myelinsegment

A myelin segment is a discrete portion of a myelinated axon that is wrapped by a single glial cell’s myelin sheath, extending between two successive nodes of Ranvier. In the central nervous system this sheath is produced by oligodendrocytes, while in the peripheral nervous system it is produced by Schwann cells. Each myelin segment is part of a continuous, compacted layer of membrane that insulates the axon and is separated from neighboring segments by gaps known as the nodes of Ranvier.

The myelin segment consists of compact myelin with relatively little cytoplasm, organized along the axon with specialized junctions at its margins called paranodal and juxtaparanodal regions. The length of a segment, and the distance to the adjacent node, varies with the diameter of the axon, the organism, and developmental stage. The overall arrangement creates alternating myelinated regions and nodes along the axon.

Functionally, myelin segments increase the speed and efficiency of electrical conduction. By insulating the axon, they reduce membrane capacitance and prevent current leakage; action potentials jump from node to node in a process called saltatory conduction. This allows rapid signal transmission with reduced energy expenditure for restoring ion gradients at the membrane.

Clinical relevance arises when myelination is disrupted. Demyelinating diseases, such as multiple sclerosis in the CNS and Guillain-Barré syndrome in the PNS, involve loss or damage of myelin segments. Remyelination can occur, particularly in the regenerating PNS, but remyelination in the CNS is often less complete.