Peristalsis is a unique pattern of smooth muscle contractions that propel ingested food distally through the esophagus and intestines. The rippling motion of muscles mixes food with gastric juices, turning it into a thin liquid. It drives the food from the esophagus (called bolus there), through the stomach (called chime at that time) to the rectum. The muscles contract in a unidirectional way to keep the food from moving retrogradely.
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Main points of gastrointestinal (GI) motility
GI motility results from coordinated contractions of the tunica muscularis, the muscular layers of the digestive tract. The muscular layer is comprised of two types of muscle bundles: circularly and longitudinally oriented. The smooth muscle cells form mechanical and electrical connections between cells to support the series of contractions.
Entry of Ca2+ through ion channels located in the plasma membrane increases the intracellular concentration of Ca2+ which is the starting point of contraction. Myosin light chain kinase is activated by Ca2+ binding to calmodulin. Subsequent phosphorylation of myosin begins the cross-bridge cycling. The dephosphorylation of myosin by Myosin phosphatase results in the relaxation of muscles, and the activity of the phosphatase is regulated by a process known as Ca2+ sensitization. The smooth muscles of the GI tract are ‘autonomous’ and spontaneous electrical activity (slow waves) is generated, that does not depend upon the input from nerves.
The natural pacemaker activity comes from interstitial cells of Cajal, which are electrically linked to the smooth muscle cells. The patterns of contractions in the gastrointestinal tract are determined by inputs from enteric motor neurons that innervate the interstitial cells and smooth muscle cells. In this animated video, we tried our best to visualize these cells and the complex process of the contractile activity of the smooth muscle.