Relationship of masseter electromyographic activity to mandible position in the freely-moving rat.

Simultaneous records of masseter-EMG activity and position of the mandible were made on 4 Wistar rats eating four food types: standard food pellets (large pellets), 45 mg precision food pellets (small pellets), bread and pudding. Masseter activity was recorded with chronic bipolar silver EMG electrodes placed within the muscle and led subcutaneously to a pedestal on the head. The transducer indicating jaw position consisted of two tiny inductance coils, placed on the head and under the mandibular symphysis. The lower coil acted as a receiving aerial for the carrier signal supplied via the upper coil, so that distance between the coils, and thus relative jaw position, was traced on the oscilloscope screen. A silent period was found in the masseter-EMG bursts similar to that found in man, where the silent period represents tooth contact and the occlusal phase of chewing (time during which teeth are in the intercuspal position) is defined as the interval between the silent period and the termination of the EMG activity in the muscle. Applying this to the rat data, opening represented 70-75 per cent of the total cycle while the combined closing and occlusion phases comprized 25-30 per cent when either type of pellet was eaten. With bread, closing and occlusal phases represented 56 per cent of the total cycle. For all three food types, muscular force, as indicated by the jaw-closing movement registered by the transducer, continued beyond the masseter EMG. Irregular jaw movements seen when the animals lapped pudding were not correlated directly with masseter activity.