müeliini

Müeliini, or myelin, is a lipid-rich insulating sheath that envelops axons in the vertebrate nervous system. In the central nervous system (CNS), myelin is produced by oligodendrocytes; in the peripheral nervous system (PNS), it is produced by Schwann cells. The sheath wraps around axons to form compact myelin, with short gaps known as nodes of Ranvier that facilitate rapid signal transmission.

The myelin membrane is highly enriched in lipids, comprising roughly 70–85% lipids and 15–30% proteins. Major CNS myelin proteins include myelin basic protein (MBP) and proteolipid protein (PLP); major PNS proteins include myelin protein zero (P0) and peripheral myelin protein 22 (PMP22). Structurally, myelin consists of compact layers formed by alternating membranes, with cytoplasmic leaflets creating cytoplasmic multicompartments and intraperiod lines. The presence of regions without cytoplasm permits tight insulation and efficient saltatory conduction.

Functionally, myelin increases conduction velocity and reduces energy expenditure by limiting the area of membrane that must be depolarized. Conduction efficiency scales with axon diameter, and the g-ratio (axon diameter to total fiber diameter) is typically around 0.6–0.7 for optimal performance.

Development and disease: myelination begins in fetal development and continues after birth, with remyelination possible after injury, mainly by oligodendrocyte precursor cells in the CNS and by Schwann cells in the PNS. Demyelinating diseases slow or block nerve signals and can cause sensory and motor deficits. Examples include multiple sclerosis in the CNS and Guillain-Barré syndrome in the PNS. Genetic leukodystrophies such as Pelizaeus–Merzbacher disease, Krabbe disease, and metachromatic leukodystrophy affect myelin formation or maintenance and lead to progressive neurological impairment.

Research continues to explore promoting remyelination and neural repair as therapeutic strategies.