FUNCTIONAL-PROPERTIES OF JAW AND TONGUE MUSCLES IN RATS FED A LIQUID DIET AFTER BEING WEANED

Decreased masticatory demands due to liquid or soft diets cause a redu ction in the growth of craniofacial bones and in the development of fe eding musculature, but the effects on masticatory function and jaw/ton gue muscle activities are unclear. The present study was undertaken to test the hypotheses that a liquid diet feeding after weaning affects the critical-period programming of mastication and the motor performan ces of jaw and tongue muscles. Thirty-six male Wistar rats were divide d into two equals groups at weaning and fed either a solid (solid-diet group) or a liquid (liquid-diet group) diet until they reached 50 day s of age. Electromyograms (EMG) of the masseter, medial pterygoid, tem poralis, anterior digastric, styloglossus, and genioglossus were recor ded while animals were naturally ingesting ordinary pellets, apple cub es, and a liquid diet. It was found that: (1) a more irregular chewing rhythm, a shorter chewing sequence, and a longer chewing cycle were f ound in the liquid-diet group, but there were no differences observed during lapping/licking between the two groups; (2) during the chewing cycles, the EMG onset of each muscle in relation to that of the masset er in the liquid-diet group nas similar to that in the lapping/licking cycles in both groups; (3) the activities of jaw elevators (masseter, medial pterygoid, and temporalis) during the chewing cycles were sign ificantly higher in the liquid-diet group; and (4) the increase in the EMG activities oi jaw elevators during pellet chewing compared with a pple cube chewing was significantly weaker in the liquid-diet group wh ereas such an Enhancement was found simultaneously in the styloglossus in the solid-diet group, and in the anterior digastric in the liquid- diet group. These findings verify that: (1) the motor output of jaw an d tongue muscles may be altered In rats fed a liquid diet after being weaned; (2) the feeding of a liquid diet to rats after being weaned ma y obstruct the functional transition from suckling to mastication; and (3) jaw elevators that develop without motor learning of mastication are inefficiency when performing functionally.