By Dr Ananya Mandal, MD
Myelin is a lipid and protein sheath-like material that forms an insulating cover that surrounds and protects nerve fibres. Myelin is vital to the normal functioning of the nervous system.
The nerve cell is made up of a cell body and a long, projecting nerve fibre called the axon that is responsible for the transmission of electrical impulses form the cell body to receiving neurons, glands and muscles.
Myelin is considered a defining feature of the vertebrates or animals with a spinal column, although similar sheaths have also evolved in some invertebrates. The myelin sheath of nerve fibres was first discovered and described by Rudolf Virchow in 1854.
Myelin is formed from supporting glial cells called the oligodendrocytes in the central nervous system and from Schwann cells in the peripheral nervous system.
The myelin is composed of about 40% water and the dry mass is composed of about 80% lipids and 20% protein. The mainly lipid composition of the myelin gives it a white hue, hence the reference to the brain’s “white matter.” The primary lipid of myelin is a glycolipid called galactocerebroside. Other major myelin constituents include myelin basic protein (MBP), proteolipid protein (PLP) and myelin oligodendrocyte glycoprotein (MOG). Within the myelin there are cross linked hydrocarbon chains composed of sphingomyelin which strengthens the myelin sheath.
The myelin sheath acts as an insulating substance that protects the nerves. As the nerve carries electrical impulses from one end to the other, the myelin helps to stop the impulse from leaving the axon and increases electrical resistance, therefore enhancing signal conduction.
Reviewed by Sally Robertson, BSc