Human Body 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.

 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.

 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.