The striated musculature of the UES, along with the cricoid and thyroid cartilages, the hyoid bone, their ligaments, and the supra- and infrahyoid musculature form a functional unit that selectively controls the opening and closing of the esophageal inlet. At rest, upper esophageal tone acts as a barrier against both entry of air into the esophagus and regurgitation of materials coming from the proximal esophagus. However, reflexive responses of the sphincter offer no substantial protection against regurgitation of stomach contents that cause substantial esophageal distension. Although esophageal distention can experimentally induce sphincter contraction, this potential protective reflex against esophago-pharyngeal reflux has not been observed during spontaneous gastroesophageal reflux episodes. Furthermore, owing to the effects of gaseous distention, regurgitation may be more likely when acid and gaseous contents gastroesophageal reflux together. Modulation of the timing and the extent of sphincter opening during swallowing is intimately correlated with the volume and physical properties of the bolus, along with coordinated activity of the tongue, pharyngeal constrictors, and hyoid musculature, which suggests continuous sensory feedback throughout the pharyngeal swallow.
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