APA Dictionary of Psychology

the classification of peripheral nerve fibers (axons) according to their diameters, speeds of conduction, and locations. They fall into three main classes. A fibers vary from 1 to 20 μm in diameter; they are myelinated fibers and conduct rapidly. B fibers occur in the preganglionic autonomic nervous system; they are myelinated but of relatively small diameter, and they conduct more slowly than A fibers. C fibers vary from 0.2 to 1.5 μm in diameter; they are unmyelinated and conduct slowly. See also velocity of conduction.