Effects of HCl on esophageal reflex reponses.

The purpose of this manuscript is to review all of the experimental studies on the effects of intraluminal HCl on defined esophageal reflexes. Intraluminal presence of HCl in the esophagus activates the slowly adapting touch sensitive mechanoreceptors of the mucosa, which are necessary for the esophageal phase of swallowing to always follow the pharyngeal phase. Intraluminal HCl sensitizes and activates the slowly adapting touch/tension mechanoreceptors of the mucosa which stimulate the pharyngeal phase of swallowing, and activate contractions of the esophagus orad of the stimulus in the striated muscle esophagus. Intraluminal HCl also sensitizes and activates the rapidly adapting mucosal mechanoreceptors which mediate relaxation of the upper esophageal sphincter. Intraluminal HCl sensitizes the slowly adapting mechanoreceptors of the muscularis that stimulate contraction of the upper esophageal sphincter, or the mechanosensitive motor neurons that relax the lower esophageal sphincter. Intraluminal HCl has greater effects on mechanoreceptors closer to the lumen. The effects of HCl on the above reflexes occur with low doses over short periods of time whereas higher doses and longer application times inhibit the esophageal mechanoreceptors and associated reflex responses. The esophagus also has chemoreceptors which are activated by HCl that cause contraction of the small airways, increase airway mucous secretion, and salivary secretion, all of which defend against the effects of supra-esophageal reflux. Therefore, the esophagus is well designed with built in defense mechanisms against normal exposure of the esophagus to HCl, however, high levels of HCl can be very destructive to esophageal receptors and their reflex responses.