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) c ells of hand and forearm muscles at different levels of EMG activity. 2. Twenty-three CM cells were recorded in six hemispheres of four trai ned monkeys. CM cells were identified by the presence of post-spike fa cilitation (PSP) in spike-triggered averages (STAs) of their target mu scles. Cell and muscle activity was recorded during performance of a l ow 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 segme nts 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, w ere compared across two to six different levels of EMG activity during the hold periods. 4. Of the forty-two CM cell-muscle combinations tes ted, 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 foun d for nineteen combinations (45 %). 5. From a low to a high level of E MG activity (0.3-8.65 % of the maximum EMG activity recorded), the abs olute amount of facilitation produced by each CM cell increased by a f actor of 1.2-32 (median value 3.7). This increase in facilitation occu rred irrespective of the presence or absence of correlation between CM cell discharge rate and target muscle activity. 6. For thirty cell-mu scle combinations in which a significant PSF could be measured at more than one level of EMG activity, the relative degree of facilitation r emained 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 act ivity. 7. The changes in PBF amplitude with level of EMG activity were also present in STAs compiled from only those spikes with long inters pike intervals (20-25 ms or greater). The results suggested that spike s with short interspike intervals did not make a significant contribut ion to the increase in PSF amplitude observed at the higher levels of EMG activity. 8. The changes in PSF amplitude with target muscle activ ity 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.