The effects of glossopharyngeal and chorda tympani nerve cuts on the ingestion and rejection of sapid stimuli: An electromyographic analysis in the rat

The present study tested the effects of bilateral section of either the chorda tympani or glossopharyngeal nerves on the production of oro-pharyngeal electromyographic (EMG) responses to intra-oral sapid stimulation. The responses of adult rats fitted with intra-oral cannulas and fine-wire electrodes in the anterior digastric (jaw opening) and thyropharyngeus (swallowing) muscles were examined following direct oral stimulation with water and 5 concentrations of sucrose, NaCl, and quinine monohydrochloride (QHCl). One group of rats was tested both before and after bilateral removal of the chorda tympani. A second group of rats was tested subsequent to bilateral removal of the glossopharyngeal nerves. A normal EMG response pattern to suprathreshold QHCl consisted of several intra-oral licks followed by a series of large amplitude mouth openings (gapes). In addition, there was a longer latency to the first swallow following QHCl stimulation compared to water stimulation. Cutting either nerve affected this rejection response to QHCl, but produced little change in the ingestive response to the other stimuli. Following chorda tympani nerve cuts, rats showed an increased latency to the first gape and a small reduction in the number of gapes across the 5 concentrations of QHCl (16%). In contrast, bilateral section of the glossopharyngeal nerves produced a much larger reduction in the number of gapes (54%), but had no effect on the latency to the first gape. In addition, the latency to swallow suprathreshold QHCl was shorter following glossopharyngeal nerve cuts. These observations suggest that gustatory receptors on the anterior tongue, innervated by the chorda tympani, initiate a rejection response, but that receptors on the posterior tongue, innervated by the glossopharyngeal nerve, are necessary for a sustained rejection sequence.