Corticomotoneuronal postspike effects in averages of unrectified EMG activity

1. Spike-triggered averaging (SpTA) of rectified electromyograms (EMGs) in awake monkeys is recognized as an effective means of establishing causal relations between cortical/cell discharge and muscle activity. The appearance of postspike facilitation (PSF) in averages of rectified EMG activity is interpreted as evidence of an underlying excitatory synaptic linkage between the trigger cell and motoneurons. Cells producing PSF are referred to as corticomotoneuronal (CM) cells. Similarly, postspike suppression (PSS) is interpreted as evidence of underlying inhibitory synaptic linkages. Studies to date have focused almost exclusively on averages of full-wave rectified EMG activity. Because the potential utility of SpTA of unrectified EMG activity has not been systematically examined, we compared postspike effects in SpTAs of rectified and unrectified EMGs for 44 cortical cells yielding 293 cell-target muscle pairs (CMPs). 2. Clear PSF was found in 110 of 293 averages of rectified EMG activity from 28 known CM cells. Forty-nine of these 110 CMPs (45%) also showed clear postspike effects (PSE) in the corresponding averages of unrectified EMGs activity. Loss of effects in averages of unrectified EMGs can be attributed to cancellation of the negative and positive phases of motor unit potentials. Mean onset latencies were similar for effects in rectified and unrectified EMGs [6.9 +/- 1.8 (SD) ms vs. 6.6 +/- 1.2 (SD) ms]. Overall, the magnitudes of effects measured as peak-to-noise ratios were also similar for effects in the two groups (8.8 vs. 8.7). The 61 CMPs that showed clear PSF in rectified EMGs but no effects in corresponding unrectified EMGs had, as a group, longer and more dispersed onset latencies and were weaker in magnitude than those that showed effects in both rectified and unrectified EMGs. Moreover, the occurrence of clear effects in averages of unrectified EMGs was correlated with the strength of PSF (75% of strong but only 29% of weak PSFs showed clear PSEs in averages of unrectified EMG activity). 3. Clear PSS was observed in 24 (8%) averages of rectified EMGs derived from nine cells. Remarkably, clear PSEs were found in 12 of the corresponding averages of unrectified EMG activity.(ABSTRACT TRUNCATED AT 400 WORDS)