The nervous system is composed of 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. It could be considered as the master control unit inside your body.
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.
 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. (See figure to right.) 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.
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.
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.
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.
  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. The autonomic system controls the action of the glands; the functions of the respiratory, circulatory, digestive, and urogenital systems; and the involuntary muscles in these systems and in the skin. Controlled by nerve centers in the lower part of the brain, the system also has a reciprocal effect on the internal secretions, being controlled to some degree by the hormones and exercising some control, in turn, on hormone production.
Two antagonistic divisions make up the autonomic nervous system: the sympathetic, or thoracolumbar, division, which stimulates the heart, dilates the bronchi, contracts the arteries, and inhibits the digestive system, preparing the organism for physical action; and the parasympathetic, or craniosacral, division, which has the opposite effects, and prepares the organism for feeding, digestion, and rest. The sympathetic division consists of a chain of interconnected ganglia (groups of nerve cells) on each side of the vertebral column, which send nerve fibers to several large ganglia, such as the celiac ganglion. They, in turn, give rise to nerves passing to the internal organs. The ganglia of the sympathetic chains are connected to the central nervous system by fine branches connecting each ganglion with the spinal cord. Fibers of the parasympathetic system arise in the brain and, with the cranial nerves, especially the vagus and accessory nerves, pass to ganglia and plexuses (networks of nerves) within the various organs. The lower part of the body is innervated by fibers arising from the lowest (sacral) segment of the spinal cord and passing to the pelvic ganglion, which gives rise to nerves for such organs as the rectum, bladder, and genital organs.
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, see Mental Conditions.
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} cerebral palsy, is associated with birth defects. 
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