Many axons are wrapped in a multilayered sheath of lipid-rich membrane called myelin. The sheath is produced by oligodendrocytes in the central nervous system and by Schwann cells in the periphery. It greatly increases the speed of action-potential conduction by allowing the impulse to jump between exposed nodes of Ranvier.

Myelin also reduces the metabolic cost of signaling and provides trophic support to the axon. Its formation is a major developmental event; myelination of human association tracts continues into early adulthood. Diseases that destroy myelin, such as multiple sclerosis, slow or block conduction and produce a wide range of neurological symptoms.

Remyelination can occur spontaneously or be enhanced by emerging therapies, although it is often incomplete. The precise matching of myelin thickness to axon diameter is regulated by activity and by signaling between axon and glia. Imaging methods that quantify myelin are increasingly used as biomarkers in both research and clinical trials.

• Fatty insulating sheath around axons
• Speeds conduction via saltatory propagation
• Produced by oligodendrocytes and Schwann cells
• Target of demyelinating diseases