The rectal expansion and concomitant sensory function on graded, isobaric, rectal distension within the interval 5-60 cm H2O was investigated in 36 healthy young volunteers. Anal pressure and electromyography (EMG) from the external anal sphincter were simultaneously recorded. Rectal distension caused an initial rapid expansion followed by transient, often repeated, reflex rectal contractions and a slow gradual increase of rectal volume. The maximal volume displaced by the first reflex rectal contraction was 18 (13) ml, which was less than 10% of the volume at 60 s. The pressure threshold for appreciation of rectal filling was 12 cm H2O (95% CL 5-15 cm H2O) and coincided with the threshold for rectoanal inhibition. Urge to defecate was experienced at 28 cm H2O (15-50 cm H2O) distension pressure, which was close to the threshold for maximal rectal contraction, also coinciding with the appearance of the external anal sphincter reflex. The interindividual variation of rectal volume on distension with defined pressures varied widely, indicating a considerable variation of rectal compliance in normal man. No correlation was found between rectal volume and sex or anthropometric variables. The relative variations in pressure thresholds for eliciting rectal sensation and rectoanal reflexes were less than the corresponding threshold volumes. It was concluded that the dynamic rectal response to distension reflects a well graded reflex adjustment ideal for a reservoir.
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