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Myelin sheath is a fatty insulating layer that surrounds nerve fibers, called axons, in the central and peripheral nervous systems. This sheath is formed by specialized cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. These cells wrap around the axon multiple times, creating a multi-layered coating rich in a lipid-protein substance called myelin.

The primary function of the myelin sheath is to accelerate the transmission of nerve impulses. It acts like the insulation on an electrical wire, preventing the leakage of electrical signals. Myelin is not continuous along the axon; instead, it is interrupted at regular intervals by gaps called nodes of Ranvier. This segmented structure allows for saltatory conduction, a process where the electrical impulse jumps from one node to the next, significantly increasing the speed of signal propagation compared to unmyelinated axons.

Myelination is a crucial developmental process that continues into adulthood. Damage or loss of the myelin sheath, a condition known as demyelination, can severely impair nerve function. Diseases like multiple sclerosis (MS) are characterized by demyelination in the central nervous system, leading to a wide range of neurological symptoms. Similarly, conditions affecting the peripheral nervous system's myelin can result in muscle weakness, numbness, and pain. The health and integrity of the myelin sheath are therefore vital for normal nervous system operation.