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
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 (called demyelination), nerves do not conduct electrical impulses normally.
Some disorders that cause demyelination affect mainly the central nervous system. Others, such as chronic inflammatory demyelinating polyneuropathy Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) Chronic inflammatory demyelinating polyneuropathy is a form of polyneuropathy that, like Guillain-Barré syndrome, causes increasing muscle weakness, but the weakness progresses for more than... read more , affect mainly nerves in other parts of the body.
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Causes of Demyelinating Disorders
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 Tay-Sachs Disease and Sandhoff Disease Tay-Sachs disease and Sandhoff disease are types of lysosomal storage disorder called sphingolipidoses and are caused by a buildup of gangliosides in the tissues in the brain. These diseases... read more , Niemann-Pick disease Niemann-Pick Disease Niemann-Pick disease is type of lysosomal storage disorder. Types A and B are sphingolipidoses and are caused by a buildup of sphingomyelin in the tissues. Type C is a lipidosis that is caused... read more , Gaucher disease Gaucher Disease Gaucher disease is a type of lysosomal storage disorder called a sphingolipidosis. It is caused by a buildup of glucocerebrosides in tissues. Children who have the infantile form usually die... read more , 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:
Drugs (such as the antibiotic ethambutol)
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
Adrenoleukodystrophy and adrenomyeloneuropathy Adrenoleukodystrophy and Adrenomyeloneuropathy Disorders that cause demyelination and have no known cause are called primary demyelinating disorders. Demyelination is the destruction of the tissues that wrap around nerves, called the myelin... read more
Neuromyelitis optica spectrum disorder Neuromyelitis Optica Spectrum Disorder (NMOSD) Neuromyelitis optica spectrum disorder affects mainly the nerves in the eyes and spinal cord, causing patches of myelin (the substance that covers most nerve fibers) and the nerve fibers under... read more (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 Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers an autoimmune disorder is not known. Symptoms vary depending... read more ). The autoimmune reaction results in inflammation, which damages the myelin sheath and the nerve fiber under it.