Men usually describe their orgasms as a pulsing sensation followed by a release of fluid and sperm. This feeling results from a coordinated wave of contractions in the prostate, urethra and penis that is triggered by stimulation of muscarinic receptors.
Sexual arousal requires muscular tension, which is why so many muscles contract involuntarily during orgasms. These contractions include clitorius (women) and penis erection, urinary bladder contractions, pelvic floor muscle contractions and more.
The Epididymis
The epididymis (plural: epididymides) is a crescent-shaped mass of tightly coiled tubules inside the scrotum that carry sperm from the testis to the ductus deferens, which leads to the urethra. The whitish, fleshy organ is made of a pseudostratified columnar epithelium lining surrounded by a layer of connective tissue and smooth muscle. Long, branching microvilli on the surface of the epididymis lining increase its sensitivity to the motions of sperm and help it absorb the excess fluid that accumulates in the tubule and dead or defective sperm.
As sperm travels through the tubule of the epididymis, it is agitated and stimulated by a series of rhythmic muscular contractions called peristalsis. These contractions slowly push the sperm along the tube, concentrating it and removing any immature or defective sperm that may be unable to fertilize an egg.
When the sperm reaches the end of the tubule, it is ready to be carried down the ductus deferens to the bladder and then to the urethra. As it moves along the ductus deferens, sperm is forced forward by rhythmic contractions of the rectus abdominis and pelvic muscles, as well as by the force of gravity.
The sperm is finally released from the tail of the epididymis into the ductus deferens as a viscous fluid known as semen. The sperm in the epididymis has been concentrated by the repeated muscular contractions of peristalsis, and its concentration is one of the main reasons that the fluid discharged during ejaculation is a thick viscous substance rather than water.
The Urethra
The urethra, or penis tube, is a long hollow tube that takes urine stored in the bladder out of the body. It is much longer in men than in women. During ejaculation, semen is pushed through this tube by the contractions of smooth muscles. This conveyor-belt-like movement takes sperm from the seminal vesicles and mixes them with fluids from the prostate gland to make a gooey substance we call semen. It then travels through the urethra out the tip of the penis.
The muscle-rich walls of the urethra are covered in layers of stratified epithelium that secrete mucus, which helps to keep the passage moist and supple. Over half of the urethra’s length is surrounded by circular, striated muscles called the external urethral sphincter.
This muscle is a major player during ejaculation, firmly contracting to prevent urine from mixing with semen. It also keeps the urethra open during an erection.
The urethra itself is made up of two parts: the bulbar or prostatic urethra and the spongy urethra that runs along the penis. The spongy urethra is fed by a pair of muscular tubes called the Cowper’s glands, which produce and supply extra fluid to help with ejaculation. The nerve that tells these muscles to contract during ejaculation comes from a group of neurons near the base of the spinal cord called Onuf’s nucleus. Once triggered, these signals can take control of the muscles in the pelvic perineum and set off a series of strong, involuntary contractions.
The Bladder
The bladder is the organ that stores urine until it is ready to be released and also helps to expel it. The bladder is surrounded by muscles that help to control it. The external urethral sphincter is a skeletal muscle that can be consciously controlled and is located at the opening to the urinary tract. The internal sphincter, which is a smooth muscle located within the bladder neck and urethra, remains closed until the brain sends signals to urinate. The muscles of the pelvic floor and the external sphincter tighten to provide the extra pressure needed to force urine out of the body.
In males, the neck of the bladder is home to a gland called the prostate gland. During orgasm, the muscles at the base of the bladder contract to help push semen out through the urethra and penis. This is known as retrograde ejaculation. In some cases, the bladder outlet may not close completely, causing semen to flow back into the bladder. This can occur if you take certain types of medications, including antidepressants.
Sperm are made in the testes and then travel through tubes called the vas deferens that go down into the rectum, up around the bladder and into the prostate gland where they are mixed with prostatic secretions. The prostate gland and the scrotum’s muscular tissues then contract in a synchronized sequence that propels the mixture of semen, prostatic fluid and sperm out of the body through the epididymis and down into the urethra.
The Semen
During sexual stimulation the muscles around the penis, the vas deferens, and the urethra contract to force semen out of the penis. This whitish fluid, called seminal plasma, contains sperm and a milky liquid from the prostate gland and seminal vesicles. It has a pleasant smell due to the presence of basic amines such as putrescine, spermine and cadaverine. These alkaline chemicals also counteract the acidity of the urine in the bladder and protect sperm DNA from acidic denaturation.
The sperm cells travel through the epididymis and ductus deferens to the prostate gland, where they are mixed with seminal fluid from the prostate gland and the seminal vesicles. The sperm then move into the urethra, where they are combined with fluid from the bulbourethral glands (which make up only about 1% of the final semen composition).
The whitish color of semen is caused by the spermatozoa that it contains. The liquid portion of the semen is a clear secretion from the prostate gland, which consists of enzymes, citric acid, lipids and acid phosphatase. It forms about 25-30% of the total composition of semen. The remaining 70% of semen is a mucus-like substance secreted by the ductus deferens and the epididymis, called pre-cum. The lubrication of the penis and the neutralization of the acidity in the urethra by this fluid is why you feel such a tingle when you ejaculate.
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