Myelin is a lipid-rich material that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses pass along the axon. The myelinated axon can be likened to an electrical wire with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, myelin ensheaths the axon segmentally: in general, each axon is encased in multiple long sheaths with short gaps between, called nodes of Ranvier. At the nodes of Ranvier, which are approximately one thousandth of a mm in length, the axon's membrane is bare of myelin.
Transmission electron micrograph of a cross-section of a myelinated PNS axon, generated at the Electron Microscopy Facility at Trinity College, Hartford, Connecticut
Myelin formed by Schwann cells in the PNS
A nerve is an enclosed, cable-like bundle of nerve fibers in the peripheral nervous system.
Cross-section of a nerve
Micrograph demonstrating perineural invasion of prostate cancer. H&E stain.