Myelin is an insulating layer, or sheath, that forms around nerves, including those in the brain and spinal cord. It is made up of protein and fatty substances.
The purpose of the myelin sheath is to allow electrical impulses to transmit quickly and efficiently along the nerve cells. If myelin is damaged, the impulses slow down. This can cause diseases such as multiple sclerosis.
Calabresi PA. Multiple sclerosis and demyelinating conditions of the central nervous system. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 419.