Myelin is an insulating layer 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 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.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.