Myeliini

Myeliini, or myelin, is a lipid-rich insulating sheath that surrounds many 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. Each oligodendrocyte extends processes to several nearby axons, creating myelin segments along them, while a single Schwann cell wraps around and myelinates a single axon segment.

Myeliini is composed predominantly of lipids, with proteins embedded in or associated with the membrane. The high lipid content forms a compact, layered sheath that minimizes current leakage and supports rapid signal transmission. In the CNS, key myelin proteins include myelin basic protein and proteolipid protein; in the PNS, major components include myelin protein zero and peripheral myelin protein 22. The sheath contains nonmyelinated gaps known as nodes of Ranvier, which host specialized ion channels that facilitate saltatory conduction.

The primary function of myeliini is to increase the speed and efficiency of electrical impulse propagation along axons. By insulating the axon and reducing membrane capacitance, myelin enables saltatory conduction, where impulses jump between nodes, markedly increasing conduction velocity. Myelin also contributes to axonal integrity and stable signaling over time.

Development and maintenance of myeliini involve tightly regulated glial cell activity. Myelination begins in late prenatal development and progresses into adolescence in humans, depending on brain region. In the PNS, Schwann cells can promote regeneration after injury by guiding regrowth, whereas CNS myelin shows more limited spontaneous repair.

Demyelination disrupts signal transmission and is a hallmark of diseases such as multiple sclerosis (CNS) and Guillain-Barré syndrome (PNS), among others, leading to sensory, motor, and cognitive impairments.