An overview of the upper esophageal sphincter.

The anatomy and physiology of the upper esophageal sphincter (UES) are complex. Much progress has been made over the past few years in our understanding this important sphincter. The closing muscles of the UES include the inferior pharyngeal sphincter, the cricopharyngeus (CP), and the cervical esophagus. The CP is composed of two parts, which may have different functions. The CP is innervated by the pharyngeal plexus and the recurrent laryngeal nerve. Tone of the UES is probably generated by numerous reflexes rather than by specific tone-generating circuitry. The major tone-generating muscle is the CP, which is highly elastic and produces more active tension the more it is stretched. The UES opens by relaxation of the closing muscles, traction by muscles attached to the hyoid bone and thyroid cartilage, and pulsion of the bolus. The various muscles of the UES behave differently during its many dynamic states, so that similar functions are accomplished by different muscles.