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Myelin

The insulation for axons in the nervous system is provided by glial cells, oligodendrocytes in the CNS, and Schwann cells in the PNS. Whereas the manner in which either cell is associated with the axon segment(s) that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.

Conduction velocity

Myelination improves the conduction velocity—the speed with which action potentials travel—in axons. Axon diameter also affects conduction velocity; fatter axons carry action potentials faster. So there are two factors that determine how fast an action potential travels down an axon: diameter and myelination (Suzuki, 2010). Axon diameter and myelination are correlated: the skinniest axons (< 1 micron in diameter) are also unmyelinated (often called C-fibers; a good example is found in the neurons that carry pain and temperature information from the skin to the spinal cord). They carry action potentials the most slowly, about 1 meter/second (walking speed). Axons fatter than 1 mm in diameter are generally myelinated. The fattest myelinated axons can carry action potentials faster than 100 m/s—race car speeds! Mechanical (pressure) sensors in our skins and motor neurons have fat, myelinated axons, so they can do their jobs rapidly.