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Previous research has shown that applications of chemical stimuli to the epiglottis produced distinct patterns of activity in the lamb superior laryngeal nerve. To determine the response characteristics of second-order neurons, we recorded from single cells in the lamb nucleus of the solitary tract (NST) while stimulating the epiglottis with 0.5 M KCl, NH4Cl, NaCl, LiCl, distilled water, 0.005 M citric acid and 0.01 N HCl. Most neurons responded to more than one of the chemical solutions. The order of effective stimuli was KCl = NH4Cl greater than distilled water greater than HCl greater than citric acid greater than NaCl greater than LiCl. An analysis of the variation in response frequency over time found that different chemical stimuli produced significantly different response patterns in NST neurons. A comparison of the mean neural response profiles of NST neurons and superior laryngeal nerve fibers for each of the stimuli found that only the response profiles elicited by NH4Cl were significantly different. In addition to their responses to chemical solutions, almost one-third of the NST neurons responded to the rinse following application of at least some of the stimuli and 80% of the neurons were excited by mechanical stimulation of the epiglottis with a soft brush. Also, a small number of neurons exhibited a rhythmic response coordinated with respiration. The majority of recording sites were located in areas of the NST linked to swallowing and respiration suggesting that the response patterns of NST neurons elicited by chemical stimulation of receptors on the epiglottis may play a role in upper airway reflexes.

Receptors located in the posterior oral cavity and on the epiglottis play an important role in the initiation of upper airway reflexes such as swallowing, gagging, coughing and apnea. Peripheral nerves which innervate these receptor areas terminate in the nucleus tractus solitarius (NTS). We have recorded the responses of 61 neurons in the lamb NTS to stimulation of the caudal tongue, palate and epiglottis with mechanical, chemical and thermal stimuli and mapped receptive field location. Although there was some overlap in the areas of the NTS from which neurons with oral cavity and epiglottal receptive fields could be recorded, a significant difference was observed in the mean recording sites of the two groups of neurons. Neurons with oral cavity receptive fields were located more rostral, lateral and ventral in the NTS than neurons with receptive fields on the epiglottis. Little convergence of sensory input onto single cells in the NTS was observed between the oral cavity and the epiglottis. Only one NTS neuron had a receptive field in both of these receptor areas. In contrast, a large number of neurons with oral cavity receptive fields received input from two receptor areas. These neurons had a receptive field on the tongue which was located directly beneath the receptive field on the palate. Mechanical stimuli were the most effective for neurons with either oral cavity or epiglottal receptive fields and thermal stimuli were the least effective. Neurons which responded to mechanical stimuli responded better to a moving stimulus than to a punctate one, and large increases in the strength of a punctate stimulus were required to elicit significant increases in response frequency. Most NTS neurons responded to more than one of the stimulus modalities. However, a significant difference in the mean number of stimulus modalities which elicited responses was observed between neurons with oral cavity and epiglottal receptive fields. The number of multimodal neurons with epiglottal receptive fields was higher than those with oral cavity receptive fields. The multimodal nature of neurons which responded to epiglottal or oral cavity stimulation combined with their location in reflexogenic areas of the NTS suggests that these neurons could be important in the integration of afferent input from the oral cavity and upper airway. If these NTS neurons are involved in the control of oral and upper airway reflexes it would be important for them to respond to as many of the stimulus cues as possible and the majority of these neurons do just that.

After injection of [3H]leucine into the nodose ganglion of a rabbit autoradiographic examination of the distribution of vagal afferent fibers in the epiglottal wall revealed many nerve bundles of labeled afferent fibers present in the submucosal plexus between the cartilage tissue and the mucosa. The labeled afferent fibers, most of which were myelinated (while a few were unmyelinated), descended towards the mucosa, and then appeared to demyelinate at the subepithelial layer of the mucosa. The labeled afferent fibers entered the mucosal epithelium, terminated as free endings in the intercellular space among the epithelial cells, and extended near the mucosal surface. Grains were also observed near the taste buds in the epithelium, which suggests that some vagal afferent fibers innervated the epiglottal taste buds.

The purpose of the present investigation was to record electrophysiological responses from single fibers in the hamster superior laryngeal nerve (SLN) that were responsive to chemical stimulation of the larynx. Twenty chemical solutions, commonly used in studies of mammalian gustatory physiology, were applied to taste buds on and around the epiglottis. These stimuli were dissolved in physiological saline. Responses were the number of impulses elicited over a 15-s period following stimulus onset, above or below the background activity elicited by a previous rinse with saline. Unlike fibers in the hamster chorda tympani or glossopharyngeal nerves, SLN units were not easily classifiable into response types. Excitatory stimuli were primarily acids and bitter-tasting stimuli, with the order of their effectiveness being urea much greater than tartaric acid greater than HCl greater than KCl greater than citric acid greater than caffeine greater than quinine hydrochloride greater than acetic acid. The sweet-tasting stimuli and most salts other than KCl were primarily inhibitory, with the order of inhibitory effectiveness being CaCl2 greater than sucrose greater than fructose greater than LiCl greater than NaNO3 greater than Li2SO4 greater than NaCl. A hierarchical cluster analysis of fibers yielded no distinct clusters, yet differing sensitivities across the fibers were suggested. SLN fibers are highly responsive to sour and bitter stimuli, although they are not sensitive to fine differences in taste quality, as are fibers in other gustatory nerves.

The occurrence, distribution and nature of calcitonin gene-related peptide (CGRP) in the tongue, epiglottis and pharynx of the rat was investigated by immunocytochemistry and radioimmunoassay. Numerous CGRP-containing nerves were found to innervate and terminate freely within the epithelium of the tongue, epiglottis and pharynx. Immunoreactive fibres were also found in the muscle layer and around blood vessels in the tongue, and in motor end plates in the muscle of the epiglottis and pharynx. Section of the trigeminal nerve induced a marked reduction in the number of immunoreactive nerves in the anterior portion of the tongue, whereas glossopharyngeal denervation results in a depletion of CGRP immunoreactivity in the posterior portion of the tongue. Immunoreactive nerves of the epiglottis and pharynx were depleted only after superior laryngeal nerve section. A subpopulation of labelled primary sensory neurones were observed in trigeminal and glossopharyngeal ganglia following injection of True blue retrograde tracer in the tongue. Most of the labelled cells were also immunoreactive for CGRP. Following systemic treatment with capsaicin, a loss of intra- and subepithelial CGRP-immunoreactive nerves was observed in all investigated tissues, while immunoreactive motor end plates remained unchanged.