Ejaculation is the emission of semen to the exterior, resulting from the contraction of muscles surrounding the male internal urogenital ducts. Ejaculation is a reflex mediated by the spinal cord, which coordinates autonomic and motor outflow, integrating these with the excitatory and inhibitory effects from descending cerebral pathways.
The normal male ejaculatory response comprises two phases: emission and expulsion, which are under autonomic and somatic control, respectively. During emission, smooth muscles of the vas deferens, the seminal vesicles and the prostate, as well as their secretions, are involved. At the end, the mixture of spermatozoa from the epididymis and the vas deferens, together with the secretions of the seminal vesicles, represents about 50% of the ejaculate; and the prostate, which secretes nearly the other half of the semen, is made available in the prostatic (posterior) urethra. Orgasm is caused when friction on the glans penis and other stimuli send signals to the brain and spinal cord.
Orgasm generally accompanies the expulsion phase, which occurs once an ejaculatory 'point of no return' has been reached. Nerves stimulate muscle contractions along the seminal vesicles, prostate, and the ducts of the epididymis and vas deferens. These contractions force semen into the urethra. Contraction of the muscles around the urethra further propels the semen through and out of the penis. The neck (base) of the bladder also constricts to keep semen from flowing backward into the bladder.
Once ejaculation takes place the arteries constrict and the veins relax, and causing the penis to become limp (detumescence). After detumescence, erection cannot be obtained for a period of time (refractory period), commonly about 20 minutes in young men.
Ejaculatory Disorders and Erectile Dysfunction