Physioanatomic entirety of external anal sphincter with bulbocavernosus muscle.

Stimulation of the glans penis evokes contraction of both the bulbocavernosus muscles (BCM) and the external anal sphincter (EAS). This synchronous contraction of the two muscles led us to study their physioanatomic relationship and possible role in erection and ejaculation. Fifteen male cadavers were studied (8 neonatal deaths and 7 adults; mean age 48 years) by dissection. The bulbocavernosus reflex action was performed in 12 healthy male volunteers (mean age 37 years) before and after anesthetizing the EAS. The response of the EAS and BCM to inferior rectal nerve stimulation was assessed in 6 men (mean age 41 years). The superficial fibers of the base loop of the EAS extended forward to the penile bulb where they were arranged into 3 groups: 1 median and 2 lateral. The median fibers, or the "retractor penis muscle," were found inserted into the corpora cavernosa and the lateral fibers, or the "compressor bulbae muscle," into the perineal membrane. Upon glans penis stimulation, both the EAS and BCM contracted synchronously with similar latency and action potentials. During EAS anesthesia, the two muscles did not respond to glans penis stimulation. They contracted simultaneously with similar latency and action potentials upon inferior rectal nerve stimulation. The BCM is an integral part of the EAS, and the muscle in its entirety is appropriately named "anogenital muscle." The muscle plays a dual and synchronous role in fecal control and sexual response.