THE PROPERTIES OF MAMMALIAN B FIBERS

Abstract

The mammalian preganglionic autonomic fibers which contribute the B elevation of the action potential differ in their properties so markedly from the A and the C fibers as to justify the designation of the B group as a distinct entity. The spike duration (1.2 msec), the size and the duration of the positive after-potential (1.5 to 4% of the spike height, duration 100 to 300 msec), the absence of negative after-potential in single responses, and the form of the recovery curve after excitation are definitive for the group. There are some points of resemblance to A fibers. The period of latent addition (0.2 msec.) is the same. The sensitivity to pH changes resembles that of A fibers. With respect to the refractory period (1.1 to 1.5 msec.) and the sensitivity to asphyxia, the B fibers come into line with the small A fibers. The basic pattern of the after-potential changes exhibited by B fibers is similar to that found in A and C fibers. Following a prolonged tetanus, the after-potentials have the same form for all fibers, but this characteristic configuration appears on different time scales, depending upon the group of fibers in question.