DISTRIBUTION AND MORPHOLOGICAL CHARACTERIZATION OF VISCEROFUGAL PROJECTIONS FROM THE LARGE-INTESTINE TO THE INFERIOR MESENTERIC AND PELVIC GANGLIA OF THE MALE-RAT

Viscerofugal neurons are enteric neurons in the myenteric plexus of the stomach and intestine that project to the prevertebral ganglia as the afferent limb of intestino-intestinal reflexes. This study characterizes viscerofugal projections to the inferior mesenteric ganglion and investigates the possibility of similar projections to the major pelvic ganglia in the male rat. The colon and rectum were examined for retrogradely labelled neurons following the injection of retrograde tracer into the inferior mesenteric or major pelvic ganglia, or following the application of tracer to the caudal end of the cut intermesenteric nerves, or either end of the cut hypogastric nerves. All labelled viscerofugal neurons were found in the myenteric plexus and were often grouped near the mesenteric attachment. The number of viscerofugal neurons projecting to the inferior mesenteric ganglion via the lumbar colonic nerves increases along the length of the large intestine with the maximum number of viscerofugal neurons found in the rectum. Some viscerofugal neurons from the distal colon and rectum reach the inferior mesenteric ganglion via the hypogastric nerves. A similar number and distribution of viscerofugal neurons project via the inferior mesenteric ganglion into the intermesenteric nerves as terminate in the inferior mesenteric ganglion. Very few viscerofugal neurons project to the neurons of the major pelvic ganglia via the rectal nerves, and no viscerofugal neurons project caudally in the hypogastric nerves to these ganglia. The majority of labelled neurons resembled Dogiel type I morphology. Thus the inferior mesenteric ganglion receives a substantial innervation from viscerofugal neurons of the large intestine, with the greatest supply from the distal colon and rectum. However, the major pelvic ganglia do not receive such a supply, suggesting that the sympathetic innervation of the distal bowel by these ganglia may be functionally different from that of the prevertebral supply.