Postspike facilitation of forelimb muscle activity by primate corticomotoneuronal cells.

2. In macaque monkeys making rampand-hold wrist movements against elastic loads we recorded activity of task-related motor cortex cells and specific wrist and finger muscles. EMG electrode pairs were implanted in six flexor and six extensor muscles identified by their anatomical location in the forelimb and by characteristic movements evoked by intramuscular stimulation. For precentral cortex cells that discharged during the wrist movements we compiled spike-triggered averages (STAs) of full-wave rectified EMG activity of five to six coactivated muscles. 2. Eighteen percent of the STAs compiled from selected cells showed evidence of postspike facilitation (PSF) of average EMG activity. Such PSF began after the cortical spike (mean onset latency, 6.7 ms), reached a peak (mean peak latency, 10.2 ms), and declined again to prespike baseline levels (mean decay time, 7.0 ms). Seventy-eight percent of the STAs showed no spike-related features, and 4% exhibited complex features. 3. The strength of the PSF was qualitatively rated as strong, moderate, or weak on the basis of clarity relative to base-line fluctuations. When quantified, strong PSF had the largest peak amplitudes relative to mean base-line levels and relative to maximal base-line fluctations. Strong PSF had slightly shorter onset latency and longer duration than moderate or weak PSF. 4. In some monkeys the cells’ projection into the pyramidal tract was tested by the collision technique. Fast pyramidal tract neurons (PTNs) produced mainly PSF with the shortest onset latencies, but also gave rise to PSF with longer latencies; slow PTNs produced PSF with longer latencies. 5. Pairs of spikes with brief interspike intervals were found to be particularly effective in generating PSF. STAs triggered from two action potentials separated by very short interspike intervals showed a net facilitation exceeding the linear sum of the PSF of isolated action potentials. 6. Most cells produced PSF in more than one of the coactivated forelimb muscles. To eliminate any possibility of redundant recording of the same facilitated motor units through different electrodes, EMG activity was cross-correlated and those records with evidence of significant cross talk were excluded from the data base. Of 370 taskrelated neurons recorded with five to six independent muscles, 27% produced strong or moderate PSF in at least one muscle. Half of these produced clear PSF in more than one muscle. The cell’s muscle field, i.e., the set of facilitated muscles, typically comprised a subset of the coactivated synergist muscles. The mean number of facilitated muscles was 2.5 for extensor cells and 2.1 for flexor cells. 7. The excitatory effects on forelimb extensor muscles tended to be stronger and more widespread than on flexor muscles. The proportion of task-related cells showing clear PSF was larger for extensor cells (37%) than flexor cells (22%). Moreover, the ratio of strong to weak PSF was twice as great for extensor cells ( 1.2) as for flexor cells (0.6). Finger extensor muscles were more strongly facilitated than wrist extensor muscles or flexor muscles.