Introduction

Most nerve fibers inside and outside the brain are wrapped with many layers of tissue composed of a fat (lipoprotein) called myelin. These layers form the myelin sheath. Much like the insulation around an electrical wire, the myelin sheath enables electrical impulses to be conducted along the nerve fiber with speed and accuracy. When the myelin sheath is damaged, nerves do not conduct impulses normally.

See the figure Insulating a Nerve Fiber.

When babies are born, many of their nerves lack mature myelin sheaths. As a result, their movements are jerky, uncoordinated, and awkward. As myelin sheaths develop, movements become smoother, more purposeful, and more coordinated. Myelin sheaths do not develop normally in children with certain rare hereditary diseases, such as Tay-Sachs disease, Niemann-Pick disease, Gaucher's disease, and Hurler's syndrome. These children may have permanent, often extensive, neurologic problems.

In adults, the myelin sheath can be destroyed by stroke, inflammation, immune disorders, metabolic disorders, and nutritional deficiencies (for example, a lack of vitamin B₁₂). Such destruction is called demyelination. Poisons, drugs (such as the antibiotic ethambutol), and excessive use of alcohol can damage or destroy the myelin sheath. If the sheath is able to repair and regenerate itself, normal nerve function may return. However, if the sheath is severely damaged, the underlying nerve fiber can die, causing irreversible damage. Nerve fibers cannot regenerate themselves.

Disorders that cause demyelination in the central nervous system (the brain and spinal cord) and have no known cause are called primary demyelinating disorders. Multiple sclerosis is the most common of these disorders.