Regulation of primary motor cortex excitability by repetitive passive finger movement frequency

Somatosensory input induced by passive movement activates primary motor cortex (M1). We applied repetitive passive movement (RPM) of different frequencies to test if modulation of M1 excitability depends on RPM frequency. Twenty-seven healthy subjects participated in this study. Motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) to left M1 were recorded from the right first dorsal interosseous muscle (FDI) to assess corticospinal excitability (experiment 1: n=15), and F-waves were measured from the right FDI as an index of spinal motoneuron excitability (experiment 2: n=15). Passive abduction/adduction of the right index finger was applied for 10min at 0.5, 1.0, 3.0, and 5.0Hz. Both 0.5Hz-RPM and 1.0Hz-RPM decreased MEPs for 2min (p<0.05), and 5.0Hz-RPM decreased MEPs for 15min compared with baseline (p<0.05); however, there was no difference in MEPs after 3.0Hz-RPM. No F-wave changes were observed following any RPM intervention. Based on the results of experiments 1 and 2, we investigated whether RPM modulates cortical inhibitory circuit using the paired-pulse TMS technique (experiment 3: n=12). Short-interval intracortical inhibition (SICI) was measured using paired-pulse TMS (inter-stimulus interval of 3ms) before and after 1.0, 3.0, and 5.0Hz-RPM. Both 1.0 and 5.0Hz-RPM increased SICI compared with baseline (p<0.05). These experiments suggest that M1 excitability decreases after RPM depending on movement frequency, possibly through frequency-dependent enhancement of cortical inhibitory circuit in M1.