myelinated

Myelinated refers to nerve fibers whose axons are wrapped in a myelin sheath, a specialized insulating covering that increases the speed of electrical conduction. In the central nervous system, myelin is produced by oligodendrocytes; in the peripheral nervous system, it is produced by Schwann cells. The myelin sheath consists of layered membrane wrapped around the axon, interrupted at intervals by nodes of Ranvier, where the axonal membrane is exposed. This segmentation enables saltatory conduction, allowing action potentials to jump from node to node and dramatically increasing conduction velocity while reducing energy expenditure for ionic pumping. Conduction velocity grows with axon diameter and the thickness of the myelin sheath; larger, heavily myelinated fibers conduct most rapidly.

Myelination begins during development and continues into adolescence in many tracts. Some small-diameter or certain sensory fibers remain unmyelinated or thinly myelinated, which supports slower signaling.

Myelin composition differs between the CNS and PNS. CNS myelin is rich in lipids and proteins such as myelin basic protein and proteolipid protein, while peripheral myelin includes other proteins like P0 and PMP22. Demyelination, whether due to disease or injury, slows or blocks conduction and can cause neurological deficits. Diseases include multiple sclerosis in the CNS and Guillain-Barré syndrome in the PNS; in both, remyelination can occur but may be incomplete, contributing to persistent symptoms.