. On the other hand, the
sensory axon starts in the vicinity of a receptor in the periphery. It probably travels with some motor axons on its way toward the spinal cord, and enters the
vertebral canal through an
intervertebral foremen. The cell body sits just inside the
intervertebral foremen, along with other cell bodies of
primary sensory neurons, in what are called
dorsal root ganglia. The central process of the sensory axon travels
dorsally and
medially, forming a
dorsal root to enter the spinal cord in the vicinity of the
dorsal horn. Within the gray matter of the spinal cord the axon can then form synapses either with
interneurons or with
spinal cord motor neurons.
The
dorsal ramus supplies the skin in the region overlying the vertebral column and the intrinsic musculature of the back. A
ventral ramus continues forward, giving off branches that supply the remainder of the skin and musculature of the trunk or it continues into the limbs.
Nerves emerging through a specific
intervertebral foremen tend to supply particular parts of the body in a manner that is consistent, by and large, from one body to another. This characteristic is referred to as
segmental organisation of
spinal nerves. In some parts of the body (thorax), this
segmental organisation is relatively simple. Each nerve emerges from an
intervertebral foremen, passes along the body wall between two ribs, and innervates the skin and musculature lying between (or adjacent to) those two ribs. In other regions the arrangement is more complex. Upon emerging from the
intervertebral foramem, nerves in some regions revert themselves and recombine, forming
plexuses.
Whether a spinal nerve enters into plexus formation or retains a simple segmental distribution, each spinal nerve innervates a particular area of skin in a predictable and orderly way. The area of skin innervated by sensory fibers from a particular spinal segment is called the
dermatome for that segment
(dermatome map). Typical dermatome maps are somewhat deceiving as there is actually considerable overlap between the territories supplied by nerves arising from adjacent spinal cord segments. This point has considerable clinical significance. The implication is that in order to achieve complete loss of sensation on an area of skin, a patient would have to do damage to (or have anesthesia applied to) three adjacent spinal nerves.
Spinal nerves: visceral componentsThe discussion above centres on
somatic components of spinal nerves, which mediate skin sensation and
proprioception (position sense) for the body wall and limbs. As well as motor innervation to skeletal (striated or "voluntary") muscle, intuition shows that there must also be nerves which are capable of relaying sensory information (particularly pain or stretch)
from the internal organs or
viscera and conveying motor outflow
to glands, smooth muscle in viscera and blood vessels, or to the (cardiac) muscle of the heart. These are the visceral components of spinal nerves. There are also visceral components to cranial nerves.
Visceral Motor System
The
autonomic nervous system portions of the
central nervous system (CNS) and
peripheral nervous system (PNS) are concerned with regulation of visceral motor functions (efferent fibres), though for a variety of reasons, this designation is now less widely accepted. The
visceral motor system is somewhat different in its plan of organisation from the
somatic motor system. This system of skeletal (striated) muscles is controlled by a motor neuron sitting in the
ventral horn of the spinal cord, which sends its axon into the
ventral root, then into a
spinal nerve. The peripheral portion of the
somatic motor system (nerve system) is a
one-neuron system. The activity in a spinal cord motor neuron does not occur independently, but rather, it is influenced by other spinal cord neurons or by descending axons of neurons whose cell bodies are in the brain.
Unlike the
somatic motor system, the peripheral component of the
autonomic (or visceral) motor system (nerve system) is a
two-neuron system. The cell body of the first neuron, called a
preganglionic neuron, is located in the
CNS. In the spinal cord, cell bodies of
preganglionic visceral motor neurons are located in the
intermediolateral cell column, rather than in the
ventral horn, where the
somatic motor neurons lie. However, as with other motor axons, axons of
preganglionic visceral motor neurons leave the spinal cord by way of
ventral roots.
The cell body of the second neuron, called a
postganglionic neuron, is located outside the
dorsal cavity, in what are called
autonomic ganglia or
visceral ganglia.
The
visceral motor system is commonly divided into
2 parts: the
sympathetic and the
parasympathetic divisions. Most of the viscera receive innervation from both sympathetic and parasympathetic fibers, but the effects produced by activity in the two divisions generally oppose one another. Activity in the
sympathetic nervous system is generally associated with an increase in the level of excitation of an organism. It is sometimes called the
"fight or flight" system. The
parasympathetic nervous system is generally thought of as
"vegetative", being concerned with the body's
recovery from exertion, or active when the body is in its
resting state. Apart from the functional differences between the sympathetic and parasympathetic divisions of the visceral motor system, there are significant differences in their anatomical organization.
