mielinréteg

Mielinréteg, also known as the myelin sheath, is a lipid-rich, multilayered covering that wraps around many axons in the vertebrate nervous system. It is produced by glial cells: oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). The sheath is segmented by gaps called nodes of Ranvier, where the axonal membrane is exposed, enabling rapid saltatory conduction of electrical impulses.

The myelin sheath acts as an insulator, dramatically increasing the velocity of action potential propagation along the axon. By wrapping the axon, it reduces membrane capacitance and minimizes ion leakage, making nerve signaling more energy-efficient. Myelin is rich in lipids and contains specialized proteins that stabilize the sheath and promote rapid signal transmission. In the CNS, major myelin proteins include MBP and PLP; in the PNS, P0 and PMP22 play prominent roles.

Development and maintenance: Myelination begins during development and continues into adolescence for many neural pathways. Oligodendrocytes can myelinate multiple axons, forming several internodes, while Schwann cells myelinate a single segment of one axon. If myelin is damaged, conduction can slow or fail. CNS remyelination can occur but is often incomplete, whereas PNS remyelination tends to be more robust.

Clinical significance: Demyelinating diseases disrupt the myelin sheath and impair neural signaling. Multiple sclerosis involves autoimmune destruction of CNS myelin, leading to neurological deficits. In the PNS, conditions such as Guillain-Barré syndrome cause rapid demyelination, resulting in weakness and sensory disturbances. Research into myelin biology informs strategies to protect, repair, or replace myelin in various disorders.