Properties of motor units in nerve-intact autografts of cat extensor digitorum longus muscles

In cats, isometric contractile properties were measured on five extensor digitorum longus (EDL) muscles and four EDL muscle grafts 150-270 days after autografting with nerves intact. Comparisons were made between the properties of whole muscles and grafts and between 36 motor units in control EDL muscles and 41 motor units in grafts. The time-to-peak twitch force (TPT) of 23 ± 1.7 (SE) ms for grafts was significantly prolonged compared with the value of 17 ± 0.7 ms observed for whole EDL muscles. The mean values for the TPT of motor units were not different from the respective values for whole grafts or for whole muscles. The maximum specific force of whole grafts of 19.7 ± 0.6 (SE) N/cm² was significantly less than the control value of 23.6 ± 0.6 N/cm², an observation consistent with all previous data on the maximum specific force of grafts and control muscles. Based on the presence or absence of sag and an index of fatigue, motor units were classified as fast fatigable (FF), fast intermediate (FI), fast fatigue-resistant (FR), and slow (S). Motor units were classified 33% FF, 22% FI, 27% FR, and 17% S in control muscles and 17% FF, 43% FI, 29% FR, and 12% S in autografted muscles. Compared with control muscles, the number of small FF units increased significantly in the autografts, but no significant difference was observed in the fatigue properties of motor units. The lack of any difference in the fatigue properties of motor units in grafts compared with those in control muscles was unexpected, considering the more rapid fatigability reported for whole grafts compared with whole muscles. There was no evidence of a decrease in the number of motor units in the grafted compared with the control muscles. Overall, the degree to which control values for motor-unit properties were regained was remarkable, considering the complete degeneration and regeneration of essentially all of the fibers in the nerve-intact autografts and the necessity for reinnervation of fibers within motor units.