Location of preganglionic cell bodiesLocation of
postganglionic cell bodies:
- the sympathetic postganglionic cell bodies are generally located near the vertebral column in paravertebral ganglia (sympathetic chain ganglia) which are located at all vertebral levels, or prevertebral (preaortic) ganglia, which are located in the abdomen anterior to the vertebral column. near the stems of the major branches of the abdominal aorta. Parasympathetic postganglionic cell bodies are generally located within or very near to the target structure.
Length and trajectory of
preganglionic axons:
- sympathetic preganglionic axons pass only from thoracolumbar levels to ganglia located near the vertebral column, whereas parasympathetic preganglionic axons pass directly from craniosacral levels to the vicinity of the target organ. Sympathetic preganglionic axons are therefore (relatively) short, and parasympathetic preganglionic axons are (relatively) long. Aside from this general rule, several important points concerning the distribution of sympathetic and parasympathetic preganglionic axons need to be made.
Other
preganglionic axons pass through the
paravertebral ganglion at their own segmental level to synapse in
prevertebral (preaortic) ganglia (
celiac,
superior mesenteric and
inferior mesenteric ganglia).
Preganglionic axons that behave this way (levels
T5-L2) are known as
splanchnic nerves on their way to the
preaortic ganglia. There are usually three major
splanchnic nerves (
on each side) arising from thoracic levels. The
greater splanchnic nerve usually arises from
T5-9 or
10. The
lesser splanchnic nerve usually arises from
T10 and
T11. The
least splanchnic nerve usually arises from
T12.
Splanchnic nerves also arise from lumbar levels (
lumbar splanchnics).
Some
splanchnic nerves do not synapse in
prevertebral ganglia at all, but continue directly to the
suprarenal (or
adrenal) glands, where they innervate cells of the
adrenal medulla directly. This would seem to be a violation of the rule that the
visceromotor system is a two neuron system. The cells of the
adrenal medulla are modified
postganglionic neurons.
Parasympathetic preganglionic axons of neurons with cell bodies located in the brain stem leave the
central nervous system (CNS) with
cranial nerves 3, 7, 9, and
10 and then may tag along with other
cranial nerves to get to their destinations. For the
parasympathetic fibers in
cranial nerves 3, 7, and
9, these are ganglia in or near the eye, the glands, and the smooth muscle of the head. Parasympathetic components of
cranial nerve 10 (
vagus = wanderer) have a more widespread territory, ultimately synapsing in ganglia in the walls of many viscera, including the heart and the digestive tract from the pharynx to the
left colic (splenic) flexure of the large intestine. The
right colic flexure is known as the
hepatic flexure. The axons of preganglionic parasympathetic neurons in spinal cord segments
S2-4 leave the
central nervous system (CNS) in the
ventral roots of spinal nerves
S2-4, then form what are known as
pelvic splanchnic nerves (or
pelvic nerves) on their way to synapse in ganglia in the walls of the
pelvic viscera.
Length and Trajectory of Postganglionic AxonsSympathetic
postganglionic axons must pass from cell bodies in
paravertebral ganglia or
prevertebral ganglia to targets in the
viscera. Therefore they are (relatively) long. Sympathetic
postganglionic axons may reach their targets by one of three means:
- Rejoining a spinal nerve and traveling to target structures in the limbs or body wall. These targets would be mainly sweat glands, smooth muscle in the walls of blood vessels and arrector pill muscles
- Tagging along with blood vessels supplying the target organ. Most postganglionic axons innervating abdominal or pelvic viscera arrive at their target organ by this route
Parasympathetic postganglionic axons must pass only from cell bodies in ganglia located in or near the target organ to their specific target cells. Therefore they are (relatively) short. When the target is a thoracic, abdominal, or pelvic
viscus, the ganglia are located in the walls of the
viscus and are so short they are never seen on gross dissection. When the target is a gland or smooth muscle in the head. however, the ganglia sit a short distance from the target, and the
postganglionic axons reach their targets by tagging along with other cranial nerves passing to the target structure.
Neurotransmitters:
- Acetylcholine is the neurotransmitter for parasympathetic preganglionic and postganglionic neurons at synapses located in parasympathetic ganglia and on target cells.
