myelinaation

Myelination is the biological process by which axons acquire a myelin sheath, a lipid-rich membrane that insulates the axon and increases the speed and efficiency of electrical signaling. The process is carried out by glial cells and is essential for rapid nerve conduction and reliable neural communication.

In the central nervous system (CNS), oligodendrocytes produce myelin by extending processes that wrap around several nearby axons, forming multiple myelin internodes. In the peripheral nervous system (PNS), Schwann cells wrap around a single axon to create the myelin sheath. Myelin reduces membrane capacitance and increases electrical resistance, enabling saltatory conduction where action potentials jump between nodes of Ranvier.

The myelin sheath consists of compact lipid-rich membrane layers with critical myelin proteins. In the CNS, prominent components include myelin basic protein (MBP) and proteolipid protein (PLP); in the PNS, proteins such as myelin protein zero (P0) and PMP22 are important. Nodes of Ranvier are gaps in the myelin where voltage-gated channels concentrate, allowing rapid depolarization. The g-ratio, the ratio of the inner axonal diameter to the total fiber diameter, is typically around 0.6–0.7, reflecting an optimal balance between thickness and conduction speed.

Developmentally, CNS myelination begins in early life and continues into adolescence, while PNS myelination proceeds postnatally. Myelination is influenced by neuronal activity and growth factors, and it plays a role in metabolic support for axons beyond insulation.

Clinical relevance includes demyelinating diseases such as multiple sclerosis (CNS) and Guillain-Barré syndrome (PNS), as well as leukodystrophies. Remyelination can occur after injury, though this process may be incomplete with age or disease.