On the transmission of sacral parasympathetic nervous influence on distal colonic and rectal motility in the cat.

Experiments were performed on cats anaesthetized with chloralose and treated with adrenoceptor blocking agents. Distal colonic and rectal motility were selectively recorded by a volumetric method. The effects of muscarinic and ganglionic nicotine receptor blockade on motor responses induced by graded efferent electrical pelvic nerve stimulation (PNS) were studied. Stimulation at low current strength evoked contractions in both the colon and the rectum, which were sensitive to atropine and to hexamethonium. High intensity stimulation elicited distal colonic contractions resistant to both atropine and hexamethonium. Similar excitatory responses to high strength PNS were also observed in the rectum, though not in all experiments. Stimulation at intermediate intensities evoked distal colonic and rectal relaxations which were resistant to atropine but blocked by hexamethonium. The results indicate that PNS influences colonic and rectal motility by activation of at least three discrete non-adrenergic nervous pathways: (1) low-threshold excitatory fibres involving nicotinic and muscarinic transmission, (2) high-threshold excitatory fibres with a non-muscarinic, non-nicotinic transmission mechanism, (3) inhibitory fibres with an intermediate stimulus intensity threshold, exerting their effect by a non-muscarinic mechanism involving a nicotinic step.