Mielinici

Mielinici refer to specialized glial cells responsible for the formation of the myelin 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), by Schwann cells. An oligodendrocyte can extend its processes to several axons, wrapping a segment of each with compact myelin. A single Schwann cell myelinates a single internode on one axon, though a nerve fiber may have many internodes formed by many Schwann cells along its length.

Developmentally, myelination begins in late gestation and continues into adolescence in humans. Oligodendrocyte precursor cells originate in the developing CNS and migrate, while Schwann cells arise from neural crest cells. This leads to the formation of myelin sheaths along axons, increasing the speed and efficiency of electrical signaling.

Structure and composition: Myelin is a multilamellar, lipid-rich sheath. In the CNS it contains high levels of cholesterol and galactolipids and proteins such as myelin basic protein (MBP) and proteolipid protein (PLP). In the PNS, the myelin proteins include P0 (MPZ) and PMP22. The compact myelin is interspersed with non-compact regions that accommodate metabolic needs and cellular transport.

Function: Myelin increases conduction velocity by reducing membrane capacitance and increasing membrane resistance, enabling saltatory conduction between nodes of Ranvier. It also provides metabolic and trophic support to axons in some contexts.

Clinical relevance: Demyelination disrupts nerve conduction, as seen in diseases such as multiple sclerosis (CNS) and Guillain-Barré syndrome (PNS). Remyelination can occur, mediated by oligodendrocyte precursor cells in the CNS and Schwann cells in the PNS, though it may be incomplete in chronic disease.