MEDizzy - A 10-year-old boy consumes a glass of milk and two cookies.

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Gastrointestinal Physiology

A 10-year-old boy consumes a glass of milk and two cookies. His LES and fundus relax while the food is still in the esophagus. Which substance is most likely to cause relaxation of the LES and fundus in this boy?

ExplanationThe fundus of the stomach and lower esophageal sphincter both relax during a swallow while the bolus of food is still higher in the esophagus. This phenomenon is called receptive relaxation. Receptive relaxation is mediated by afferent and efferent pathways in the vagus nerves. Nitric oxide is the neurotransmitter thought to mediate receptive relaxation at the smooth muscle cell. Motilin is a gastrointestinal hormone that mediates migrating motility complexes (also called housekeeping contractions); these contractions occur between meals. Gastrin and histamine do not have significant effects on smooth muscle contraction or relaxation at physiological levels.