Motor unit function in skeletal muscle autografts of rats

Abstract

Standard and nerve-intact grafts of the extensor digitorum longus (EDL) muscles of rats do not fully recover control values for maximum tetanic tension (P o). We compared the physiologic properties of motor units from standard and nerve-intact EDL grafts of rats with those of control EDL muscles. Standard grafts were completely removed and replaced in their original site. For nerve-intact grafts all physical connections were severed except for the nerve. Isometric contractile properties of whole muscles and single motor units were measured in situ 75 and 50 days after surgery for standard and nerve-intact grafts, respectively. Motor units from both types of grafts showed a mean and distribution for time-to-peak twitch tension (TPT) not different from control motor units. The absolute P o of nerve-intact grafts showed a greater recovery than the standard grafts, but was still significantly lower than the control value. The average decrease in the P o of motor units from nerve-intact grafts of 16% was not different from the decrease in the P o of the total EDL graft which suggests no loss of motor units in nerve-intact grafts. In standard grafts, the 57% decrease in P o for the whole muscle was attributable to a 20% decrease in the average tension development per motor unit and a 45% decrease in the number of motor units. These differences suggest a more complete reinnervation of the nerve-intact grafts than standard grafts.