Glossopharyngeal and tectal influences on tongue-muscle motoneurons in the Japanese toad

Abstract

Anuran tongue-muscle motoneurons receive excitatory inputs both from the glossopharyngeal nerve afferents and from the optic tectum. As a step toward elucidating the neural bases for controlling the tongue movements, we searched intracellularly for the neuronal pathways from the glossopharyngeal afferents to the tongue-muscle motoneurons in paralyzed Japanese toads. The electrical stimuli applied to the glossopharyngeal nerve (ipsilateral lingual branch) evoked polysynaptic excitatory postsynaptic potentials (EPSPs) in tongue-protractor motoneurons and mixed mono- and polysynaptic EPSPs in tongue-retractor motoneurons. Furthermore, we investigated the mode of the convergency of the excitatory inputs from the glossopharyngeal nerve and those from the optic tectum. A spatial facilitation was observed between the tectal EPSPs and the glossopharyngeal EPSPs in some motoneurons tested. These results suggest the existence of common excitatory premotor interneurons, on which the tectal descending volleys and the glossopharyngeal afferent volleys converge.