Demyelination is the major underlying factor responsible for the symptoms of multiple sclerosis (MS). Demyelination is the destructive removal of myelin, an insulating and protective fatty protein which sheaths nerve cells (neurons). More specifically, the myelin is wrapped around the long extensions of neurons called axons. During MS relapses, patches of white matter in the central nervous system that normally contain tracts of myelinated neurons become inflamed and lose their myelin. These patches of demyelination are known as lesions.
The cause and precise mechanism of demyelination is not clearly understood but there is good evidence that the body's own immune system is at least partially responsible. Acquired immune system cells called T-cells are known to be present at the site of lesions. Other immune system cells called macrophages (and possibly mast cells as well) also contribute to the damage.
Myelin is produced by special "glial cells" in the central nervous system called oligodendrocytes. Oligodendrocytes and axons have a many to many relationship - that is one oligodendrocyte produces myelin for several axons and one axon has several oligodendrocytes producing its myelin. In MS, it is not just the myelin that is destroyed but also these oligodendrocytes and occasionally even the axons themselves.
Axons use an electrochemical mechanism to transmit nerve impulses - the action potential. This requires sodium and potassium ions to pass through a semi-permeable membrane around the nerve. It is believed that the myelin not only insulates and encases this electrochemical process but also actively assists it. When axons become demyelinated, they transmit the nerve impulses 10 times slower than normal myelinated ones.
During periods of MS remission, the oligodendrocytes repair the damaged axons in a process called remyelination. However, very often the oligodendrocytes are also destroyed which delays or prevents remyelination from happening. Additionally, another kind of glial cell, called astrocytes, cause scar tissue to form in place of the myelin. Scar tissue does not perform the same function as the myelin. As the disease reaches its more advanced phases, the axons themselves are often destroyed as well.
Multiple Sclerosis: Etiology