Evidence for a dual pelvic nerve influence on large bowel motility in the cat

Abstract

The effects of efferent electric pelvic nerve stimulation on colorectal motility and blood flow with emphasis on the motor responses in consecutive colonic and rectal segments were studied in anesthetized cats. Selective pharmacological blockade and graded nerve stimulations might reveal the presence of functionally differentiated efferent fibers controlling colonic motility. Pelvic nerve stimulation induced immediate and sustained colorectal contractions and a simultaneous increase of the over-all colonic blood flow. The excitatory responses declined immediately on cessation of a shortlasting stimulation (< 2 min); after a longlasting one, the rectal contraction was maintained for several min. The colonic contraction on pelvic nerve stimulation remained unchanged after atropine but was delayed in onset. In the transverse and distal colon it was preceded by a relaxation which was most pronounced in the distal part. The vasodilator response was unchanged. After atropine the rectal segment showed a purely relaxatory response. Despite continuous pelvic nerve stimulation the relaxation vanished, and rectal volume returned to resting level within 3-5 min. On cessation of such a prolonged stimulation there was a marked rectal after-contraction. The excitation thresholds for the efferent nerve fibers eliciting these different responses could not be separated. The motility and the vasodilator responses were not influenced by adrenergic or by serotoninergic blockade. Apparently, direct preganglionic stimulation of the cat pelvic nerves activates intramural cholinergic excitatory neurons as well as non-cholinergic excitatory neurons, and furthermore, non-adrenergic non-cholinergic inhibitory neurons, which together result in most complex colonic and rectal motor responses. From a functional point of view these centrally controlled responses may well be independently controlled by separate preganglionic neurons though they do not differ concerning excitation thresholds. The effects are consistent with a dual function of the distal colon and rectum. Such a dual parasympathetic influence on the large bowel simulates the vagal control of the stomach, where specific vagal relaxatory fibers convey a reflex widening of the corpus-fundus reservoir during food intake.