 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.^[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.^[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.^[ref]