Rectal sensitivity assessed by a reflexologic technique: further evidence for two types of mechanoreceptors

We previously showed that slow-ramp rectal distensions induce graded inhibi tions of the somatic nociceptive RIII reflex recorded from the lower limb, which correlated with both distension volume and visceral sensation. In con trast, rapid phasic rectal distensions induced facilitatory or biphasic eff ects (i.e., facilitations followed by inhibitions) depending on the level o f distension. To examine the role of mucosal and serosal rectal mechanorece ptors in these viscerosomatic interactions, we analyzed, in six healthy vol unteers, the effects of both types of rectal distension on the RIII reflex after topical application of lidocaine or placebo administered in a double- blind and crossover fashion. Inhibitions of the RIII reflex induced by both slow-ramp and rapid distensions were strongly reduced after administration of lidocaine but not after placebo. In contrast, facilitations of the RIII reflex observed during the initial phase of rapid distensions were not mod ified after lidocaine or placebo applications. These results suggest that i nhibitions, but not facilitations, of the nociceptive RIII reflex triggered by rectal distensions depend preferentially on the activation of superfici al mucosal receptors. This reflexologic technique might thus represent an i nteresting tool for studying the role of the different rectal mechanorecept ors involved in visceral sensations.