An update on the physiology of the components of the upper esophageal sphincter.

The upper esophageal sphincter is composed of both opening and closing muscles. The opening muscles include the thyrohyoideus and the geniohyoideus, and the closing muscles include the cricopharyngeus, thyropharyngeus, and cervical esophagus. The relative contribution of each muscle to the opening and closing of the UES varies with the physiologic state. Although the cricopharyngeus may be the primary muscle of the UES, it cannot account for all of the observed functions of the UES.