Projections to the trigeminal, facial, ambiguus, and hypoglossal motor nuclei were determined by using horseradish peroxidase histochemistry. Most of the afferent projections to these motor nuclei were from the brainstem reticular formation, frequently in areas adjacent to other synergetic motor nuclei. The reticular formation lateral to the hypoglossal nucleus and reticular structures surrounding the trigeminal motor nucleus projected to each of these other brainstem motor nuclei involved in oral-facial function. Afferent projections to these motor nuclei also were organized along the rostrocaudal axis. Within the reticular formation most of the afferent projections to the trigeminal motor nucleus originated rostral to the majority of neurons projecting to the hypoglossal and ambiguus nuclei, which in turn were rostral to the primary source of reticular afferents to the facial nucleus. In comparison, projections from the sensory trigeminal nuclei and nucleus of the solitary tract were sparse. The interneuron pools that project to the orofacial motoneurons provide one further link in understanding the brainstem substrates for integrating oral and ingestive behaviors.

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Previous behavior studies (Grill & Norgren, 1978) demonstrated that gustatory stimuli produce stereotyped orofacial movements that constitute the observable concomitants of ingestion and rejection. For further clarification of the relation between these orofacial movements (the buccal phase of ingestion) and the act of swallowing (the pharyngeal phase), electromyographic responses to intraoral sapid stimulation were recorded from a subset of orofacial and pharyngeal muscles in a freely moving chronic preparation. Activity in a jaw opening muscle (anterior digastric), a facial muscle (zygomatic), tongue protruder (genioglossus), tongue retractor (styloglossus), and a pharyngeal constrictor used in swallowing (thyropharyngeus) differentiated between ingestive sequences to water (W), sucrose (S), and NaCl (N) and a rejection response elicited by quinine monohydrochloride (Q). Ingestion responses to W, S, and N consisted of rhythmic alterations between genioglossus and styloglossus activity (intraoral licks) accompanied by episodic bursts of pharyngeal constrictor activity (swallowing). Both bout duration and the number of swallows increased at higher concentrations of S and N. In contrast, Q stimulation elicited a rejection response, characterized by several licks and followed by long duration contractions of the zygomatic and anterior digastric muscles (gapes). During gapes, styloglossus activity rather than genioglossus activity was simultaneous with that of the anterior digastric. At higher concentrations of Q, the latency to gape decreased and the latency to swallow increased. The earliest components of the response to S, N, or Q were virtually indistinguishable from one another, results suggesting that tactile (fluid) stimulation initiates the ingestive sequence and that gustatory stimuli modulate this ongoing activity.

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