The aim of this study was to describe the control of tension by rate modulation of single motor units in reinnervated muscle. Single fast-twitch motor units were isolated from medial gastrocnemius (MG) muscles in two groups of anesthetized adult cats: one in which the MG nerve was left untreated and another in which that nerve was sectioned and immediately sutured together 10-33 mo before study. Together with conventional measures of isometric contractile properties, units were tested with the use of computer-controlled feedback regulation of stimulation rate to maintain tension during continuous isometric contraction at a constant submaximal level [25% of maximal tension (P(max))]. For motor units from both groups, stimulation rate began to decline after target tension was attained and then settled at lower values for variable durations before rapidly increasing, usually within the last 5% of the contraction's duration, until reaching the experimentally selected limit of 100 pulses/s (pps). Measures of the declining phase in stimulation rate occurring at the beginning of sustained contraction were not significantly different in comparison of untreated versus reinnervated muscles. These measures included 1) the magnitude of the decrease in rate, 2) the minimum rate attained, and 3) the time taken to reach minimum stimulation rate expressed as a fraction of endurance time (E(t), total duration of the sustained contraction). Most fast-twitch units from reinnervated muscles fell within normal limits for both endurance time and the number of stimuli applied over that period. Mean stimulation rate (E(t)/number of stimuli) for the untreated and reinnervated groups was slightly but significantly different [18 pps ± 1 (SE) and 21 pps ± 1, respectively; P < 0.05]. In confirmation of our earlier report, control data demonstrated a significant negative association (Spearman correlation coefficient, r(s) = -0.77; P < 0.001) between P(max) and E(t). That relation was not as clear among fast-twitch units pooled from all reinnervated muscles, although a significant correlation (r(s) = -0.35; P < 0.005) resulted from the tendency for units with low E(t) to have low P(max). Correlation coefficients calculated per muscle were not significant (P > 0.05) for any of the five reinnervated muscles (r(s) range from 0.15 to -0.46) but were significant (P < 0.05) for each untreated muscle (lowest r(s) = -0.73). The sample of units from reinnervated muscles was readily distinguished from control by the presence of units with relatively low values for both P(max) and E(t). After nerve section and muscle reinnervation, recovery of the complex temporal patterns in stimulation rate normally used to maintain tension indicated that activity-dependent aspects of motor-unit force production were reestablished in self-reinnervated muscle. The functional consequence of this recovery is that tension may be controlled through normal patterns of rate modulation if motoneurons are appropriately activated. In contrast, the normally strong relationship between tension and endurance time was not recovered, and this may alter the quality of contraction controlled by motor-unit recruitment.
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