The effect of external urethral sphincter contraction on the cavernosus muscles and its role in the sexual act.

Objectives/aim: A study of the effect of external urethral sphincter contraction on ischio-/bulbo-cavernosus muscles could not be traced in the literature. We investigated the hypothesis that external urethral sphincter contraction induces cavernosus muscles' contraction.
Methods: Twenty-one healthy volunteers (age 37.6 +/- 9.7 SD years, 12 men, nine women) were studied. The electromyographic response of the ischio- and bulbo-cavernosus muscles to external urethral sphincter stimulation was recorded before and after anesthetization of the external urethral sphincter, and the ischio- and bulbo-cavernosus muscles; the response was also recorded using normal saline instead of lidocaine.
Results: Upon external urethral sphincter stimulation (five square pulses, 1 ms duration, 53.8 +/- 10.2 mA threshold), both cavernosus muscles exhibited increased electromyographic activity with a mean amplitude of 386.2 +/- 44.9 microV for the ischio-cavernosus and 318.4 +/- 36.6 microV for the bulbo-cavernosus muscle. The mean latency read 16.8 +/- 1.3 ms for the ischio-cavernosus muscle and 15.7 +/- 1.2 ms for the bulbo-cavernosus muscle. Neither the ischio- nor the bulbo-cavernosus muscle responded to stimulation of the anesthetized external urethral sphincter, but both responded after saline administration. Likewise, the anesthetized ischio- and bulbo-cavernosus muscles showed no response to external urethral sphincter stimulation but responded after saline had been injected.
Conclusions: Increased electromyographic activity of the two cavernosus muscles on external urethral sphincter stimulation presumably denotes contraction of these two muscles and that this action is probably reflex, mediated through the 'sphinctero-cavernosus-reflex'. Cavernosus muscles' contraction assists in the erectile and ejaculatory mechanisms. It is suggested that this reflex be included as an investigative tool in the diagnosis of erectile and ejaculatory disorders, provided further studies are performed.