Selective Stimulation of Peripheral Axons

Abstract

Electrical nerve stimulation may be a highly useful tool for the in vivo study of skeletal muscle contractile properties in experimental animals. Unfortunately, currently available stimulation techniques fail to provide contraction under conditions which are similar to the known recruitment properties of various muscles. A frequently used technique delivers to the nerve brief rectangular pulses of suprathreshold amplitude while varying the pulse frequency (rate coding). In essence, all the motor units are always active, which is a significant deviation from the “size principle” mode (Henneman, Somjen, and Carpenter, 1965) under which units are recruited according to their size, small ones first and then progressively larger ones. Attempts to elicit a more favorable condition by increasing the stimulus pulse amplitude from subthreshold to suprathreshold are also deficient, since the larger axons have lower excitation thresholds (Blair and Erlanger, 1933) and are always activated first in a “reverse recruitment” mode. Such an approach yields large initial force increments and results in fast setting fatigue which is characteristic of large motor units.