Axons thicker than one micron in the CNS and peripheral nervous system (PNS) are myelinated. Myelin is a spiral sheet of cell membrane wrapped around the axon.
In the CNS, myelin is produced by oligodendroglial cells and in the PNS by Schwann cells. Each oligodendrocyte makes multiple segments of myelin that wrap around many axons.
Each Schwann cell makes one segment of myelin. This is one reason why peripheral myelin regenerates more efficiently. Nodes of Ranvier are points of discontinuity between adjacent myelin sheaths in which the axon is not covered by myelin. Unmyelinated axons are covered by Schwann cell cytoplasm, but there is no spiraling of Schwann cell membrane around them.
The structure of central and peripheral myelin is essentially the same. Myelin is composed of 70% lipids and 30% protein. There are some important differences in myelin proteins between CNS and PNS. These differences explain why an allergic reaction against PNS myelin does not cause central demyelination and vice versa; and why inherited metabolic disorders of myelin proteins that affect peripheral nerves do not damage central myelin. On the other hand, lipids are similar between PNS and CNS myelin. For this reason, metabolic disorders of myelin lipids, such as metachromatic leukodystrophy, affect both, the central white matter and peripheral nerves.
The myelin sheath acts as an electrical insulator, preventing short-circuiting between axons. More important, it facilitates conduction. The nodes of Ranvier are the only points where the axon is uncovered by myelin and ions can be exchanged between it and the extracellular fluid. Depolarization of the axonal membrane at the nodes of Ranvier boosts the action potential that is transmitted along the axon and is the basis of saltatory (jumping) conduction.
References
Yool DA, Edgar JM, Montague P, Malcolm S. The proteolipid protein gene and myelin disorders in man and animal models. Hum Mol Genet. 2000 Apr 12;9(6):987-92. PMID: 10767322