Characteristics of motor units in muscles of rats grafted with nerves intact.

Extensor digitorum longus (EDL) muscles of rats were grafted with the nerves intact. Sixty days after grafting, single motor units were isolated and characterized in terms of fatigability and morphology. The distribution of fatigue indexes for motor units in control muscles revealed two main peaks, whereas in grafts no clear peaks were observed, fatigue indexes being distributed more evenly over the entire range. This difference in the distribution of fatigue indexes of motor units may explain the increased resistance to fatigue observed for whole grafts compared with whole EDL muscles. An inverse linear relationship was found between maximum tetanic tensions and the fatigue indexes of motor units from control and grafted EDL muscles. The distribution of fiber areas for single motor units was broader in grafts than in control EDL muscles, but the mean innervation ratio was not different. In approximately 20% of the units mapped in grafts, fibers were clustered tightly within a small portion of the total cross section, suggesting axonal sprouting during regeneration.