Phasic and rhythmic responses of the oral musculature to mechanical stimulation of the rat palate

Activity of the anterior and posterior digastric muscles, the intrinsic tongue muscles and the masseter muscles was recorded electromyographically. Muscle-activity patterns depended on the site of stimulation, the strength of the stimulus and its duration. Five types of reflex patterns were distinguished: type A: on-response in the digastric muscles (latency about 7ms) and in the intrinsic tongue muscles (latency about 15 ms); type B: on-off response in the digastric and intrinsic tongue muscles; type C: on-response followed by long-lasting low-level activity in the digastric and intrinsic tongue muscles; type D: on-off response and long-lasting low-level activity in the digastric and tongue muscles; type E: rhythmic activity in all muscles under investigation preceded by one of the other reflex types. In the type-E response, the frequency of the bursts was about 4.3 Hz; the masseter bursts were not in phase with those of the digastric muscles; the intrinsic tongue muscles showed constant activity with superimposed rhythmic bursts. By increasing the stimulus strength or the stimulus duration, there was a gradual shift from simple, phasic, reflex responses to more complex reflex patterns. This holds for all palatal sites. However, rhythmic activity was most easily elicited in the region of the incisive papilla, less easily in the antemolar region; no rhythmic activity was found in the intermolar region. Thus stimulus duration is a crucial factor in eliciting rhythmic oral activity in the rat.