Effects of rectal distensions on nociceptive flexion reflexes in humans

We previously showed that gastric distension inhibits the somatic nociceptive flexion RIII reflex. To explore further the viscerosomatic interactions, we tested in the present study the effects of rectal distensions on RIII reflexes. Rapid and slow-ramp rectal distensions were performed in 10 healthy volunteers with an electronic barostat. The RIII reflex was continuously recorded from the lower limb during both types of distension and from the upper limb during rapid distensions. The visceral sensations were scored on a graded questionnaire. Rapid distensions facilitated the RIII reflex recorded from the lower limb, but at the highest distension level, facilitation was followed by inhibition. Slow-ramp distension induced gradual inhibition of the RIII reflex, which correlated with both distension volume and visceral sensation. RIII reflex recorded from the upper limb was also inhibited by rapid rectal distensions. Reflex inhibitions were probably related to the activation of pain modulation systems. One plausible explanation for the facilitatory effects, observed only at the lower limb, is the convergence of rectal and reflex afferents at the same levels of the spinal cord. The differential effects of rapid and slow-ramp distensions suggest the activation of two distinct populations of mechanoreceptors by these two modes of distension.