Physioanatomical relationship of the external anal sphincter to the bulbocavernosus muscle in the female.

Both external anal sphincter (EAS) and bulbocavernosus muscle (BCM) have been shown anatomically and physiologically to constitute one muscle in males. We investigated the hypothesis that the EAS and BCM have similar anatomical pattern in females. The study consisted of cadaveric dissection, electromyographic recordings and inferior rectal nerve stimulation. Bulbocavernosus reflex action was performed in 16 healthy women before and after anesthetization of the EAS and BCM. The EAS extended forward across the perineal body and became continuous with the BCM in the labia majora. Glans clitoris (GC) or inferior rectal nerve stimulation effected synchronous EAS and BCM contractions with identical action potentials. GC stimulation while the EAS or BCM was anesthetized produced neither EAS nor BCM response. Similarly, stimulation of the anesthetized GC produced no EAS or BCM response. The BCM and EAS apparently constitute a single muscle, which seems to play dual and yet synchronous roles in fecal control and sexual response.