Differential blocking of motor fibers by direct current

Differential blocking of alpha motor fibers was investigated in single fiber recordings when slowly rising direct current was applied to the gastrocnemius nerve in the cat. 18 out of the 26 (69.2%) L7 ventral root filaments, each of which contained 2-5 single alpha, or gamma motor fibers, or both, showed consecutive blocking from thicker to thinner fibers with increase of polarizing current. In the remaining 8 filaments (30.8%) thicker and thinner fibers were blocked almost simultaneously, or the order was reversed. The relation between the strength of the blocking current and conduction velocity is summarized on 30 alpha and 41 gamma fibers. The blocking current of each single fibers was shown to be inversely proportional to 2.01 power of conduction velocity (correlation coefficient -0.71). A regression line was also drawn for alpha fibers alone, its correlation coefficient being -0.60. Concerning prolongation of latencies of 82 single fibers (39 alpha and 43 gamma), it was shown that the conduction velocity, the smaller was the prolongation of latencies. In no experiments did the prolongation of latencies of single alpha fibers exceed the latency range of the mass alpha volley. Moreover, single alpha fibers were blocked earlier than or almost simultaneously with the mass alpha volley.