PROPERTIES OF MOTOR UNITS IN A HETEROGENEOUS PALE MUSCLE (M. GASTROCNEMIUS) OF THE CAT

Abstract

The properties of motor units in M. gastrocnemius of the cat were investigated by isolating and stimulating filaments of ventral roots containing only 1 axon of that muscle. The conduction velocities of the axons ranged from 57.9-108.0 m/sec. The isometric tensions ranged from 0.5-120.0 g with a mean of 35.5 g. 33% of all units fell into the 0-10 g group; there were decreasing numbers of units in each higher decade. Contraction-times ranged from 17.8-129 msec, with a peak between 30-40 msec. In general, large units contracted rapidly, small units slowly. The tensions developed at 5, 10, 20 and 50/sec. were compiled and represented graphically. A relationship between the maximum tetanic tension of a unit and the conduction velocity of its axon was found: slowly conducting fibers supplied small units, rapidly conducting fibers large units. The data on soleus and gastrocnemius were analyzed in an attempt to identify 3 types of motor units corresponding to the 3 kinds of muscle fibers present in these muscles.