Jaw-jerk reflex activity in relation to various clenching tasks in man.

In order to investigate whether the mandibular stretch (jaw-jerk) reflex is modulated in a task-dependent manner, jaw-jerk reflexes were elicited in eight subjects during clenching with unilateral and bilateral tooth support, respectively. The reflexes were examined in the electromyographic (EMG) activity recorded by means of surface electrodes and were elicited by means of small transient jaw displacements at a constant value of 80 microns. Low levels of background EMG were applied ranging from 1 to 30% maximal voluntary contraction (MVC) as controlled by means of visual feedback. Linear relationships were observed between reflex amplitude and level of background EMG. The slope in these relationships served as a measure of reflex gain. For the masseter as well as the anterior temporal muscles, the reflex gain, averaged over both sides, was larger during clenching with unilateral tooth support than with bilateral tooth support (P < 0.05). Furthermore, the gain was larger on the side without tooth support during unilateral clenching and larger on the side without visual feedback of elevator muscle activity during bilateral clenching. It can be concluded that the jaw-jerk reflex is modulated to subserve the stabilization of the mandible, with the reflex sensitivity being larger the more that alternative stabilizing factors such as mechanical tooth contact, visual feedback and feedback from periodontal pressure receptors around the teeth are lacking. The reflex modulation may be of functional importance in stabilizing the mandible during its movement in the chewing process, as the food is predominantly placed unilaterally between the antagonistic teeth during individual chewing cycles.