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. 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, and they 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. T he 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.