I INTRODUCTION Nervous System, those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms.
II ANATOMY
AND FUNCTION
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 generalized activity, or they may
be highly efficient and rapidly conducting units. The specific response of the
neuron—the 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.
A Nerve
Cell
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; it is especially important in
the transmission of nerve impulses from the region of the cell body to other
cells. See Neurophysiology.
B Simple
Systems
Although all many-celled animals have some kind of
nervous system, the complexity of its organization 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 organized 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.
C Vertebrate
Systems
Vertebrate animals have a bony spine and skull in which
the central part of the nervous system is housed; 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; 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; both are also in contact with other parts of the body
through the nerves. The distinction made between the central nervous system and
the peripheral nervous system 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 (see Reflex).
In the skin are cells of several types called receptors; 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 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 (see Muscle). 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, which are chains of interconnected nerve cells, stimulated at one end and capable of bringing about movement or glandular secretion at the other.
D The
Nerve Network
The cranial nerves connect to the brain by passing
through openings 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; 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 regions
of the trunk and to the limbs. They interconnect extensively, thereby forming
the brachial plexus, which runs to the upper extremities; and the lumbar plexus,
which passes to the lower limbs.
E Autonomic
Nervous System
Among the motor fibers may be found groups that carry
impulses to viscera. These fibers 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 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.
III DISORDERS
OF THE NERVOUS SYSTEM
Consideration of disorders of the nervous system is the
province of neurology; psychiatry deals with behavioral disturbances of a
functional nature. The division between these two medical specialties cannot be
sharply defined, because neurological disorders often manifest both organic and
mental symptoms. For a discussion of functional mental illness, Mental Illness.
Diseases of the nervous system include genetic malformations, poisonings, metabolic defects, vascular disorders, inflammations, degeneration, and tumors, and they involve either nerve cells or their supporting elements. Vascular disorders, such as cerebral hemorrhage or other forms of stroke, are among the most common causes of paralysis and other neurologic 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. For example, meningitis, or infection of the meninges investing the brain and spinal cord, 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; viruses causing encephalitis attack the brain.
Inflammations of the nervous system are named according to the part affected. Myelitis is an inflammation of the spinal cord; neuritis is an inflammation of a nerve. It may be caused not only by infection but also by poisoning, alcoholism, or injury. Tumors originating in the nervous system usually are composed of meningeal tissue or neuroglia (supporting tissue) cells, depending on the specific part of the nervous system affected, but other types of tumor may metastasize to or invade the nervous system (see Cancer). In certain disorders of the nervous system, such as neuralgia, migraine, and epilepsy, no evidence may exist of organic damage. Another disorder, cerebral palsy, is associated with birth defects.
Contributed By:
Frederick A. Mettler