Several factors can cause cell death and more or lesser degeneration of the nervous system. These factors may be gene mutations, viral infections, psychotropic drugs, metal poisoning, etc.
The best known degenerative nerve diseases are multiple sclerosis, Parkinson's disease, Huntington's disease and Alzeheimer disease.
Multiple sclerosis is a disorder in which demyelination of isolated areas of the nerves of the eyes, brain, and spinal cord occurs.
The term multiple sclerosis derives from multiple areas of healing (sclerosis) that represent many foci of demyelination in the nervous system. The neurological signs and symptoms of multiple sclerosis are so diverse that the doctor may not diagnose it when the first symptoms occur. Because the disease often worsens slowly over time, affected individuals have relatively good health periods (remissions) alternating with periods of weakness (exacerbations). About 400,000 Americans, mostly young adults, have the disease.
The cause of multiple sclerosis is unknown, but it is suspected that a virus or some unknown antigen is responsible for somehow triggering an autoimmune process, usually early in life. Then the body, for some reason, produces antibodies against its own myelin. Antibodies produce inflammation and damage the myelin sheath. Heredity seems to play a role in multiple sclerosis. About 5% of affected individuals have a sister or brother who also has the disease and about 15% of them have an affected close relative. Environmental factors also play a role. Sclerosis affects 1 in 2,000 individuals who spend the first decade of their lives in temperate climates, but it only affects 1 in 10,000 individuals born in tropical regions.