REFLEX CHANGES IN THE MASTICATORY MUSCLES WITH LOAD PERTURBATIONS DURING CHEWING HARD AND SOFT FOOD

Reflex changes in masticatory muscles were investigated in naturally b ehaving rabbits while chewing soft food (bread) and hard food (raw ric e). To study peripheral control mechanisms of mastication, reflex chan ges in masticatory muscles were correlated with the jaw movement traje ctories. When the hard food was tested, the closing muscle was activat ed isometrically and the antagonist activity was evident during closur e. Under such conditions, the so-called masseteric inhibitory periods (MIPs) and digastric short bursts (DSBs) were found in the closing pha se, which was not the case with soft food. The reflex changes in the m asticatory muscles were similar to those in an unloading reflex or in a reflex after tooth tap. Precise comparison of the EMG and the moveme nt orbit showed that DSB with MIP was preceded by a trough in the clos ing velocity which bottomed at 8 +/- 1 ms (mean +/- S.D.: n = 9) befor e the DSB onset. These results suggest that the DSB with MIP may be a reflex change generated by periodontal mechanoreceptor stimulation aft er the upper and lower teeth come together with hard food, which have a role in regulatory mechanisms of mastication when the teeth are sudd enly loaded by hard food.