Possible factors which may affect phase durations in the natural chewing rhythm.

To study central and peripheral control mechanisms maintaining rhythmical jaw behaviors, chewing movements with different foods in texture were obtained in the freely behaving rabbit. Jaw movement trajectories and muscle activities (masseter, digastric, thyrohyoid) were recorded and the durations in total cycle, fast closing (FC), slow closing (SC), and opening (OP) phases were obtained as well as the burst duration in the muscles. Durations varied cycle-by-cycle and among the foods, however, the total cycle duration was found to have little differences among the foods tested. Regression analyses were applied to seek time relations to the change in total cycle duration with the duration in its constituent phases. Results suggested that changes in the total cycle duration may be due to those in the SC duration (power phase) with hard food (heavy load), but due to those in the OP duration (reverse phase) with soft food (light load). The duration of the FC was fairly constant for all the foods tested. In conclusion, the chewing rhythm may be controlled centrally to be independent of the load, and chewing cycles may begin at the middle of the opening phase.