Further evidence of incomplete neural control of muscle properties in cat tibialis anterior motor units

To examine the influence of a moteneuron in maintaining the phenotype of the muscle fibers it innervates, myosin heavy chain (MHC) expression, succinate dehydrogenase (SDH) activity, and cross-sectional area (CSA) of a sample of fibers belonging to a motor unit were studied in the cat tibialis anterior 6 mo after the nerve branches innervating the anterior compartment were cut and sutured near the point of entry into the muscle. The mean, range, and coefficient of variation for the SDH activity and the CSA for both motor unit and non-motor unit fibers for each MHC profile and from each control and each self-reinnervated muscle studied was obtained. Eight motor units were isolated from self-reinnervated muscles using standard ventral root filament teasing techniques, tested physiologically, and compared with four motor units from control muscles. Motor units from self-reinnervated muscles could be classified into the same physiological types as those found in control tibialis anterior muscles. The muscle fibers belonging to a unit were depleted of glycogen via repetitive stimulation and identified in periodic acid-Schiff-stained frozen sections. Whereas muscle fibers in control units expressed similar MHCs, each motor unit from self-reinnervated muscles contained a mixture of fiber types. In each motor unit, however, there was a predominance of fibers with the same MHC profile. The relative differences in the mean SDH activities found among fibers of different MHC profiles within a unit after self-reinnervation and those found among fibers in control muscles were similar, i.e., fast-2 < fast-1 ≤ slow MHC fibers. Furthermore, the ranges in fiber CSA and SDH activity within a motor unit having a mixture of fiber types were similar to those found among a sample of nondepleted fibers that presumably represented all fiber types within the muscle. These results demonstrate that the normal interrelationships among the MHC isoform, the SDH activity, and the CSA within a fiber were maintained or reestablished in reinnervated units, but all fibers within each of the units did not convert and become a homogeneous population of fibers. Thus the SDH activity and CSA of muscle fibers are shown to be more associated with the MHC phenotype of the fiber than with the motor unit type.