Effect of sensory input from the tongue on jaw movement in normal feeding in the opossum.

Opossums were presented with solid and liquid foods. The movements of the jaw and tongue were recorded cineradiographically together with recordings of the EMG activity in muscles opening the jaw and moving the base of the tongue (hyoid). The jaw opening in each cycle was in two stages--01 and 02; 01 had a constant amplitude irrespective of the food ingested. Ingestion of liquid (which involved continuous accumulation of a liquid bolus in the valleculae prior to swallowing) was associated with cycles of oral movement in which 02 was small; tongue retraction was associated with this opening. In contrast, solid and semisolid food ingestion was associated with large angles of jaw opening in 02 that also coincided with the tongue retraction. In this latter case a characteristic pattern of EMG activity, in which all the muscles moving the hyoid were simultaneously active, was added to the pattern seen in lapping; this additional activity had an EMG pattern that was consistent with a jaw opening reflex. The findings contrast with other reports that the jaw opening reflex is suppressed in mastication. Experimentally induced tongue contact with a variety of solid surfaces during lapping (an activity involving accumulation of a liquid bolus in the valleculae) induced neither increased jaw opening nor the additional EMG pattern. However, in situations when there was no bolus in the valleculae, additional jaw opening activity was elicited when the tongue contracted solids intra- or extra-orally. It is suggested that the ability of sensory input, from the anterior tongue, to elicit a jaw opening reflex and to change the type of jaw/tongue cycle was dependent upon the extent of bolus accumulation in the valleculae and therefore indirectly upon the consistency of the food.