Gastrointestinal smooth muscle cells contract as a unit because of anatomic and electrical coupling. Smooth muscles may contract and relax over a few seconds (phasic), or contractions may last from minutes to hours (tonic). Material moves through the esophagus and entire gastrointestinal (GI) tract from regions of higher to regions of lower intraluminal pressure. Primary peristaltic contractions are initiated in the esophagus by swallowing and are responsible for moving most material through the esophagus; secondary peristaltic contractions initiated by distension and local reflexes remove any "leftover" material.
The principal motility function of the orad (proximal) stomach is receptive relaxation, which is mediated by a vagovagal reflex and allows the stomach to store ingested material. The principal activity of the caudal (distal) stomach is mixing and emptying. Gastric contractions are triggered by regularly (3-5 min) occurring depolarizations called slow waves.
Small intestinal motility is characterized by brief, irregular contractions that are interrupted during fasting approximately every 90 min by a wave of intense contractions that sweeps the entire length of the small intestine. After a meal, these
Essential Medical Physiology, Third Edition
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