|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.
In demyelination, the optic nerve function is impaired. There is a decreased
conduction of the nerve impulses resulting in decreased vision, colour
vision, relative afferent pupillary defect and central scotoma.