The activity of thirty-one single motor units (SMUs) was recorded from forearm and hand muscles of three volunteers. The excitability of the rhythmically firing motoneurones supplying these SMUs was examined after voluntary discharge using transcranial magnetic stimulation (TMS). 2. The magnetic stimulus was delivered either at a fixed delay (range: 1-60 ms) after SMU discharge (triggered mode) or at random with respect to voluntary SMU discharge (random mode). Post-stimulus time histograms (PSTHs) of responses to 50-100 stimuli were constructed for each experimental condition. 3. In the triggered mode, the probability of response to TMS progressively decreased as the spike-to-stimulus interval was shortened. Shortening of the interval also resulted in redistribution of responses within the different subpeaks characterizing the short-latency response of motor units to TMS: the relative response probability of the first subpeak decreased with the shorter spike-to-stimulus intervals. 4. In the triggered mode, the probability of SMU responding to TMS was significantly higher when the firing rate of the motor unit was increased from a slow rate (< 10 impulses s-1) to a faster rate (> 12 impulses s-1), irrespective of the spike-to-stimulus interval. In contrast, in the random mode, the response probability was greater at the slower discharge rate. 5. The higher excitability of motoneurones at the fast rate in the triggered mode is consistent with findings in cat motoneurones suggesting a shallower after-hyperpolarization, but other mechanisms could contribute. Furthermore, our results suggest that there is an asymptotic recovery in the excitability of slow firing motoneurones towards the end of the interspike interval.