Visceral AfferentsVisceral sensation (at least insofar as it is consciously perceived) is largely limited to pain. This may be due to over distension of a
viscus, or spasm of smooth or cardiac muscle. A striking characteristic of visceral pain is that very often it is perceived in a body part other than where it is being produced. For example, the pain of a heart attack is often perceived as a pain radiating down the left arm. The pain of early appendicitis is often localized in the umbilical region of the abdominal wall. Pain which is produced in a
viscus, but localized to the body wall or limbs is called
referred pain and is a notable characteristic of
visceral sensation. The anatomical basis of
referred pain is poorly understood. It is thought that, since
visceral sensation is not usually consciously perceived, when a spinal cord segment is bombarded with pain input from an injured or inflamed
viscus, the information is interpreted as arising from body areas from which that segment generally receives input. That is, the pain is perceived as arising from the
dermatome innervated by that spinal segment.
Aside from the special features described above,
visceral afferents are highly similar to
somatic afferents. That is, they are a one-neuron system, with a sensory axon arising near a receptor in a and passing all the way to the
dorsal horn of the spinal cord. As with somatic sensory neurons, the cell body of a visceral sensory neuron sits in a
dorsal root ganglion and the central process of the sensory axon passes to the spinal cord by way of a
dorsal root. Afferents arising from a
viscus will usually pass back to the
central nervous system (CNS) by tagging along with an autonomic nerve. This means that visceral sensory fibers may be found passing through autonomic ganglia, but they
DO NOT synapse in those ganglia.
Enteric nervous systemAside from postganglionic parasympathetic neurons found in the walls of the gut, the walls of the viscera of the gastrointestinal tract have been shown to contain a network of millions and millions of nerve cells which play an important role in controlling gut
motility. This network forms what is known as the
enteric nervous system. Estimates are that the
enteric nervous system may contain more nerve cells than the spinal cord! The details of the anatomy of the
enteric nervous system are still poorly understood, however, and this is an area of active investigation. It is not yet clear how many
neurotransmitters are utilised by enteric neurons, but the evidence suggests that
serotonin is a major
neurotransmitter in this system.
When a spinal segment is not in its normal position, it partly closes nerve openings between the vertebrae, which in turn causes the nerves to be pinched. This will cause a reduction in the flow of nerve energy to some part of the body. When this occurs the organs and tissues which the pinched nerve supplies cannot function properly and results in pain. Susceptibility to disease will result. Misalignment should be promptly checked and corrected. Nerves do not give off a flow of nerve energy and are gland cells that produce and release a hormone that causes the inhibition or the contraction of muscle cells and the inhibition or enhancement of secretion by a gland cell that includes another nerve cell. That is all they do, no more, no less. They do not actually conduct electricity or any other form of energy.
When a nerve cell undergoes its function of secreting a hormone, changes occur in its outer cell membrane that allow electrically-charged ions to move in and out of the cell in a step-wise fashion along the full extent of the nerve. This is what really occurs when a nerve is described as "conducting an impulse" or "firing." A spinal nerve at the intervertebral opening is actually a thin tube of connective tissue containing the extensions of millions of nerve cells. These extensions are the axons that are also described as "fibers." This latter term is misleading because it implies a certain firmness such as fine wires would have. Nothing could be more incorrect. The axons are delicate, flimsy structures. Since they are merely elongated or drawn out parts of cells they need nourishment along with the cells that make up their sheaths. Therefore, delicate blood vessels are contained in what is called a nerve at the visible level. If compression of a nerve does not directly kill the axons, the axons may die because the compression cuts off the flow of blood in the vessels of the nerve. Compression of a nerve cell anywhere along its extent can cause it to secrete its hormone. If it is a sensory nerve cell, it can cause the brain to experience pain. If it is a motor nerve cell, the hormone can cause a muscle cell to contract.
Nervous systemThe
nervous system is concerned with those elements within the animal organism that transmits nerve impulses or activates muscle mechanisms. The reception of stimuli is the function of special sensory cells. The conducting elements of the nervous system are cells called neurons. These may be capable of only slow and generalised activity or they may be highly efficient and rapidly conducting units. The specific response of the neuron (nerve impulse) and the capacity of the cell to be stimulated make this cell a receiving and transmitting unit capable of transferring information from one part of the body to another.
Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as
axons and
dendrites. The
dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The
axon, by contrast, is usually a single elongated extension and is especially important in the transmission of nerve impulses from the region of the cell body to other cells. Although all many-celled animals have some kind of nervous system, the complexity of its organisation varies considerably among different animal types. In simple animals such as
jellyfish, the nerve cells form a network capable of mediating only a relatively stereotyped response. In more complex animals, such as shellfish, insects, and spiders, the nervous system is more complicated. The cell bodies of neurons are organised in clusters called
ganglia. These clusters are interconnected by the neuronal processes to form a
ganglionated chain. Such chains are found in all vertebrates, in which they represent a special part of the nervous system, related especially to the regulation of the activities of the heart, the glands, and the involuntary muscles.
