Origins and courses of the nervous branches to the male urethral sphincter

The striated sphincter of the male urethra, the so-called rhabdosphincter, contributes significantly to urethral closure pressure. It is generally agreed that the somatic nerve fibers from the pudendal nerve innervate the rhabdosphincter, and the autonomic nerve fibers innervate the smooth muscle of the urethra. Although it is difficult to clearly identify the rhabdosphincter macroscopically, we minutely investigated the nerve branches to the urethral sphincter muscle region in 10 male pelvic halves. In addition, the origins and courses of the pudendal plexus in 88 male pelvic halves were investigated. To this region were given branches of the pudendal nerve and the pelvic plexus. The branches from the pelvic plexus to the region generally originated from S4 as the lowest branch of the pelvic splanchnic nerve, and ran along the rectal attachment of the levator ani. The caudal root of the pelvic splanchnic nerve formed a common trunk with the nerve to the levator ani (94%). Various connections were sometimes observed between the pudendal nerve and the branches medial to the levator ani. It is suggested that the somatic nerve fibers from the nerve to the levator ani or from the pudendal nerve might also join the nerve branches to the region from the pelvic plexus.