myelinskida

Myelinskida, or myelin sheath, is a multilayered, lipid-rich coating that envelops many axons in the vertebrate nervous system. In the central nervous system (CNS), myelin is produced by oligodendrocytes; in the peripheral nervous system (PNS), by Schwann cells.

Composition and structure: myelin is rich in lipids, with cholesterol and sphingolipids forming the compact layers, and proteins such as myelin basic protein, proteolipid protein, and myelin-associated glycoprotein that help stabilize and organize the sheath. The arrangement creates insulating layers around the axon, interrupted at nodes of Ranvier where the axon membrane is exposed.

Function: the myelin sheath electrically insulates axons and enables saltatory conduction, dramatically increasing conduction velocity while reducing metabolic energy expenditure. This arrangement allows rapid and efficient transmission of nerve impulses over long distances.

Development and anatomy: myelination begins during fetal development and continues after birth, with CNS myelination extending into early adulthood. The g-ratio, the ratio of the inner axon diameter to the total fiber diameter, is typically about 0.6–0.7, reflecting an optimal balance between speed and axon size. In the CNS, a single oligodendrocyte can myelinate multiple axons, whereas in the PNS, a Schwann cell myelinates a single axon segment.

Remyelination and clinical relevance: remyelination can occur after injury, more effectively in the PNS than in the CNS. Demyelinating diseases, such as multiple sclerosis (CNS) and Guillain-Barré syndrome (PNS), disrupt myelin, slowing or blocking nerve conduction and producing various neurological symptoms.