Vertebrate SystemsVertebrate animals have a bony spine and skull in which the central part of the nervous system is housed and where the peripheral part extends throughout the remainder of the body. That part of the nervous system located in the skull is referred to as the brain and that found in the spine is called the
spinal cord. The brain and the
spinal cord are continuous through an opening in the base of the skull and both are also in contact with other parts of the body through the nerves. The distinction made between the
central nervous system (CNS) and the
peripheral nervous system (PNS) is based on the different locations of the two intimately related parts of a single system. Some of the processes of the cell bodies conduct sense impressions and others conduct muscle responses, called reflexes, such as those caused by pain. In the skin are cells of several types called receptors and each is especially sensitive to particular stimuli. Free nerve endings are sensitive to pain and are directly activated. The neurons so activated send impulses into the
central nervous system (CNS) and have junctions with other cells that have axons extending back into the periphery. Impulses are carried from processes of these cells to motor endings within the muscles. These
neuromuscular endings excite the muscles, resulting in muscular contraction and appropriate movement. The pathway taken by the nerve impulse in mediating this simple response is in the form of a
two-neuron arc that begins and ends in the periphery. Many of the actions of the
nervous system can be explained on the basis of such
reflex arcs (chains of interconnected nerve cells) stimulated at one end and capable of bringing about movement or glandular secretion at the other.
Nerve NetworkThe
cranial nerves connect to the brain by passing through openings
(foramina) in the skull, or cranium. Nerves associated with the spinal cord pass through openings in the vertebral column and are called
spinal nerves. Both cranial and spinal nerves consist of large numbers of processes that convey impulses to the central nervous system and also carry messages outward; the former processes are called
afferent, the latter are called
efferent. Afferent impulses are referred to as sensory and efferent impulses are referred to as either
somatic or visceral motor, according to what part of the body they reach. Most nerves are mixed nerves made up of both sensory and motor elements. The cranial and spinal nerves are paired. The number in humans are
12 and
31, respectively.
Cranial nerves are distributed to the head and neck regions of the body, with one conspicuous exception: the
tenth cranial nerve, called the
vagus. In addition to supplying structures in the neck, the
vagus is distributed to structures located in the chest and abdomen. Vision, auditory and vestibular sensation, and taste are mediated by the second, eighth, and seventh cranial nerves, respectively.
Cranial nerves also mediate motor functions of the head, the eyes, the face, the tongue, and the larynx, as well as the muscles that function in chewing and swallowing.
Spinal nerves, after they exit from the vertebrae, are distributed in a bandlike fashion to various regions of the trunk and to the limbs. They interconnect extensively, thereby forming the
brachial plexus, which proceeds to the upper extremities: the
lumbar plexus proceeds to the lower limbs.
Autonomic Nervous SystemAmong the motor fibres may be found groups that carry impulses to viscera. These fibres are designated by the special name of
autonomic nervous system. That system consists of two divisions, more or less
antagonistic in function, that emerge from the
central nervous system (CNS) at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the
sympathetic ganglionated chain, courses through the
spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.
Consideration of disorders of the
nervous system is the province of
neurology. Psychiatry deals with behavioural disturbances of a functional nature. The division between these two medical specialities cannot be sharply defined, because neurological disorders often manifest both organic and mental symptoms. Diseases of the
nervous system include genetic malformations, poisonings, metabolic defects, vascular disorders, inflammations, degeneration, and tumours, which involve either nerve cells or their supporting elements. Vascular disorders, such as
cerebral haemorrhage or other forms of stroke, are among the most common causes of paralysis and other neuralgic complications. Some diseases exhibit peculiar geographic and age distribution. In temperate zones,
multiple sclerosis is a common degenerative disease of the
nervous system, but it is rare in the Tropics. The
nervous system is subject to infection by a great variety of bacteria, parasites, and viruses.
Meningitis is an infection of the
meninges investing the brain and spinal cord and can be caused by many different agents. On the other hand, one specific virus causes
rabies. Some viruses causing neurological ills affect only certain parts of the
nervous system. For example, the virus causing
poliomyelitis commonly affects the spinal cord and viruses causing
encephalitis attack the brain.
