1. Mammalian taste receptors are distributed within several distinct subpopulations, innervated by branches of cranial nerves VII, IX, and X. Most gustatory electrophysiology has focused on input from the fungiform papillae on the anterior portion of the tongue, carried by the chorda tympani branch of the VIIth nerve. However, laryngeal taste buds in the hamster are as numerous as those in the fungiform papillae. Gustatory fibers in the hamster's chorda tympani and glossopharyngeal nerves have been well characterized. In comparison with these taste fibers, much less is known about the chemical sensitivities of fibers innervating laryngeal taste buds. 2. Action potentials were recorded from 65 individual fibers in the superior laryngeal nerve (SLN) of the hamster. Stimuli were distilled H2O and five concentrations each of sucrose, NaCl, HCl, and quinine hydrochloride (QHCl). All stimuli except the NaCl series were made in physiological saline (0.154 M NaCl) and were delivered from the laryngeal side of the epiglottis via a tracheal cannula. Responses were quantified as the number of impulses in 10 s minus the responses in the preceding 10 s of baseline activity during a rinse with physiological saline. 3. Distilled H2O, HCl, and NaCl were by far the most excitatory stimuli, with mean responses across all cells 5-10 times greater than those evoked by sucrose or QHCl. The order of effectiveness of the strongest concentrations of the stimuli was H2O > 0.03 M HCl > 1.0 M NaCl >> 0.03 M QHCl > 1.0 M sucrose. 4. The mean concentration-response function for NaCl was U shaped, with the greatest number of impulses to distilled H2O and 1.0 M NaCl. The responses diminished as the concentrations approached physiological levels (0.154 M NaCl), where there was no response, and increased as NaCl concentration rose above this level. Increasing concentrations of HCl above 0.0003 M elicited increasing responses in these fibers. 5. The mean time course of the responses to distilled H2O and to hypotonic NaCl solutions (0.01 and 0.03 M) peaked in the first few seconds and then declined slowly. This was distinct from the time course of the responses to hypertonic NaCl concentrations (0.3 and 1.0 M), which increased gradually throughout the 10-s response period. Responses to HCl peaked in the initial second and then decayed rapidly to a slowly declining plateau. These distinctively different time courses suggest different receptor mechanisms for water, salt, and acid stimuli. 6. The across-fiber pattern of the responses to hypotonic NaCl solutions correlated strongly to that elicited by distilled H2O. The strongest concentrations of HCl evoked patterns that were highly correlated with one another, as did the two strongest concentrations of QHCl. Thus stimuli similar in quality produced comparable patterns of activity across these fibers. 7. The fibers were classified according to their best response to distilled H2O, 0.03 M HCl, 1.0 M NaCl, 0.03 M QHCl, and 1.0 M sucrose, resulting in 26 H2O-, 20 HCl-, 17 NaCl-, and 2 QHCl-best cells. Although the fibers could be grouped in this fashion into best-stimulus classes, a hierarchical cluster analysis did not suggest distinct clusters of fibers, as are seen in other gustatory nerves. A factor analysis of the fiber response profiles resulted in three orthogonal common factors, representing fibers predominantly sensitive to H2O, HCl, and NaCl, with many profiles distributed continuously between these extremes. Thus three sets of sensitivities (to water, salt, or acid) arise from the laryngeal receptors and are distributed across these first-order afferent fibers in an overlapping, continuous fashion. 8. The relative sensitivities of hamster VIIth and IXth nerve fibers have suggested different functional roles for these two gustatory nerves, with the VIIth relatively more sensitive to appetitive stimuli (sucrose and NaCl) and the IXth to aversive stimuli (HCl and QHCl). The SLN is predominantly sensitive to water, salt, and acid. These and other considerations support the notion that the SLN is particularly important for providing information about changes in the chemical milieu of the larynx. This information is probably more important for airway protection than for the processing of gustatory quality. Thus gustatory afferent input from the various populations of taste buds provides several kinds of information, which may subserve a number of functional roles.
