Sensory and motor responses to rectal distention vary according to rate and pattern of balloon inflation.

Anorectal motor activity and rectal sensation were recorded in 12 normal male subjects during ramp distention of the rectum with water and air at randomized rates of 10, 20, 50, and 100 mL/min and during intermittent rapid distention with air. There were no significant differences between the results of ramp inflation with water or with air, and the repeated infusion of the same medium yielded reproducible results. Ramp distention induced sigmoid pressure-volume profiles. Different sensations occurred at specific points on the pressure-volume curve and were maintained until succeeded by the next sensation. Initial perception of the distention occurred during the initial steep pressure increase, the sensation of wind occurred during the plateau phase, and the desire to defecate occurred at the onset of the final rapid ascent. Rectal sensations were induced at lower volumes at low infusion rates when the slope of the pressure-volume relationship was shallower than at high infusion rates. This suggests that the receptor triggering rectal sensation is not a simple volume or pressure receptor, but is more likely to be a slowly adapting mechanoreceptor lying parallel to the circular muscle of the rectal wall. During rapid intermittent distention, the rectal volumes required to elicit rectal sensations were lower than during ramp distention, although the pressure-volume curve was steeper. Moreover, sensations often only lasted a short period of time but recurred on deflation. These data suggest activation of an additional population of rapidly adapting or high threshold mechanoreceptors. Anal relaxation was always evoked by intermittent rectal distention and was almost always associated with a rectal sensation and an increase in external anal sphincter activity. In contrast, anal relaxation could be absent or delayed during ramp inflation, especially at lower infusion rates, suggesting that internal sphincter can maintain continence for a long period of time while the rectum is slowly filling. Rectal sensation and concomitant external anal sphincter activity was not associated with anal relaxation during ramp inflation; most subjects felt the sensation long after the pressure reached its lowest level. However, under all circumstances the onset of rectal sensation was associated with an increase of external anal sphincter electrical activity. In conclusion, the rectal sensory and anorectal motor responses to distention depend on the rate and pattern of distention, which may activate a different population of receptors. Results from different laboratories cannot be compared directly unless the pattern and rate of distension are the same.