Glossopharyngeal-hypoglossal nerve reflex of the frog in metamorphosis

In adult frogs, a reflex discharge is elicited from the hypoglossal nerve by mechanical and chemical stimuli to the centre of the tongue surface and electrical stimuli to the glossopharyngeal nerve innervating tongue. However, there has been no report concerning the embryological stage at which this reflex develops. The metamorphosis of a frog is divided into three parts, premetamorphic, metamorphic and climax stage. In the climax stage, the forelimbs are lengthened and the tail gradually shortened. The glossopharyngeal-hypoglossal nerve reflex arose in the early climax stage, during which the tongue appeared on the floor of the mouth. At this stage, the conduction velocities of the first compound action potential of the glossopharyngeal and hypoglossal nerves were about 8.7 and 9.3 m/s, respectively. The latency of the reflex was long, especially the central latency. The response of the glossopharyngeal nerve and the reflex discharge from the hypoglossal nerve upon mechanical stimulation of the tongue were smaller in the tadpole than in the adult frog. As metamorphosis proceeded, the conduction velocity of these nerves increased; the latency shortened, especially the central latency; the reflex response to mechanical stimulus increased. These results suggest that, although the glossopharyngeal-hypoglossal nerve reflex is developed in the early climax stage, it is completed only in the adult frog. Therefore, adult frogs living mainly on land may reject and swallow prey by this reflective tongue movement.