Nervous System Organisation

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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 fibre, 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 system

The 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 system

The 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 components

For 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.

• Cell bodies of spinal cord motor neurons sit in the ventral horn of the spinal gray matter
• Cell bodies of primary sensory neurons sit in dorsal root ganglia aggregations just inside the intervertebral foramina
• The central process of the primary sensory neuron running between the intervertebral foremen and the spinal cord is part of a dorsal root
• The axon of the spinal cord motor neuron, where it runs between the ventral horn and the intervertebral foremen, is part of a ventral root
• Where the dorsal and ventral roots come together and exit through the intervertebral foremen, they form a spinal nerve
• Dorsal roots are sensory, ventral roots are motor. Spinal roots are mixed

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 fibres 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 components

The 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 fibres, 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 bodies

• the sympathetic preganglionic cell bodies are located in the spinal cord from T1 levels to L2 (or 3). The sympathetic division is often referred to as the thoracolumbar division of the visceral motor system. Parasympathetic preganglionic cell bodies are located in the brain stem and in the spinal cord at S2 - levels. The parasympathetic division is often referred to as the craniosacral division of the visceral motor system.

Location 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.

• sympathetic preganglionic axons leave the central nervous system (CNS) as part of spinal nerves T1-L2. All sympathetic preganglionic axons leave spinal nerves soon after their exit from the intervertebral foramen and enter the sympathetic chain ganglion at their own segmental level by way of a white ramus communicans (so called because preganglionic axons are generally myelinated and have, theoretically, a glistening white appearance.) Some preganglionic axons synapse with a postganglionic neuron in the paravertebral ganglion at their own level and levels T2-T4. Other preganglionic sympathetic axons do not synapse in the ganglion at their own segmental level. Some preganglionic axons ascend or descend in the sympathetic chain to synapse in paravertebral ganglia at cervical or lower lumbar and sacral levels.

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 fibres 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 Axons

Sympathetic 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

• Passing directly from the paravertebral ganglion to the target organ. This is the case for sympathetic innervation of thoracic viscera

• 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:

• Preganglionic sympathetic neurons use acetylcholine as their neurotransmitter at synapses in prevertebral ganglia or paravertebral ganglia (sympathetic chain ganglia), but epinephrine or norepinephrine is the neurotransmitter where the postganglionic axon synapses with the target organ.

• Acetylcholine is the neurotransmitter for parasympathetic preganglionic and postganglionic neurons at synapses located in parasympathetic ganglia and on target cells.

Visceral Afferents

Visceral 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 fibres may be found passing through autonomic ganglia, but they DO NOT synapse in those ganglia.

Enteric nervous system

Aside 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 "fibres." 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 system

The 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 Systems

Vertebrate 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 Network

The 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 System

Among 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.