Glossary
myelin sheath
The myelin sheath is fatty insulation wrapped around some axons that speeds the impulse by letting it jump between the nodes of Ranvier.
The myelin sheath is a wrapping of fatty material around an axon, and its job is speed. Myelin is laid down in segments by Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system, and between one segment and the next there is a bare gap called a node of Ranvier. Because myelin insulates the membrane, the impulse cannot travel through it; instead it is regenerated at each node, so it appears to jump from gap to gap rather than crawl along the whole axon. The same fiber therefore carries a signal many times faster than an unmyelinated one of the same diameter, which is how a large body wires long distances without needing enormous nerves. The impulse does not weaken along the way, because each node rebuilds it at full size. Loss of myelin is the mechanism in demyelinating disease: the jumps fail, and the signal slows or stops.
Taught in
Related terms
- neuron — A neuron is a nerve cell that receives, carries and passes on information as an electrical impulse, and its working parts are the dendrites, the cell body and the axon.
- reflex arc — A reflex arc is the pathway a reflex travels — receptor, sensory neuron, a synapse in the CNS, motor neuron and effector — without waiting for the brain.
- action potential — An action potential is the all-or-nothing reversal of a neuron's membrane voltage that carries a signal along its axon.
Source: The anatomy and physiology guide in this course, whose organ and tissue descriptions follow the US National Cancer Institute's SEER Training Modules and MedlinePlus consumer health pages. Both are US federal government material, not subject to copyright in the United States. The definition above is written for this course in our own words. (SEER Training Modules accessed 2026-10-26; definition written 2026-10-27)