Comparison of Hypoglossal and Reticular Formation Neural Activity During Gustatory-Evoked Ingestion and Rejection

Abstract

Neurons in the reticular formation (RF) lateral to the hypoglossal nucleus (mXII) contribute to a distribution of brainstem neurons that project to the oral motor nuclei [1]. This region of the medullary RF, implicated as part of a central pattern generator (CPG) for swallowing [2], appears contiguous with a CPG for rhythmic masticatory function [3]. Other neuroanatomical studies indicate that this region of the RF may also receive projections from oro-sensory nuclei, including the nucleus of the solitary tract [4,5] and the trigeminal sensory complex [6], and is thus well situated to integrate oro-sensory signals with ingestive function. Acute neurophysiological studies, in fact, have shown that a small subset of neurons in the caudal RF lateral to mXII are active during both cortically induced fictive mastication and swallowing induced by superior laryngeal nerve stimulation [7]. This suggests that the CPGs for mastication/licking and swallowing share some premotor neurons. The purpose of the present study was to determine the relationship between neural activity during ingestion (licks and swallows) and rejection (gapes) of gustatory stimuli from neurons in the medullary RF compared to a population of mXII neurons in awake, freely-moving animals.