Functional anatomy and physiology of the upper esophageal sphincter.

Upper esophageal sphincter (UES) refers to the high-pressure zone located in between the pharynx and the cervical esophagus. The physiological role of this sphincter is to protect against reflux of food into the airways as well as prevent entry of air into the digestive tract. UES is a musculocartilaginous structure with its anterior wall being formed by the full extent of the posterior surface of the cricoid cartilage and arytenoid and interarytenoid muscles in the upper part. Posteriorly and laterally the cricopharyngeus (CP) muscle is a definitive component of the UES. CP has many unique characteristics: it is tonically active, has a high degree of elasticity, does not develop maximal tension at basal length, and is composed of a mixture of slow- and fast-twitch fibers, with the former predominating. These features enable the cricopharyngeus to maintain a resting tone and yet be able to stretch open by distracting forces, such as a swallowed bolus and hyoid and laryngeal excursion. CP, however, constitutes only the lower one third of the entire high-pressure zone. The thyropharyngeus (TP) muscle accounts for the remaining upper two thirds of the UES. The UES pressure is not entirely the result of myogenic activity, as a component of the pressure is the result of passive elasticity of the tissues. The opening of the UES involves relaxation of CP and TP muscles and forward movement of the larynx by the contraction of hyoid muscles. The UES function is controlled by a variety of reflexes that involve afferent inputs to the motorneurons innervating the sphincter. These physiological reflexes elicit either contraction or opening of the UES. Inability of the sphincter to open leads to difficulty in swallowing. Opening of the sphincter without associated CP relaxation leads to the clinical syndrome of cricopharyngeal bar.