Myelin serves a very important purpose in the mammalian central nervous system. It is a membrane made from proteins and lipids. In addition to this, myelin is primarily located on axons of neurones (myelin sheath) and in the form of a layer.
Diseases associated with myelin breakdown
And many other demyelinating diseases, where the body's own immune system attacks the myelin forming the myelin sheath and so nerves become un-insulated. Un-insulated nerves transmit action potentials very slowly, due to the lack of saltatory conduction.