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 nerve signals (electrical impulses) to be conducted along the nerve fiber with speed and accuracy. When the myelin sheath is damaged, nerves do not conduct electrical impulses normally. Sometimes the nerve fibers are also damaged.
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. Nerve fibers in the central nervous system (brain and spinal cord) cannot fully regenerate themselves. Thus, these nerve cells are permanently damaged.
Insulating a Nerve Fiber
Some disorders that cause demyelination affect mainly the central nervous system. Others, such as chronic inflammatory demyelinating polyneuropathy, affect mainly nerves in other parts of the body.
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 disease, and Hurler syndrome. These children may have permanent, often extensive, neurologic problems.
In adults, the myelin sheath can be damaged or destroyed by the following:
Such destruction is called demyelination.
Some disorders that cause demyelination have no known cause. These disorders are called primary demyelinating disorders. The most common of these disorders is
Other primary demyelinating disorders include
Neuromyelitis optica spectrum disorder (neuromyelitis optica)
Sometimes primary demyelinating disorders develop after a viral infection or vaccination against a viral infection. A likely explanation is that the virus or another substance somehow triggers the immune system to attack the body’s own tissues (autoimmune reaction). The autoimmune reaction results in inflammation, which damages the myelin sheath and the nerve fiber under it.