Mechanism of ejection during ejaculation: identification of a urethrocavernosus reflex.

The ejaculatory mechanism involves 2 reflexes: the "glans-vasal," which seems to bring the semen to the posterior urethra (emission phase of ejaculation), and the "urethromuscular" which ejects it to the exterior (ejection phase). This study investigated the mechanism of bulbocavernosus muscle (BCM) contraction, once the seminal fluid reaches the bulbous urethra. The study included 14 healthy male volunteers (mean age 37 +/- 10.2 SD years). To test the response of the BCM to urethral distension, a 10F balloon-tipped catheter was introduced into the prostatic urethra and filled with saline in increments of 0.25 mL: a needle electrode recorded the response. The balloon was then withdrawn to lie in the membranous. bulbous, and pendulous urethra and the test was repeated at each site. The latency of the muscle response was calculated. The BCM response to each of the anesthetized bulbous urethra and anesthetized BCM was recorded. Distension of the prostatic, membraneous, or pendulous urethra effected no BCM EMG response. Bulbous urethral distension with 0.25 mL of saline also produced no muscle response, whereas distension with 0.5 mL and up to 1.5 mL caused increased EMG activity of the BCM. The muscle response augmented with the increase of the distending volume. The mean latency was 10 +/- 1.3 ms and showed no significant change (p > .05) with the different distending volumes. Neither the anesthetized bulbous urethra nor the anesthetized BCM responded to bulbous urethral distension. The BCM contraction upon distension of the bulbous urethra is probably reflex and mediated through the urethrocavernosus reflex. Small-volume distension did not effect BCM contraction. The latter presumably propels the semen from the posterior to the pendulous urethra. It is suggested that the urethrocavernosus reflex be included in current andrologic investigations for patients with ejaculatory disorders.