myelinointi

Myelinointi, or myelination, is the biological process by which axons are insulated with a lipid-rich myelin sheath. The myelin increases the speed of electrical signal transmission along neurons and reduces the energy required to propagate impulses, supporting rapid and efficient neural communication.

In the central nervous system (CNS), myelination is produced by oligodendrocytes, which extend their processes to wrap several nearby axons with multiple layers of myelin. In the peripheral nervous system (PNS), Schwann cells myelinate individual axons. The myelin sheath is segmented into internodes, separated by gaps known as nodes of Ranvier, where voltage-gated ion channels are concentrated.

Conduction along a myelinated axon is saltatory: action potentials jump from node to node, greatly increasing conduction velocity. Myelin also reduces membrane capacitance and ion leakage, lowering the metabolic cost of signaling. The relative thickness of the myelin sheath to the axon diameter is expressed by the g-ratio, typically around 0.6–0.7 for optimal conduction.

Developmentally, myelination begins late in prenatal life and continues into young adulthood in many neural pathways. Neuronal activity, growth factors, and hormones influence myelination. In the CNS, oligodendrocyte precursor cells differentiate into mature oligodendrocytes; in the PNS, Schwann cells differentiate and wrap axons accordingly.

Damage to myelin disrupts neural conduction and can cause neurological symptoms. Demyelinating diseases include multiple sclerosis in the CNS and Guillain-Barré syndrome in the PNS, among others. Remyelination can occur in some contexts, but repair may be incomplete with aging or disease.