The Colo-Anal Reflex
The propagating motor patterns in the present study induced by prucalopride show a close temporal association with anal sphincter relaxation indicative of the colo-anal reflex, an independent neurally-controlled reflex involving autonomic sacral neural pathways (29, 30) (46) (43) (35) (37). The relaxation was often complete unlike the recto-anal inhibitory reflex (RAIR) which is characterized by a transient involuntary relaxation of the internal anal sphincter in response to rectal balloon distension. The internal anal sphincter will be inhibited by direct activation of intrinsic inhibitory nerves (12) (30) (42) and via parasympathetic activation of myenteric inhibitory nerves (20). Relaxation of the external anal sphincter can be achieved through decrease in the discharge frequency of sacral motor neurons innervating the sphincter (20). The pudendal nerve fibers that reach the external anal sphincter have cell bodies in the Onuf’s nucleus and can be stimulated by parasympathetic nerves, likely via interneurons, from the sacral defecation center (44) (31) (7) resulting in external anal sphincter relaxation.