THE ROLE OF ASCENDING EXCITATORY AND DESCENDING INHIBITORY PATHWAYS IN PERISTALSIS IN THE ISOLATED GUINEA-PIG SMALL-INTESTINE

1. The effects of experimental manipulations to alter the activation o f ascending excitatory and descending inhibitory reflexes on peristals is were studied in isolated segments of guinea-pig small intestine. 2. The normal site of initiation of the peristaltic contraction, the ora l end, was always shifted to a point just anal to a crush which interr upted enteric neuronal pathways. 3. Shortening the functional length o f the intestine by a series of crushes, thus effectively reducing the length of enteric neuronal refer pathways, led to a progressive increa se in the threshold distension for triggering the peristaltic contract ion. 4. A sleeve placed around the intestine to prevent it from being distended by fluid led to a shift in the site of initiation of the per istaltic contraction to a point just anal to the sleeve. Furthermore, sleeves placed in the anal half of the intestine were able to stop pro pagation of the contraction. 5. The effect of these manipulations on p eristalsis suggests that ascending excitatory and descending inhibitor y enteric pathways, activated by distension, determine the pattern of peristaltic activity. The peristaltic contraction is initiated at the oral end as a result of the summation of ascending excitatory inputs a nd the relative absence of inputs from descending inhibitory pathways at this point. The magnitude of the distension for triggering this con traction is determined by a balance between ascending excitatory and d escending inhibitory inputs to the circular muscle. Propagation of the circular muscle contraction requires the activation of ascending exci tatory pathways at each point along the intestine and the sequential i nactivation of the descending inhibitory reflex pathways anal to the c ontraction. The propagation of the circular muscle contraction stops w hen there is no longer a sufficient distension stimulus ahead.