Somatic innervation of the human bulbocavernosus muscle.

OBJECTIVE: To demonstrate the somatic reflex innervation of the bulbocavernosus muscle (BCM), the principal muscle for ejaculation.
METHODS: Genitourinary electrodiagnostic testing utilizing modifications of the standard bulbocavernosus reflex was performed in 13 healthy male volunteers ages 20-43.
RESULTS: Bulbocavernosus muscle contraction was elicited by stimulation of the dorsal nerve of the penis, from both the penile skin and from the anterior urethra, and following stimulation of the perineal nerve. Latencies were variable depending on the point of stimulation.
CONCLUSIONS: All 3 afferent pathways synapse on pudendal motoneurons in the conus medullaris, and provide for peripheral reflex control of BCM contractions. Based on the latencies of the urethral evoked responses, urethral innervation differs from penile shaft innervation, each having a distinct population of the dorsal nerve of the penis (DNP) fibers. The presence of an electrically-defined pathway from the anterior urethra to the BCM suggests that somatic afferents from the anterior urethra are involved with the ejaculatory reflex. These somatic reflexes are components of normal ejaculatory function. The findings contribute to understanding the neurophysiology of ejaculation, and may be applicable to the evaluation of ejaculatory disorders.