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.