Intracellular microelectrodes and organ bath techniques were used to study spontaneous cyclic electrical and mechanical activity in the rat colon. Electron microscopy and immunohistochemical studies showed two major populations of interstitial cells of Cajal (ICC): one associated with Auerbach's plexus (ICC-AP) and one with the submuscular plexus (ICC-SMP). The ICC-SMP network partly adhered to the submucosa when removed and was generally strongly damaged after separation of musculature and

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... ucosa. Similarly, longitudinal muscle removal severely damaged AP. Two electrical and mechanical activity patterns were recorded: pattern A, low-frequency (0.5-1.5 cycles/min), high-amplitude oscillations; and pattern B, highfrequency (13-15 cycles/min), low-amplitude oscillations. Pattern A was recorded in preparations with intact AP but absent in those without intact AP. Pattern B was recorded in preparations with intact SMP but was absent in those lacking SMP. With full-thickness strips, the superimposed patterns A and B were recorded in circular muscle. When longitudinal muscle mechanical activity was recorded, only pattern A was present. We conclude that two pacemakers regulate rat colonic cyclic activity: the ICC-SMP network (responsible for cyclic slow waves and small-amplitude contractions) and the ICC-AP network (which may drive the cyclic depolarizations responsible for high-amplitude contractions). This is the first report showing consistent slow wave activity in the rodent colon. interstitial cell of Cajal; slow waves; smooth muscle THE MEMBRANE POTENTIAL OF smooth muscle cells from the gastrointestinal tract usually displays rhythmic slow waves. These slow waves correlate with cyclic contractions in many gastrointestinal smooth muscles. For a long time, the cyclic activity was believed to be "myogenic" in origin because neural blockers were not able to modify rhythmicity. Interstitial cells of Cajal (ICC) were described for the first time by Cajal (3). In 1982 (28), it was suggested that ICC were probably