myelinate

Myelination is the process by which axons are wrapped with myelin sheaths, insulating their membranes and allowing faster electrical signaling. In the central nervous system (CNS), oligodendrocytes extend processes to wrap multiple nearby axons; in the peripheral nervous system (PNS), Schwann cells wrap myelin around single axons. The resulting sheath is a compact, lipid-rich layer with non‑myelinated cytoplasmic loops at the paranodes. Myelin increases conduction velocity primarily through saltatory conduction, reduces membrane capacitance, and raises membrane resistance, enabling rapid, energy-efficient impulse transmission along long axons.

Myelin is rich in lipids, including cholesterol and sphingolipids, and in specific proteins. CNS myelin contains myelin basic protein and proteolipid protein, while PNS myelin includes peripheral myelin protein 22. The myelin sheath is periodically interrupted by nodes of Ranvier, gaps where voltage-gated sodium channels cluster to regenerate action potentials and maintain rapid signaling.

Developmentally, myelination follows region- and age-dependent schedules. In humans, CNS myelination begins in the late fetal period and continues into adolescence for many tracts; PNS myelination commonly proceeds after birth. The thickness of myelin is matched to axon diameter, often described by the g‑ratio, roughly 0.6 to 0.7.

Remyelination can occur after injury. In the CNS, oligodendrocyte precursor cells contribute to repair, though remyelination may be incomplete; in the PNS, Schwann cells more readily drive remyelination. Demyelinating diseases, such as multiple sclerosis, involve loss of myelin and disrupted conduction, highlighting the importance of myelination for normal neural function.