Physiological Pharmaceutics

152 Physiological PharmaceuticsOnce food waste reaches the sigmoid colon, it remains there until it is ready to beexcreted from the body. As the faecal material enters the rectum, the walls distend toaccommodate it. When sufficient pressure occurs within the distended rectal cavity, the urgeto eliminate wastes begins. When receptors of the nervous system within the rectal wall arestimulated by its stretching, they send impulses to the anal canal, chest and abdominal-wallmuscles, and the medulla of the brain, which makes the individual conscious of the need todefaecate.Electrical activityChanges in the electrical potential of the smooth muscle coat are often used to assessgastrointestinal motility. Two types of electrical activity have been recorded in the colon i)slow wave activity and ii) spike potentials. The former consist of regular phasicdepolarisations of the cell membrane which originate in the circular muscle layer of thetransverse colon 14 . When compared with that of the stomach and small intestine, colonicslow wave activity is of low frequency and irregular. The slow wave pacemaker in theproximal or transverse colon maintains co-ordinated regular activity throughout the colon,and appears, at least in the cat, to migrate towards the caecum 15 16 . The retrogradepropagation of slow waves in the proximal segment allows longer mucosal exposure for theintraluminal contents resulting in more complete absorption 17 . In man, the dominant slowwave frequency is 11 cycles per minute in the transverse and descending colon, slightly lessin the caecum, ascending and sigmoid colon, whilst that in the rectum is the highest observedin the gastrointestinal tract at 17 cycles per min.Spike potentials may be superimposed on the slow waves or may exist as burstsunrelated to slow wave activity, and are thought to initiate functional colonic contractions.Spike bursts of long duration (>10s) increase after eating and may increase luminal transit.Short duration spike bursts (