Changes in jaw-jerk on different levels of jaw closure and teeth-clenching in humans.

We investigated how the jaw-jerk in the human masseter muscle is modulated in relation to the level of jaw closure (JC) and teeth clenching. Electromyographic (EMG) activity was recorded with surface electrodes. Background EMG activity of the masseter muscle was kept at three low teeth clenching levels with visual feedback. The level of JC was changed in six steps along the habitual path of closure relative to the mean maximal jaw opening during gum chewing by inserting a bite block between the upper and lower molars. The jaw-jerk was evoked by applying mechanical stimulation of about 20 N with a hammer to the bite-fork placed on the lower molars on one side in each condition of combination of a level of JC with a level of teeth-clenching. At the resting condition the excitability of the jaw-jerk increased with JC, while at weak voluntary teeth clenching it then decreased and increased again as the jaw was progressively closed. It is suggested that the excitability of the jaw-jerk would increase toward the occlusal position, which in turn would contribute to smooth masticatory movements. In addition, the mode of modulation of the jaw-jerk was studied in a subject with skeletal malocclusion.