The following is a simple explanation of multiple sclerosis, a nervous system disorder.
Insulation covering the nerve is called the myelin sheath.
This myelin sheath is destroyed, or attacked, in MS people. Thus the nerve is bared and the messages do not get through properly. Another example of this is much like an electrical cord, covered by insulation. If you strip the insulation away, the electrical current goes astray. So it is with our nerves.