The influence of single monkey cortico-motoneuronal cells at different levels of activity in target muscles.

1. This study assessed the facilitation by cortico-motoneuronal (CM) cells of hand and forearm muscles at different levels of EMG activity. 2. Twenty-three CM cells were recorded in six hemispheres of four trained monkeys. CM cells were identified by the presence of post-spike facilitation (PSF) in spike-triggered averages (STAs) of their target muscles. Cell and muscle activity was recorded during performance of a low force (0.2-1.5 N) precision grip task between the index finger and thumb. The hold periods of this task lasted 1-1.5 s and provided segments of steady EMG activity. 3. The discharge activity of each CM cell, and the amplitude of the PSF produced in one or two target muscles, were compared across two to six different levels of EMG activity during the hold periods. 4. Of the forty-two CM cell-muscle combinations tested, twenty (48%) showed a significant increase in CM cell discharge rate with increased target muscle EMG activity (P < 0.001); three (7%) showed significant negative correlation; and no correlation was found for nineteen combinations (45%). 5. From a low to a high level of EMG activity (0.3-8.65% of the maximum EMG activity recorded), the absolute amount of facilitation produced by each CM cell increased by a factor of 1.2-32 (median value 3.7). This increase in facilitation occurred irrespective of the presence or absence of correlation between CM cell discharge rate and target muscle activity. 6. For thirty cell-muscle combinations in which a significant PSF could be measured at more than one level of EMG activity, the relative degree of facilitation remained constant in nine, increased in thirteen and decreased in seven combinations. In some cases saturation effects were evident. For ten combinations PSF was observed at high but not at low levels of EMG activity. 7. The changes in PSF amplitude with level of EMG activity were also present in STAs compiled from only those spikes with long interspike intervals (20-25 ms or greater). The results suggested that spikes with short interspike intervals did not make a significant contribution to the increase in PSF amplitude observed at the higher levels of EMG activity. 8. The changes in PSF amplitude with target muscle activity are probably explained best by changes at the spinal motoneuronal level, which set the response to the CM input. These changes may also reflect differences in the strength of synaptic connectivity made by a CM cell within the motoneurone pool of the target muscle.