Dissociation of the ascending excitatory reflex from peristalsis in the guinea-pig small intestine

Localized distension of the intestine evokes an ascending excitatory reflex and a descending inhibitory reflex in the circular muscle layer. The sequential activation of these two reflexes is believed to underlie the motor pattern of peristalsis, which is responsible for the co-ordinated propulsion of intestinal contents. In this study we have shown that the initiation of peristalsis involves mechanisms additional to those mediating the ascending excitatory reflex. A short length of guinea-pig small intestine was mounted in a partitioned organ bath so that the lumen was occluded by the partition, but neuronal continuity was maintained. The anal segment was distended by intraluminal fluid infusion to evoke a peristalsis; in the oral segment, an isotonic transducer was used to record circular muscle contractions due to ascending excitatory reflexes. Stepwise distension of the anal segment with 5 μl increments at 10s intervals, or with a large, single-step infusion, elicited both the ascending excitatory reflex and peristalsis, when carried out at 3 min intervals. The threshold volume for the ascending excitatory reflex was smaller than the threshold for peristalsis with either incremental or single-step distensions. The ascending excitatory reflex appeared with a shorter delay than peristalsis. Tetrodotoxin (0.6 μM) or hexamethonium (100 μM) added to the oral compartment abolished the ascending excitatory reflex but not peristalsis. These drugs abolished both the ascending excitatory reflex and peristalsis when added to the anal compartment. When stimuli were delivered at 1 min intervals, peristalsis failed completely after the first trial, but the ascending excitatory reflex persisted, at a slightly reduced amplitude. When the anal segment was distended to just-subthreshold volume, electrical field stimulation (0.25-0.5 ms, 1-5 Hz for 1 s), delivered at 3 min intervals, evoked ascending excitatory responses but not peristalsis. Higher frequency stimulation (10 Hz) consistently evoked both peristalsis and the ascending excitatory responses. When trains of electrical stimulation were repeated at 1 min intervals, peristalsis quickly failed, but the ascending excitatory response persisted, although reduced in amplitude. The initiation of peristalsis can be dissociated from the ascending excitatory reflex by its threshold volume, by the duration of distension or the intensity of electrical stimulation required, and by its susceptibility to fatigue with repeated mechanical or electrical stimuli. This suggests that the ascending excitatory reflex may be part of the mechanism underlying the initiation of peristalsis, but that additional mechanisms must also be involved. Peristalsis should not be regarded as a reflex response but rather as an all-or-nothing motor pattern, triggered by mechanical stimulation, similar to other co-ordinated motor patterns in vertebrates and invertebrates.