Recovery of rat tibialis anterior motor unit properties following partial denervation

The recovery of selected mechanical, morphological, and metabolic properties of rat tibialis anterior fast motor units was determined following partial denervation (n = 7) or partial denervation and hemispinal cord transection (n = 5) and compared with age-matched control units (n = 7). Following 1-12 months of recovery, the mechanical properties of each unit were measured and the fibres depleted of glycogen by using standard ventral root filament stimulation techniques. Quantitative histochemical techniques were used to determine cross-sectional area and the activities of succinate dehydrogenase and alpha-glycerolphosphate dehydrogenase in individual unit fibres. Partial denervation increased the mean fibre area but decreased alpha-glycerolphosphate dehydrogenase activity. Succinate dehydrogenase was unchanged in the denervated groups. The variability in area and enzymatic activities among the unit fibres was unchanged. However, the interrelationship between the enzymes was altered by both denervation procedures. Succinate dehydrogenase activity was directly related to fatigue resistance and inversely related to tetanic tension across the units. These findings suggest that a motor unit reestablishes many of its properties despite marked changes to the composition of the unit brought about by partial denervation. In addition, a reduction in the neuromuscular activity of units during reorganization had a limited effect on recovery.