Ejaculation is a complex and pleasurable process that is heavily controlled by the nervous system. Five groups of neurons lying in different spinal segments command the physiological events that lead to ejaculation.
When stimulation of the clitoris or penis reaches a peak, it triggers a series of muscle contractions that move sperm and fluids into the urethra and out of the body, producing orgasm.
Muscles of the Urethra
The urethra is surrounded, over about half of its length, by circular striated muscle forming the external urethral sphincter (or rhabdosphincter). These muscles firmly contract during ejaculation to propel semen throughout the urethra. The urethral bulb, which contains small glands secreting mucus called Cowper’s glands, is also closely invested by these muscle fibers. The glands of the urethra, together with the bulboglandularis muscle, make up the bladder neck.
The muscular structure of the urethra consists of an inner epithelial layer, a spongy submucosa, and outer smooth and fibroelastic connective-tissue layers that form a tube-like shape. The spongy submucosa is rich in vascular plexus and augments urethral occlusive pressure.
During sexual activity, the pudendal nerve conveys motor inputs to the pelviperineal striated muscles. The levator ani, ischiocavernosus, and bulbospongiosus muscles play a major role in ejaculation. The urethral sphincter is tightened by the contractions of these muscles to prevent urine from leaking out.
Circular striated muscles of the urethra and UVS-like striated muscle are innervated by the same nerve. This suggests that these muscles act in tandem to orchestrate the complex neural control of micturition. Vibratory stimulation of the glans penis induces synchronized contractions of these muscles in rats, including the urethral sphincter, suggesting that they are involved in triggering ejaculation in humans as well. Moreover, intense vibratory stimulation of the glans penis in spinal cord-injured patients has been shown to induce ejaculation in more than 50% of them.
Semen
A sticky fluid called semen comes out of your penis during ejaculation. It contains sperm and can contain up to 500 million of them. Typically, a guy produces 3 to 5 milliliters of semen each time he ejaculates. Each milliliter contains about 20 million sperm. The sperm travel through the epididymis, a torturously coiled tube topping the testes, then into the vas deferens, a long tube that stores and transports semen from the testicles, up around the bladder, into the urethra and then out of the penis.
Nerves that run from your genitals to parts of the spinal cord control this reflex circuit, which has two phases: emission and expulsion. In the first, your vas deferens contract to squeeze sperm toward the base of your penis, through the prostate gland and into the urethra. In the second, muscles at the base of your penis contract every 0.8 seconds and force or shoot the semen out of your penis in several spurts.
The friction caused by sexual stimulation of the glans of the penis and other parts of the genitals triggers impulses in the central nervous system that cause this dance of muscle contractions. It may seem chaotic and uncontrolled but, in reality, it is a tightly choreographed court dance that integrates three different branches of the nervous system and can lead to a show-stopping eruption.
Urethral Bulb
The urethra is a cylinder with an inner space that can expand and contract, allowing it to convey seminal fluid from the prostate gland to the outside world. The urethra also acts as a conduit for urine, directing it away from the bladder toward the external orifice of the penis. The urethra is supported by a muscular layer consisting of the bulbospongiosus and ischiocavernosus muscles, as well as the two crura of the penis and the corpus spongiosum (also known as the corpus cavernosum urethrae). These muscles contract voluntarily and semi-voluntarily during erection and rhythmically during orgasm. Each contraction creates a pressure peak within the urethral bulb, which transmits its effect to the urethra beyond it. During ejaculation, the muscle contractions of the bulbospongiosus muscle cause this increased pressure to expel up to five milliliters of semen into the air.
Semen accumulates inside the spongy corpus spongiosum, which runs along the ventral surface of the penis between the two crus. The spongy mass is more soft and flexible than the rigid erectile structure of the two corpora cavernosa that run alongside it. The base of the corpus spongiosum flares out slightly to form a bulbous expansion called the glans of the penis. The urethra extends through the glans and out the other end of the penis. It receives ducts from the ejaculatory duct and the prostatic urethra (the longer membranous portion that carries urinate into the bladder). A hood of loose retractable skin, known as the prepuce, covers the glans. It is attached to the penis by a longitudinal fold of tissue, the frenulum. The urethral bulb and its lining are lubricated by the semen of the prostate gland and by the bulbourethral glands of the genitals.
Bladder
The bladder is a hollow muscular, subperitoneal organ that collects urine and holds it until your brain sends signals to urinate. The bladder wall has smooth muscle fibers that are interwoven in many directions and are collectively called the detrusor muscle. When this muscle contracts, it expels urine through the urethra.
As your bladder fills up with urine, it stretches the walls of the bladder. This stretch is sensed by bladder afferent neurons that project to the spinal cord and other parts of the body. These afferents are responsible for triggering the urinal stimulus needed to trigger urination.
During ejaculation, the urethral sphincters and pelvic muscles contract to push semen out of the penis. The sperm then travels down the urethra to the testicles where they are released for fertilization. The prostate, the urethra, and the testicles are all protected by the muscular pelvic floor muscles (see below).
These muscles are tightened during Kegel exercises that strengthen the pelvic floor and help control your bladder, bowel, and erections. The muscles and tissues of the pelvic floor extend from your tailbone in the back to your pubic bone in the front. They are supported by the muscles of your lower abdomen, the urethra that takes urine out of your body, and the rectum that allows you to poop.
