COVARIATION BETWEEN HUMAN INTRINSIC HAND MUSCLES OF THE SILENT PERIODS AND COMPOUND MUSCLE ACTION-POTENTIALS EVOKED BY MAGNETIC BRAIN-STIMULATION - EVIDENCE FOR COMMON INHIBITORY CONNECTIONS

Transcranial magnetic stimuli at different stimulus intensities were a pplied in six healthy subjects to test the hypothesis that, in differe nt intrinsic hand muscles, the duration of the resultant cortically ev oked silent periods (C-SPs) from each stimulus would be positively cor related between muscles, indicating a common inhibitory mechanism. A f igure-of-eight coil discharging through a Magstim 200 stimulator deliv ered 25 stimuli at each stimulus intensity at a minimum of five intens ities ranging from 55% to 160% of the individual resting motor thresho ld. In each subject, simultaneous surface recordings from pairs of mus cles were made from the first dorsal interosseous (FDI), opponents pol licis (OP), abductor pollicis brevis (APB) and abductor digiti minimi (ADM). The C-SP durations within all three muscle pairs tested were hi ghly correlated (P<0.001). The amplitude of the pre ceding compound-mu scle action potentials (CMAPs) was positively correlated between FDI a nd OF, but not between APE and ADM or FDI and ADM. C-SP duration was l inearly related to stimulus intensity, but did not correlate with the latency or amplitude of the preceding CMAP. SPs elicited by peripheral nerve stimuli in pairs of hand muscles did not co-vary significantly. The results provide evidence that inhibitory influences of cortical o rigin are distributed widely to intrinsic hand muscles. In contrast, c ovariation of excitatory effects only appears between muscles synergis tically involved in a motor task.