The myelin sheath is a fatty insulating layer that wraps around the axons of many neurons. It greatly increases the speed at which electrical impulses travel.

In the central nervous system, oligodendrocytes produce myelin; in the peripheral nervous system, Schwann cells perform the same function. The sheath is interrupted at regular intervals by nodes of Ranvier, where the axonal membrane is exposed. Action potentials jump from node to node in a process called saltatory conduction, which is both faster and more energy-efficient than continuous propagation along an unmyelinated axon.

Loss of myelin, as in multiple sclerosis, slows or blocks conduction and produces a wide range of neurological symptoms. Myelination continues through childhood and adolescence, particularly in pathways that support higher cognitive functions. The presence or absence of myelin is one of the factors that determine how rapidly a given neural circuit can operate.

When a signal races along an axon at high speed, the myelin sheath is what makes the speed possible.