Development, anatomy, and physiology of the upper esophageal sphincter and pharyngoesophageal junction

Abstract

The upper esophageal sphincter (UES) functions to close or to open the esophago-pharyngeal junction as needed. The UES closing muscles include the cervical esophagus, cricopharyngeus (CP), and inferior pharyngeal constrictor, but the primary functional muscle of the UES is the CP. The UES opening muscles include anteriorly the superior and inferior hyoid muscles and posteriorly the stylopharyngeus, palatopharyngeus, and pteropharyngeus. The UES is opened intermittently during various functions by relaxation of its closing muscles, contraction of its opening muscles, and bolus pulsion. The UES closing muscles contain two sets of muscle fibers: an inner layer of slow-twitch fibers and outer layer of fasttwitch fibers. It is hypothesized that these two fiber types serve the two basic functions of the UES closing muscles: slow tone generation and rapid reflex responsiveness. The UES motor and sensory functions are controlled by branches of the glossopharyngeal and vagus nerves. The motor nerve of the CP in animals is the pharyngoesophageal branch of the vagus nerve and may be the recurrent laryngeal nerve in humans. The nucleus ambiguus is the primary motor nucleus of the UES, and the nucleus tractus solitarius is the primary termination site of UES afferents. The UES opens and closes in complex patterns well coordinated with laryngeal movement to prevent aspiration during swallowing, belching, and vomiting. The UES tone increases during various digestive or respiratory tract reflexes to prevent the insufflation of air into the esophagus or the pharyngeal reflux of esophageal contents with possible aspiration. The specific actions of individual muscles of the UES differ among its various functions. The UES is fully functional as early as 33 weeks postmenstrual age although the specific parameters may differ.