Study of the mechanism underlying the difference in motility between the large and small intestine: the "single" and "multiple" pacemaker theory.

The common movement of the small intestine (SI) is peristalsis and of the large intestine (LI) are giant migrating contractions. The mechanism underlying the difference in the type of motility between the SI and LI is yet to be elucidated; the current communication investigated this point. 7 pigs (4 male) were anesthetized, abdomen opened and a balloon-ended catheter was introduced into the right colon through ileotomy. Balloon was filled in increments of 10 ml of saline and the EMG activity of the colonic longitudinal and circular muscle layers was recorded before and after myotomy performed between 2 of the 3 electrodes fixed to the colon. The balloon was then withdrawn, located in the terminal ileum and distended in increments of 2 ml; the ileal EMG activity was registered before and after myotomy done between 2 of the 3 electrodes applied to the ileum. The LI showed slow waves or pacesetter potentials (PPs) and action potentials (APs) which had the same frequency, amplitude and conduction velocity from the 3 electrodes of the same animal. The waves were recorded from the longitudinal and not the circular muscle coat. Upon LI distension, the electric activity increased and was recorded also from the circular muscle. At 40-50 ml distension, the balloon was dispelled to the transverse colon. Electric activity from SI was similar to that of the LI, but was not the same from the 3 electrodes; it diminished aborally. It increased with increasing balloon distension until, at 8-10 ml distension, the balloon moved slowly aborally. Electric waves were recorded proximally but not distally to colonic myotomy, and the balloon moved up to the cut. In the SI, waves were recorded both proximally and distally to the ileal myotomy, and the balloon moved across the cut. The fact that the colonic electric waves displayed the same variables from the 3 electrodes and that they were not recorded distally to the colonic myotomy, would suggest the presence of a "single" colonic pacemaker, probably situated in the cecum. This is in contrast to the hypothesis of the "multiplicity" of the small intestinal pacemakers, which is based on the fact that the electric activity diminished as the waves propagated aborally and on the existence of slow waves distal to the ileal myotomy. The concept of "single" and "multiple" pacemakers explaining the difference in the motility of small from that of large gut needs further studies.