Myelins

Myelins are the insulating layers that surround many axons in the vertebrate nervous system. In the central nervous system (CNS), these sheaths are formed by oligodendrocytes; in the peripheral nervous system (PNS), they are produced by Schwann cells. The myelin sheath is a lipid-rich multilamellar membrane that wraps around axons, creating a compact, electrically insulating cover.

Functionally, myelins increase the speed of electrical signal transmission along axons through saltatory conduction, allowing impulses to jump from one node of Ranvier to the next. This insulation also reduces the cellular energy required to restore ion gradients after each action potential, by limiting membrane area that must be repolarized.

Structure and components include distinct internodal segments separated by nodes of Ranvier. The sheath’s lipid-rich composition features cholesterol and various phospholipids, along with specialized proteins such as myelin basic protein and proteolipid proteins that stabilize the dense, compact structure.

Development and maintenance involve progressive myelination after birth, continuing into adolescence in humans. Myelination timing varies by region and neuron type. In the CNS, remyelination after injury or disease is primarily mediated by oligodendrocyte precursor cells, though regenerative capacity is limited. In the PNS, Schwann cells can more readily re-form myelin after damage.

Clinical relevance centers on demyelination, which disrupts conduction and produces neurological deficits. Diseases include multiple sclerosis and other leukodystrophies affecting the CNS, and Guillain-Barré syndrome or other demyelinating conditions affecting the PNS. Treatments focus on disease management and, where possible, promoting remyelination and functional recovery.