PHYSIOLOGY UPDATE The mechanisms of human ejaculation – a critical analysis

The current mechanism for human ejaculation is a two-part model consisting of a postulated ‘‘pressure chamber’’ created in the prostatic urethra by initial closure of both distal and proximal sphincters with secretions loaded in by adrenergic-mediated smooth muscle contractions of the vas deferens and capsules of the internal genital organs. The pressure build up is claimed then to ‘‘trigger’’ the intermittent relaxation/contraction of the distal sphincter and activate the contractions of the striated pelvic musculature, especially that of the bulbocavernosus, which forcefully expels the semen along the urethra by powerful, rhythmical spurts. A number of difficulties with this model are examined critically against experimental findings which indicate that the so-called pressure chamber is unlikely to be the valid trigger for ejaculation. The most recent finding of a specialised group of lumbar sacral neurons in the spinal cord of rat that function as the spinal ejaculation generator is more likely to be the ‘‘trigger’’ for ejaculation but confirmation that these cells also exist in the human cord is waiting.