Anatomy and innervation of the rhabdosphincter of the male urethra.

The rhabdosphincter of the male urethra and its innervation are still a subject of controversy. Essentially, two concepts of its anatomy can be found in the literature. Some authors describe the rhabdosphincter as part of the urogenital diaphragm caudal to the prostate, others as a striated muscle that extends from the base of the bladder to the "urogenital diaphragm." In anatomic histological studies, the rhabdosphincter and its innervation were examined by means of anatomical dissections and serial anatomical as well as histological sections of 19 male pelves, including 8 fetal specimens. The rhabdosphincter presents as a vertical structure extending from the bulb of the penis to the region of the bladder neck along the prostate and the membranous urethra. Inserting dorsally in the perineal body via a broad tendinous raphe, the striated muscle fibers form an omega-shaped loop around the anterior and lateral aspects of the membranous urethra. The existence of a "urogenital diaphragm" and a strong, circular, striated "external sphincter urethrae" completely encircling the urethra caudal to the apex of the prostate cannot be confirmed by anatomical and histological investigations. The rhabdosphincter is supplied by branches of the pudendal nerve after leaving the pudendal canal.