AVERAGE BUT NOT CONTINUOUS SPEED MATCH BETWEEN MOTONEURONS AND MUSCLEUNITS OF RAT TIBIALIS ANTERIOR

1. Properties of single motoneuron/muscle-unit combinations were deter mined for tibialis anterior (TA) in rats anesthetized with pentobarbit al. The TA observations were systematically compared with those obtain ed earlier by the use of the same techniques from rat medial gastrocne mius (MG). 2. TA motoneurons were investigated with regard to afterhyp erpolarization (AHP; total duration 32-74 ms, amplitude 0.39-4.96 mV) and axonal conduction velocity (41-79 m/s). TA muscle-unit measurement s included the time course of the isometric twitch (time-to-peak force 10.8-18.0 ms; total duration 42-92 ms), the maximum tetanic force (22 -217 mN), and a measure of fatigue sensitivity (fatigue index 5-100%). The range of twitch and AHP durations (''speed range'') was markedly smaller in the present TA material than for MG. 3. The mean duration o f the TA motoneuronal AHP (49 +/- 8 ms, mean +/- SD) was close to that of its muscle-unit twitch (56 +/- 12 ms). Thus an ''average'' speed m atch existed between TA motoneurons and their muscle fibers. 4. For TA there was no correlation between the time courses of AHP and twitch. Thus there was for TA no ''continuous'' speed match between the motone urons and their muscle fibers. 5. For TA twitches or AHPs studied sepa rately, there was a significant correlation between different time cou rse measures. Furthermore, compared with TA units having relatively fa st twitches, those with slower twitches tended to show 1) a smaller ma ximum tetanic force and 2) a greater AHP amplitude. Fatigue-resistant units tended to have slower twitches than fatigue-sensitive ones. 6. T he findings suggest the presence, for TA, of a dissociation between tw o independently controlled aspects of the commonly occurring AHP versu s twitch duration match: in TA there is only an ''average'' match, whe reas for MG there is an ''average'' as well as a ''continuous'' match (true also within same speed range as TA). Comparisons between the ten sion-frequency relation for the TA muscle and the twitch and AHP durat ions for TA units suggested that, in a muscle with this limited range of speed variation among its units, the average match would suffice fo r ensuring an adequate fit between the motoneuronal frequency coding a nd the rate gradation of muscle-unit force.