Nerve cells send signals along long wires called axons, which are insulated with a fatty substance called myelin. The myelin sheath allows signals to be transmitted rapidly from the nervous system to the rest of the body. When myelin is destroyed -- called demyelination -- nerve signals are slowed or stopped. In the central nervous system -- the brain and spinal cord -- myelin cannot repair itself after it is damaged. However, in the peripheral nervous system -- the nerves that travel to the arms and legs -- myelin can often regrow. A number of diseases affect the myelin sheath in the central and peripheral nervous system.
Multiple sclerosis is the most common chronic disabling disorder of the central nervous system in young adults, according to a November 2014 article in "American Family Physician." MS is an autoimmune demyelinating disease, which means that a person’s immune system attacks his own body -- in this case the myelin sheath in the brain and spinal cord. Symptoms may include fatigue, sensitivity to heat, numbness and tingling, bladder problems, changes in vision, muscle weakness and depression. MS can follow a relapsing-remitting course -- with attacks of worsening symptoms that are followed by partial or complete recovery. It can also follow a progressive course, in which the disease steadily worsens.
Other Autoimmune Disorders of the Myelin Sheath
Guillain-Barre syndrome, or GBS, occurs when the body's immune system attacks the myelin sheath in the peripheral nervous system -- the nerves outside the brain and spinal cord. GBS usually begins with muscle weakness in the legs that spreads up the body. The muscles of breathing may become involved. GBS can also affect the nerves involved in unconscious body functions -- leading to abnormal heart rhythms, blood pressure fluctuations and difficulties with passing urine. The cause of GBS is not known, though it sometimes happens after vaccinations or surgery.
Other autoimmune disorders of the myelin sheath include transverse myelitis, which involves an attack on the spinal cord, and acute disseminated encephalomyelitis, or ADEM, which involves both the brain and spinal cord.
Genetic Disorders of the Myelin Sheath
In certain inherited diseases, the myelin sheath does not develop properly, or it becomes damaged with time. One of the best-known of these disorders is X-linked adrenoleukodystrophy, or X-ALD, which more severely affects males. In X-ALD, an abnormal fatty substance accumulates in the brain, causing damage to the myelin sheath. Symptoms include progressive loss of thinking skills, muscle weakness and seizures. The disease also affects the adrenal gland, which regulates blood pressure and the body's ability to respond appropriately to stress.
Other Disorders of the Myelin Sheath
Progressive multifocal leukoencephalopathy, or PML, is an often fatal demyelinating disease of the brain caused by the John Cunningham, or JC, virus. PML occurs in people with severely weakened immune systems, including those with HIV. It has also been seen in people with autoimmune diseases -- like MS or psoriasis -- who take a type of medication known as a monoclonal antibody.
Central pontine myelinolysis, or CPM, involves damage to the myelin sheath in the brainstem. It usually occurs due to rapid correction of low sodium levels. In its most severe form, CPM can lead to the locked-in syndrome, which causes paralysis of all muscles of the body except the eyes.