ON THE FORCE AND SIZE OF MOTOR UNITS IN THE RABBIT'S SARTORIUS MUSCLE

Abstract

In 7 sartorius muscles of the rabbit, the total number of muscle fibers was detd. The values found were between 5216 and 7150 fibers; the mean was 6282 fibers. The mean force developed by the heterolateral sartorius muscles was 133 g. In 5 expts., the nerve fibers in the motor nerve of the m. sartorius were counted. Figures between 70 and 117 were detd. with a mean of 95 fibers. Assuming that 1/3 to 1/2 of the fibers in the motor nerve are sensory, the mean sartorius motor unit can be estimated to consist of 100 to 125 muscle fibers, producing a tension of 2.2 to 2.7 g. during faradic stimulation. In the histograms of sartorius motor nerves which had been deafferented by excision of the appropriate spinal ganglia, and which could be expected to be free of sensory and regenerated fibers, all nerve fibers were more than 5[mu] in diam. Two groups of fibers were present, one with a maximum around 7[mu], the other with a maximum around 14[mu]. Regenerated fibers originating from an anterior root damaged during removal of the ganglia may be present in the sartorius motor nerve within 5 wks. Later (usually after 70 to 75 days) regenerated fibers which are probably mostly sensory may grow into this nerve from other sources. Complete deafferenting, without damage to the anterior roots, and absence of regenerated fibers in the motor nerve are conditions for the detn. of force and size of the motor unit. These conditions are so exacting that they have in the present expts. never been met. In most expts. the motor unit produced considerably more tension (up to 8.8 g.) during faradic stimulation, than the value for the normal unit estimated above (2.2-2.7 g.). This is probably due to fiber adoption of the intact units in muscles partially dener-vated during the removal of the ganglia.