= Rectoanal inhibitory reflex
The <rectoanal inhibitory reflex> is relaxation of the <internal anal sphincter> in response to distension of the <rectum>. Because the <internal anal sphincter> is <smooth muscle>, this relaxation is distinct from voluntary control of the <external anal sphincter>. In <manometry>, a rectal balloon supplies a controlled distension while anal <pressure> sensors record the response over time.<Rectoanal sensorimotor response in humans during rectal distension by Sherrie De Ocampo and others>[2007, Methods and Results: graded balloon distensions and six-sensor recordings in 23 healthy subjects]{ref}
A fall in <pressure> can coexist with a contractile response elsewhere or at another time. The same study measured inhibitory and contractile reflexes and asked subjects about sensation; a relaxation trace therefore cannot simply be equated with the conscious urge to empty the <rectum>.<Rectoanal sensorimotor response in humans during rectal distension by Sherrie De Ocampo and others>[Results: inhibitory, contractile and perception-associated sensorimotor responses]{ref}
A complementary experiment recorded tension in isolated human <internal anal sphincter> strips. A donor of <nitric oxide> relaxed the strips; blocking its synthesis suppressed relaxation evoked by electrical nerve stimulation, and <arginine> reversed the inhibition in a stereospecific control. These interventions support a role for <nitric oxide> in nerve-mediated relaxation. An isolated strip identifies a mechanism in the effector <muscle>; it does not reproduce the whole intact reflex circuit.<Nerve mediated relaxation of the human internal anal sphincter: the role of nitric oxide by T. O'Kelly and others>[1993, abstract: superfused strips, field stimulation, synthesis inhibitors and L- versus D-arginine]{ref}
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