Swallowing reflex is known to be evoked by gastroesophageal regurgitation or esophageal stimulation in animal studies. However, details regarding the stimulating material, bolus size, and stimulation area remain unclear for the stimulation-induced type of swallowing reflex in human. Here, we evaluated the effects of different kinds of stimulation via water and air injection of the esophagus on the initiation of the swallowing reflex. Nine healthy individuals participated in this study. A fiberoptic endoscope was passed transnasally and a thin catheter for injection was passed through the other side. The tip of the catheter was placed at the upper, upper middle, lower middle, or lower region of the esophagus and the rate of injection was controlled at 0.2 mL/s. Swallowing-reflex latency was calculated as the time from injection via air or thin/thick fluid until the onset of white-out in endoscopic images. Reflex latency was significantly shorter when injection occurred at the upper region of the esophagus than at the lower region, for both thin and thickened fluids (p<0.01). At the upper region of the esophagus, the latency was significantly shorter after injection with thin fluid than with thickened fluid (p<0.05). Injection with air did not induce the swallowing reflex at all sites. These findings suggest that while the swallowing reflex is evoked by varying types of stimulation via fluid injection of the esophagus in human, sensitivity is greatest in the upper region of the esophagus compared with the lower region, and can vary depending on the injecting material.
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