Excitatory innervation of the rabbit rectococcygeus muscle by enteric nerves from the terminal large intestine.

The rectococcygeus muscle extends from the longitudinal layer of the external muscles of the terminal colon, anchoring the large intestine to the coccygeal vertebrae. The muscle is thought to assist the colon during defaecation, drawing the large intestine partly into the pelvic cavity. In the rabbit, many fine nerves run between the wall of the colon and a ganglionated nerve plexus on the ventral face of the rectococcygeus muscle. Extracellular recordings of evoked and spontaneously occurring multiunit discharges from these nerve bundles indicated that they may take their origin from the enteric nervous system of the large intestine. Enteric nerves (as we have called them) conducted bursts of spontaneously occurring multiunit discharges, each burst associated with a twisting movement of the rectococcygeus and the colon. Electrical stimulation of enteric nerves elicited cholinergically mediated contractions of the rectococcygeus; however, nerve stimulation failed to elicit relaxation of muscle tone induced by carbachol. Therefore, enteric nerves appeared to supply the rectococcygeus with an excitatory innervation only. These excitatory nerves appeared to remain uninterrupted by a ganglion synapse as they travelled to the rectococcygeus. Stimulation of enteric nerves caused the left and right hemirectococcygeal muscle strips to twist and deform the colo-rectal region to which they were attached. From these results, it is suggested that the rectococcygeus may delay the movement of luminal contents from colon to rectum by forming a weak valve which partially occludes the colo-rectal region. The enteric nerves to the rectococcygeus may assist in the maintenance of continence.(ABSTRACT TRUNCATED AT 250 WORDS)