Chorda tympani and vagus nerve convergence onto caudal brain stem neurons in the rat

Neurons responsive to chorda tympani (CT) and cervical vagus (CV) nerve stimulation were identified electrophysiologically in the caudal brain stem of the anesthetized rat. All identified dually responsive (CT/CV) neurons included in this study were orthodromically activated by both CT and CV stimulation (n = 80). These cells were located mainly in the lateral nucleus tractus solitarius (nts) or more ventrally in the region of nucleus ambiguus (amb). No CT/CV cells were found to lie clearly within the dorsal motor nucleus of the vagus nerve. Among CT/CV neurons in nts, there was no correlation of response latency to CT stimulation versus response latency to CV stimulation, while CT/CV neurons found in the region of amb demonstrated a good correlation between the two stimuli and suggested that sensory convergence occurred prior to the level of amb. Conditioning pulses applied to CV were able to alter the neural response characteristics to CT test pulses in a majority of the units tested. It is concluded that oropharyngeal afferents converge on brain stem neurons that are also responsive to vagal afferent input. These dually responsive CT/CV neurons are implicated in the integration of sensory information relevant for cephalic phase reflexes.