Myelin

Myelin is a substance that forms the coating of axons in the brain and the rest of the central nervous system. This coating is also known as the myelin sheath. It is a substance made of about 80% lipid and 20% protein. Its main function is to speed the relay of electricity messages in the nervous system. It is milky white and slippery in appearance and texture, giving rise to the term "white matter" in the brain.
Myelin is an important part of proper neural function. In later life, a process called demyelination can occur, causing poor neural function. Demyelination may be a cause of Alzheimer's disease. Multiple sclerosis happens when myelin is damaged.