Myelization

Myelization, or myelination, is the developmental process by which axons acquire a myelin sheath, a multilayered, lipid-rich membrane that insulates the axon. In the central nervous system, oligodendrocytes extend processes to multiple surrounding axons and wrap their membranes around them to form internodes; in the peripheral nervous system, Schwann cells myelinate single axons. Myelin is composed largely of lipids with specialized proteins such as myelin basic protein and proteolipid protein in the CNS, and peripheral myelin protein 22 and P0 in the PNS. The myelin sheath increases the axonal membrane resistance and decreases capacitance, enabling saltatory conduction where action potentials hop between nodes of Ranvier. This markedly increases conduction velocity and allows precise temporal coordination of neural signals.

Developmentally, myelination begins in late prenatal life in humans and continues well into adolescence, with sensory and motor pathways typically myelinating earlier than higher-order association tracts. Oligodendrocyte progenitor cells differentiate into mature oligodendrocytes that wrap many times around axons to form internodes; Schwann cells wrap around a single axon in the peripheral nerve. The thickness of myelin around an axon is often described by the g-ratio, the ratio of the inner axon diameter to the outer fiber diameter; optimal values are about 0.6–0.7.

Myelination can be influenced by neural activity, hormonal signals, and metabolic factors. Remyelination can occur after injury, but efficiency declines with age. Disruption of myelination underlies several neurological diseases, most notably multiple sclerosis and other demyelinating disorders, as well as certain leukodystrophies; peripheral demyelinating conditions include Guillain–Barré syndrome. Diagnosis relies on clinical assessment, imaging, and sometimes nerve conduction studies, while therapies focus on reducing inflammation and promoting repair.