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lower esophageal sphincter

(cardiac sphincter, gastroesophageal sphincter, LES) A specialized segment of the circular muscle layer of the distal (low end) esophagus, where it meets the stomach. It is not under voluntary control. Together with the crural diaphragm, it is involved in swallowing and vomiting, and contributing to the antireflux barrier protecting the esophagus from the caustic gastric content. (Muscles at the gastroesophageal junction) • lower esophageal sphincter: smooth muscles, internal lower esophageal sphincters, controlled by autonomic nerves (vagus nerves). • crural diaphragm: skeletal muscle, external lower esophageal sphincters, controlled by somatic nerves (phrenic nerves). During swallowing or belching, these muscle must relax briefly in order to allow passage of food or intragastric air. These swallow-induced relaxation and prolonged transient lower esophageal sphincter relaxation (TLESR) result from innervation of the inhibitory motor neurons of the sphincter. The main neurotransmitter released by the inhibitory motor neurons is nitric oxide. The axons that supply this have bodies situated in the dorsal motor nucleus of the vagus nerve. (Summary on sphincter relaxation) • Swallowing food (in): swallow-induced relaxation • Belching intragastric air (out): prolonged transient lower esophageal sphincter relaxation (Examples of dysfunction) • achalasia: Impaired relaxation by reduction or absence of the inhibitory innervation. Dysphagia and stasis of food in the esophagus. • gastroesophageal reflux disease (GERD): Failure of the antireflux barrier. Increased exposure of the esophagus to gastric acid. Cf. upper esophageal sphincter