Functional differences between afferent fibers in the hypogastric and pelvic nerves innervating female reproductive organs in the rat.

1. The uterus, cervix, and vaginal canal are innervated by afferent fibers in the hypogastric and pelvic nerves. Four studies compared the innervation territory and sensitivity to peripheral stimuli of the two sets of fibers in adult virgin rats. 2. Innervation territory was studied anatomically by injecting different fluorescent dyes into different parts of the reproductive, lower urinary, and lower digestive tracts and examining retrogradely labeled neurons in dorsal root ganglia. It was also studied electrophysiologically in anesthetized rats by summing potentials evoked in branches of the two nerves by electrical stimulation of different parts of the reproductive tract. 3. In both studies sensory innervation of the reproductive tract shifted from the pelvic to the hypogastric nerve (i.e., shifted entry into the spinal cord from the L6-S1 to the T13-L3 dorsal root ganglia, respectively) as the dye or stimulating electrode shifted from the vaginal entrance to the uterine horns, with fibers from both nerves densely innervating the cervix region (i.e., entering the spinal cord through both sets of ganglia). The anatomic results suggested that the regions innervated by fibers in one nerve might also be innervated by a small component of normally quiescent fibers in the other nerve. 4. Response sensitivity was studied electrophysiologically by simultaneously recording multiunit activity in branches of the hypogastric and pelvic nerves in two ways. First, in intact, anesthetized rats, activity was recorded during mechanical stimulation of the reproductive tract (distension of the vagina and uterus, probing the cervix). Second, in an in vitro organ preparation of the uterus and vagina, activity was recorded during chemical stimulation through the uterine artery with bradykinin, serotonin, NaCN, CO2, and KCl. 5. Pelvic nerve fibers were markedly more sensitive than hypogastric nerve fibers to uterine and cervical mechanostimulation. Similarly, pelvic nerve fibers were more likely to respond or responded more vigorously than hypogastric nerve fibers to all chemical stimuli (except KCl). 6. These results provide strong evidence that afferent fibers in the pelvic and hypogastric nerves of nulliparous adult rats subserve different functions in reproduction and sensation. Pelvic nerve fibers seem closely tied to sensory and behavioral processes associated with mating and conception, whereas hypogastric fibers seem closely tied to pregnancy and nociception, with fibers in both nerves serving functions during parturition.