Repeated clenching causes plasticity in corticomotor control of jaw muscles.

This study tested the effect of short-term tooth-clenching on corticomotor excitability of the masseter muscle using transcranial magnetic stimulation (TMS). Fifteen subjects with normal stomatognathic function participated. All subjects performed a tooth-clenching task (TCT) on five consecutive days. The TCT consisted of 10, 20, and 40% of maximum voluntary contraction in a randomized order within 1 h. All subjects underwent TMS in four sessions: pretask day 1 (baseline), post-task day 1, pretask day 5, and post-task day 5. Motor-evoked potentials (MEPs) from the masseter and the first dorsal interosseous (FDI) muscles were obtained using TMS in four sessions. Motor thresholds decreased, after the TCT, for the masseter muscle MEPs. Masseter muscle MEPs were dependent on stimulus intensity and on session, whereas FDI muscle MEPs were only dependent on stimulus intensity. Post-hoc Tukey tests demonstrated significantly higher masseter muscle MEPs post-task on day 5 with 80 and 90% stimulus intensity and above when compared with pre- and post-task day 1 values. Our results suggest that the performance of repeated TCTs can trigger neuroplastic changes in the corticomotor control of the jaw-closing muscles and that such neuroplastic changes may contribute to the mechanism underlying the clinical manifestations of tooth clenching.