A comparison of jaw-opener and jaw-closer muscle activity in humans to overcome an external force counteracting jaw movement.

In contrast to the jaw-closer muscles, no or very few spindles are present in the jaw-opening digastric muscle. Therefore sensory feedback to the digastric muscle may be different from feedback to the jaw-closer muscles, resulting in a different reaction when jaw movement is perturbed. This possible difference was investigated by comparing the reaction of the digastric muscle when jaw opening is perturbed, with the reaction of the masseter muscle when jaw closing is perturbed. Subjects made rhythmic, 1-Hz open-close movements of the jaw under control of a metronome. During jaw opening (digastric muscle) or, in the other experiments, during jaw closing (masseter muscle), an external force counteracting jaw movement could appear. Series of movements without the force were unexpectedly alternated by series with the force. In both muscles sensory induced activity started approximately 25 ms after the onset of the force and consisted of two phases. In the masseter muscle the maximum of the first increase was reached significantly sooner (37 +/- 2 ms SEM) than in the digastric muscle (54 +/- 3 ms). The second increase appeared much sooner in the masseter muscle (73 +/- 4 ms) than in the digastric muscle (159 +/- 10 ms). When the force was expected, in both muscles an increase in preprogrammed muscle activity was observed. Also an increase in reflex activity, generated before 120 ms after the onset of the force, was observed, compared with when the force appeared unexpectedly. The relative increase in reflex activity was approximately 2 times larger than the relative increase in preprogrammed activity. Therefore, the increase in reflex activity when the force was expected may have been caused not only by an increase in recruitment, but also by an increase in the gain of the reflex loops. Reflex activity relative to preprogrammed activity was on average 4 times larger in the masseter muscle than in the digastric muscle. This indicates that the masseter muscle can react more adequately to disturbances of jaw movement than the digastric muscle.