A myelin sheath is a protective layer that develops over nerves in the central nervous and peripheral nervous systems. Myelin sheaths are composed of fatty tissues and proteins. In the central nervous system, these sheaths are produced by oligodendrocytes, which can lay a myelin sheath over a number of axons. In the peripheral nervous system, myelin sheaths are produced by Schwann cells, which place a myelin sheath over a single axon.
In addition to protecting nerves, the myelin sheath helps to ensure signal transmission in the nerves.
In nerves that have healthy myelin sheaths, nerve signals can travel up to 60 meters a second.
Damage to the myelin sheaths (demyelination) causes signal transmission in the nerves to slow and is thought to contribute to degenerative conditions such as Multiple Sclerosis.
Some nerves do not have myelin sheaths, including olfactory nerves and some nerves that register sense data.
Research is underway that is attempting to discover ways to regenerate myelin sheaths as a treatment for Multiple Sclerosis and other demyelinating diseases.