Mechanisms of Pelvic Organ Cross-Talk: 2. Impact of Colorectal Distention on Afferent Nerve Activity of the Rat Bladder

Purpose: Pelvic organ neural cross-talk is mediated by central and peripheral pathways. We investigated the effects of colorectal distention at the peripheral level on mechanosensitive bladder single unit afferent activity. Materials and Methods: We used 16 female Sprague-Dawley (R) rats in this study. The single unit afferent activity of nerves originating from the bladder were identified by electrical stimulation of the pelvic nerve and by bladder distention. They were classified by conduction velocity as C or A delta fibers. After measuring single unit afferent activity during constant bladder filling without colorectal distention, measurements during bladder filling were repeated with colorectal distention at 20, 40 and 60 mm Hg. To evaluate the convergence of bladder and colorectal afferent fibers, rapid colorectal distention was performed with an empty bladder. Bladder afferent fibers with an increased response to rapid colorectal distention were defined as convergent fibers and the others were defined as nonconvergent fibers. Results: A total of 30 single units (19 C and 11 A delta fibers) were isolated. Continuous colorectal distention increased the bladder single unit afferent activity of C and A delta fibers in a pressure dependent manner. C fibers could be divided into convergent (12) and nonconvergent (7) fibers but all A delta fibers were nonconvergent. The facilitatory effect of continuous colorectal distention on bladder single unit afferent activity was absent in nonconvergent C fibers. Conclusions: In rats the convergence of bladder and colorectal peripheral nerves was electrophysiologically identified only in mechanosensitive C fibers and not in A delta fibers. Continuous colorectal distention probably increased the bladder single unit afferent activity of C fibers through direct convergence at the peripheral level between the bladder and sensory ganglia. It probably increased the bladder single unit afferent activity of A delta fibers through an alternative pathway.