Functional Impact of the Rhabdosphincter Branch of the Pelvic Nerve on the Membranous Urethra in Comparison to That of the Pudendal Nerve in Male Rabbits

Abstract

Introduction: The innervation of the membranous urethra (MU) is still under debate. We analysed the functional effects of electrostimulation of the rhabdosphincter branch of the pelvic nerve (RBP) on the MU in a standardized male rabbit model, paying attention both to the efferent and the possible afferent effects of stimulation. Material and Methods: Six male rabbits (chinchilla bastards) were included in this study. Pudendal nerve branches as well as pelvic nerve branches were exposed bilaterally in all animals. Randomized electrostimulation trials of both the pudendal nerve fibres and the RBP were carried out using a biphasic signal (0.3 mA, 200 μs). The stimulation frequency ranged from 10 to 40 Hz in a randomized pattern. Changes in MU pressure were measured urodynamically. The pressure changes occurring as a result of stimulation were compared in both nerve structures. At the end of the stimulation trial, the RBP and the pudendal nerve were dissected at different positions before repeating the stimulation and the MU pressure recording. Results: The mean MU baseline pressure without stimulation was 24 cm H₂O (range 21–27) in all animals. During unilateral pudendal stimulation, the mean pressure response rose by highly significant values (p < 0.005) compared to baseline. The pressure response was frequency dependent. Stimulation of the RBP resulted in a small but significant change of the MU pressure in the lower frequency range (10 and 20 Hz) (p > 0.05). On performing stimulation at higher frequencies, there was no significant difference from baseline. The pressure response of the MU during stimulation of the intact RBP did not differ significantly from the response after dissection of the RBP. However, after dissecting the pudendal nerve, the MU pressure response to RBP stimulation was diminished. Conclusion: Our results confirm the primacy of the pudendal nerve in the innervation of the MU. Stimulation of the RBP, however, may produce an afferent signal which is transmitted to the pudendal nerve, thus resulting in a pressure increase of the MU and contributing towards continence. We showed the importance of the RBP to maintain full urinary continence in rabbits. Our results suggest that substantially higher continence rates can be achieved through a nerve-sparing procedure in radical prostatectomy. Nerve-sparing radical prostatectomy incidentally also protects the RBP from injury.