Sympathetic efferent pathways projecting to the vagina in the dog

The level of the thoracolumbar sympathetic outflow projecting to the vagina and the laterality of the signals passing through the splanchnic nerves to the vagina have not been well understood. This study was undertaken to explore the sympathetic innervation of the canine vagina by measuring intraluminal pressure of the vagina and contraction of the vaginal wall by electrical stimulation of each of the thoracolumbar splanchnic nerves with or without transection of unilateral hypogastric nerve (HGN). The 2nd-4th lumbar splanchnic nerve (LSN) stimulation elicited elevation of intraluminal pressure of the vagina and contraction of the vaginal wall. In contrast, intermesenteric plexus stimulations caused no response in both the vaginal pressure and the wall itself. On the right side, 12 of 27 LSNs that were stimulated elicited elevation of vaginal pressure and 16 of 32 LSN stimulations caused contraction of the wall, 10 increases in tension and 6 reductions. On the left side, 14 of 26 LSN stimulations elicited elevation of vaginal pressure and 12 of 27 LSN stimulations caused contraction of the wall, 5 increases in tension and 7 reductions. After transection of the right HGN, 6 of 14 right LSN stimulations elicited elevation of vaginal pressure and 10 of 15 right LSN stimulations caused contraction of the wall, 9 increases in tension and 1 reduction. After transection of the left HGN, 13 of 19 left LSN stimulations elicited elevation of vaginal pressure and 10 of 18 left LSN stimulations caused contraction of the wall. 4 increases in tension and 6 reductions. The results indicate that each of the L2-L4 LSNs on either side sends signals to the canine vagina via multiple routes with a crossing site at the level of the caudal mesenteric plexus (CMP).