ANATOMY AND INNERVATION OF THE RHABDOSPHINCTER OF THE MALE URETHRA

The striated sphincter of the male urethra and its innervation are sti ll a subject of controversy. Essentially, two concepts of its anatomy can be found in the literature. Some authors describe the rhabdosphinc ter as part of the urogenital diaphragm caudal to the prostate, others as a striated muscle which extends from the base of the bladder to th e ''urogenital diaphragm.'' In a combined anatomic-histologic study th e striated sphincter and the pudendal nerve were examined by means of anatomical dissections and serial anatomical as well as histological s ections of 12 male pelves. Furthermore, radical prostatectomy was perf ormed in a cadaver specimen; subsequently, the so-called ''urogenital diaphragm'' was excised and then examined histologically. The varying number of striated muscle fibers caudal to the prostate is of particul ar interest. In fetuses, there are abundant striated muscle fibers dor sal to the membranous urethra, where they are arranged as a circular c ollar around the urethra. In the adult male, hardly any striated muscl e fibers can be found dorsal to the urethra; in a majority of cases th is region is devoid of striated muscle fibers. Inserting dorsally in t he perineal body, the fibers form an omega-shaped loop around the ante rior 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 a pex of the prostate could not be confirmed by our anatomical and histo logical investigations. Our study shows that the striated muscle fiber s run in a cranial direction from the bulb of the penis to the base of the bladder along the anterior and lateral aspects of the prostate an d the membranous urethra. Further dissection studies revealed that the rhabdosphincter is supplied by branches of the pudendal nerve after l eaving the pudendal canal. (C) 1996 Wiley-Liss, Inc.