Except in the esophagus and the proximal poion of the stomach, the smooth muscle of the gastrointestinal tract has spontaneous rhythmic fluctuations in membrane potential between about -65 and -45 mV. This basic electrical rhythm (BER) is initiated by the interstitial cells of Cajal, stellate mesenchymal pacemaker cells with smooth muscle-like features that send long multiply branched processes into the intestinal smooth muscle. In the stomach and the small intestine, these cells are located in the outer circular muscle layer near the myenteric plexus; in the colon, they are at the submucosal border of the circular muscle layer. In the stomach and small intestine, there is a descending gradient in pacemaker frequency, and as in the hea, the pacemaker with the highest frequency usually dominates. The BER itself rarely causes muscle contraction, but spike potentials superimposed on the most depolarizing poions of the BER waves do increase muscle tension. The depolarizing poion of each spike is due to Ca2+ influx, and the repolarizing poion is due to K+efflux. Many polypeptides and neurotransmitters affect the BER. For example, acetylcholine increases the number of spikes and the tension of the smooth muscle, whereas epinephrine decreases the number of spikes and the tension. The rate of the BER is about 4/min in the stomach. It is about 12/min in the duodenum and falls to about 8/min in the distal ileum. In the colon, the BER rate rises from about 2/min at the cecum to about 6/min at the sigmoid. The function of the BER is to coordinate peristaltic and other motor activity; contractions occur only during the depolarizing pa of the waves. After vagotomy or transection of the stomach wall, for example, peristalsis in the stomach becomes irregular and chaotic.REF: GANONG&;S REVIEW OF MEDICAL PHYSIOLOGYKIM BARRETT, HEDDWEN BROOKS, SCOTT BOITANO, SUSAN BARMAN, TWENTY-THIRD EDITION PAGE NO:470