EFFECTS OF RECTAL DISTENSIONS ON NOCICEPTIVE FLEXION REFLEXES IN HUMANS

We previously showed that gastric distension inhibits the somatic noci ceptive flexion RIII reflex. To explore further the viscerosomatic int eractions, we tested in the present study the effects of rectal disten sions on RIII reflexes. Rapid and slow-ramp rectal distensions were pe rformed in 10 healthy volunteers with an electronic barostat. The RIII reflex was continuously recorded from the lower limb during both type s of distension and from the upper limb during rapid distensions. The visceral sensations were scored on a graded questionnaire. Rapid diste nsions facilitated the RIII reflex recorded from the lower limb, but a t the highest distension level, facilitation was followed by inhibitio n. 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 r ectal distensions. Reflex inhibitions were probably related to the act ivation of pain modulation systems. One plausible explanation for the facilitatory effects, observed only at the lower limb, is the converge nce of rectal and reflex afferents at the same levels of the spinal co rd. The differential effects of rapid and slow-ramp distensions sugges t the activation of two distinct populations of mechanoreceptors by th ese two modes of distension.