For some reason, in some people, this myelin degenerates. When it degenerates it leaves a hardened area. If one could take out the nervous system and look at it, the demyeliminated areas look scarred, sclerotic. Thus the process has been named sclerosis. Because it occurs in spotty areas, it is dubbed multiple sclerosis.
People have more myelin than is necessary.
In much the same manner that one can lose a kidney and get along some myelin can be lost without creating a problem. Depending on how much myelin is lost and where it is lost, one gets symptoms. If myelin is lost in the brain or connections carrying sensitive, numbness results. If is lost in the brain or connections carrying coordination, klutsyness results. If the myelin is lost in the connections of strength, weakness results.