Regional gradient of initial inhibition and refractoriness in esophageal smooth muscle

The influence of two successive vagal stimuli on esophageal contractions was studied by recording intraluminal pressures in the smooth muscle portion of the opossum esophagus. The esophageal contraction in response to the first or second stimulus in a pair of vagal stimuli was inhibited depending on the interstimulus interval, the frequency of the stimulus, and the esophageal site. The esophageal contraction in response to the first vagal stimulus was inhibited by a second vagal stimulus if the latter stimulus was applied before the peak of the first contraction. This phenomenon is termed initial inhibition. Initial inhibition is a graded phenomenon. It was greater at higher frequencies than at lower frequencies (p less than 0.001), and was significantly greater in the distal esophagus than in the proximal esophagus (p less than 0.01). The term "refractoriness" has been used to denote inhibition of the second esophageal contraction by the first. Refractoriness was observed during and beyond the duration of the first esophageal response. Refractoriness was also observed at all esophageal levels; however, the interstimulus intervals that demonstrated refractoriness were significantly greater in the distal than in the proximal esophagus (p less than 0.01). Refractoriness was complete (effective refractory period) during the ascending phase of the first contraction. Refractoriness was incomplete after the peak of the contraction (relative refractory period). These studies show gradients of durations and degrees of initial inhibition and refractoriness along the esophagus. The gradient is responsible for the peristaltic nature of esophageal contraction. The gradients of initial inhibition and refractoriness determine esophageal response to multiple successive swallows.