Low Threshold Associated With Slow Conduction Velocity: Studies of Human Motor Axons

Introduction TEXTBOOKS of physiology and monographs on axonology state, explicitly or implicitly, that nerve fibers of large diameter (and rapid conduction velocity) are more excitable than smaller ones. This statement is not open to question when applied, for example, to threshold differences between A and C fibers, or even between A-α and Aγ axons. The concept of size-determination of excitability appears to have become so generalized, however, that if one fiber is larger than another it is assumed ipso facto to have a lower threshold to electrical stimulation. In this paper we shall show that this concept is not true in the case of a small bundle of motor axons of homogeneous function, in which the constituent fibers are fairly similar in respect to conduction velocity. We have found thresholds for skeletomotor fibers of the human upper and lower extremities to be lower for fibers of slow conduction velocity