Mechanism of ejection during ejaculation: Identification of a urethrocavernosus reflex

The ejaculatory mechanism involves 2 reflexes: the "glans-vasal," which see ms to bring the semen to the posterior urethra (emission phase of ejaculati on), and the "urethromuscular" which ejects it to the exterior (ejection ph ase). 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 te st the response of the BCM to urethral distension, a 10F balloon-tipped cat heter was introduced into the prostatic urethra and filled with saline in i ncrements of 0.25 mt: a needle electrode recorded the response. The balloon was then withdrawn to lie in the membranous, bulbous, and pendulous urethr a and the test was repeated at each sire. The latency of the muscle respons e was calculated. The BCM response to each of the anesthetized bulbous uret hra and anesthetized BCM was recorded. Distension of the prostatic, membran eous, or pendulous urethra effected no BCM EMG response. Bulbous urethral d istension with 0.25 mt of saline also produced no muscle response. whereas distension with 0.5 mL and up to 1.5 mL caused increased EMG activity of th e BCM. The muscle response augmented with the increase of the distending vo lume. The mean latency was 10 +/- 1.3 ms and showed no significant change ( p >.05) with the different distending volumes. Neither the anesthetized bul bous urethra nor the anesthetized BCM responded to bulbous urethral distens ion. The BCM contraction upon distension of the bulbous urethra is probably reflex and mediated through the urethrocavernosus reflex. Small-volume dis tension did not effect BCM contraction. The latter presumably propels the s emen from the posterior to the pendulous urethra. It is suggested that the urethrocavernosus reflex be included in current andrologic investigations f or patients with ejaculatory disorders.