Abstract
In the present study, we investigated how taste stimuli applied to the anterior tongue surface and hypopharyngeal area affected swallowing behaviors in humans. Twenty-nine healthy volunteers were instructed to repeat swallowing as quickly as possible, and swallowing intervals between the fourth and ninth swallow were recorded. During the swallowing test, a taste stimulant was applied to either the oral or hypopharyngeal cavity. For pharyngeal stimulation, distilled water (DW) or 0.3 M NaCl solution was applied at 0.2 mL/min; 0.3 M NaCl solution was used because it is known to completely inhibit water-sensitive responses in the pharyngeal region. For oral stimulation, DW, NaCl solution, or umami solution was applied at the same infusion rate. For NaCl and umami solution, the concentrations of sodium ion used were 6 mM, 40 mM, or 240 mM. The concentration of sodium ion was varied to observe effects on swallowing behaviors. During pharyngeal 0.3 M NaCl or DW stimulation, a wide variation in swallowing intervals was seen among the subjects. The mean swallowing interval was significantly shorter during DW stimulation compared with 0.3 M NaCl stimulation. The longer the swallowing interval with infusion of 0.3 M NaCl solution, the stronger the facilitation of swallowing by DW stimulation. During oral taste stimulation, swallowing interval for umami solution was significantly shorter in a concentration-dependent manner, but this was not the case for NaCl solution. Subjects were then divided into three groups depending on the swallowing interval during 0.3 M NaCl pharyngeal stimulation, high (< 7s), middle (7-11s), or low (> 11s) performance groups. Only middle group exhibited concentration-dependent facilitation by umami stimulation. In addition, the facilitatory effect was different between oral and pharyngeal stimulation, in that there was no significant relationship between the swallowing interval and facilitatory effect with umami solution applied to the oral region. The current results demonstrated facilitatory effects of umami solution applied to the oral region on swallowing behaviors but the effect was not just summative on the swallowing center and the nature of taste inputs on the swallowing network should be discussed.
