Excitability changes in human hand motor area induced by voluntary teeth clenching are dependent on muscle properties

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To investigate whether the early effects of voluntary teeth clenching (VTC) among the first dorsal interosseous (FDI), abductor digiti minimi (ADM), and abductor pollicis brevis (APB) muscles are differently modulated depending on their different muscle properties, we examined the responses of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation with selected current directions and by brainstem magnetic stimulation (BMS). Although MEP responses with anterior-medially (AM) current direction (preferentially elicited I1-waves) were facilitated in all three muscles, those responses with posterior-laterally (PL) current direction (preferentially elicited I3-waves) were different among FDI, ADM, and APB muscles. That is, MEP response in FDI and APB muscles were significantly reduced, whereas those responses in ADM muscle were not significantly reduced. Further, inhibitory effects of VTC in FDI muscle were more potent than those in ADM or APB muscles. On the other hand, the responses to BMS were unchanged by VTC in all three muscles, suggesting that the modulations of MEP were attributed to the cortical origin. Based on the our previous findings that the inhibitory connections in FDI muscle are more potent than those in ADM muscle, the cortical effects of VTC among three hand muscles are differently modulated, depending on muscle properties, presumably the extents of inhibitory connections to corticospinal tract neurons. Further, considering that functional capacity in FDI muscle is higher than that in ADM or APB muscles, the cortical inhibitory effect of VTC might contribute to sophisticated regulation of the motor outputs even during VTC.

Excitability changes in human hand motor area induced by voluntary teeth clenching are dependent on muscle properties

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