Voluntary teeth clenching facilitates human motor system excitability

Objective: Voluntary contraction of the teeth is a common maneuver used to facilitate peripheral monosynaptic reflexes. It was the aim of this study to determine the site along the neuraxis where this effect occurs. Methods: Focal transcranial magnetic stimulation (TMS) was used to measure recruitment curves, motor thresholds and intracortical inhibition and facilitation from the right first dorsal interosseus (FDI) and tibialis anterior (TA) muscles in seven normal volunteers. Changes in excitability in subcortical structures during teeth clenching were studied using F waves, H reflexes, and brainstem magnetic stimulation. Results: Recruitment curves of FDI and TA showed significant facilitation during voluntary teeth clenching indicating an overall enhancement in the motor system excitability. Teeth clenching additionally resulted in decreased intracortical inhibition in the FDI but not in TA, pointing to an intracortical site of enhancement for the hand. Motor evoked potentials (MEPs) following stimulation at the brainstem level and F waves in FDI and soleus H reflex amplitude were also facilitated by teeth clenching, indicating a subcortical site for this effect for the upper and lower extremity. M wave amplitudes did not change. Conclusions: The teeth clenching maneuver had a similar facilitatory effect on upper and lower extremities. Cortical and subcortical sites contribute to this effect in a hand muscle while only subcortical sites were identified in this facilitatory effect on the lower extremity.