Erection is associated with sexual arousal emanating from sexually related psychic and/or physical stimuli. During sexual arousal, impulses from the genitalia, together with nerve signals originating in the limbic system, elicit motor impulses in the spinal cord. These neuronal impulses are carried by the parasympathetic nerves in the sacral region of the spinal cord via the cavernous nerve branches of the prostatic plexus that enter the penis. Those signals cause vasodilation of the arterioles and corpora cavernosa. The smooth muscles in those structures relax, and the blood vessels dilate and begin to engorge with blood. The thin-walled veins become compressed by the swelling of the blood-filled arterioles and cavernosa, restricting blood flow. The result is a reduction in the outflow of blood from the penis, and blood is trapped in the surrounding erectile tissue, leading to engorgement, rigidity, and elongation of the penis in an erect position.
Semen, consisting of sperm and the associated fluids, is expelled by a neuromuscular reflex that is divided into two sequential phases: emission and ejaculation. Emission moves sperm and associated fluids from the cauda epididymis and vas deferens into the urethra. The latter process involves efferent stimuli originating in the lumbar areas (L1 and L2) of the spinal cord and is mediated by adrenergic sympathetic (hypogastric) nerves that induce contraction of smooth muscles of the epididymis and vas deferens. This action propels sperm through the ejaculatory ducts and into the urethra. Sympathetic discharge also closes the internal urethral sphincter, which prevents retrograde ejaculation into the urinary bladder. Ejaculation is the expulsion of the semen from the penile urethra,- it is initiated after emission. The filling of the urethra with sperm initiates sensory signals via the pu-dendal nerves that travel to the sacrospinal region of the cord. A spinal reflex mechanism that induces rhythmic contractions of the striated bulbospongiosus muscles surrounding the penile urethra results in propelling the semen out of the tip of the penis.
The secretions of the accessory glands promote sperm survival and fertility. The accessory glands that contribute to the secretions are the seminal vesicles, prostate gland, and bulbourethral glands. The semen contains only 10% sperm by volume, with the remainder consisting of the combined secretions of the accessory glands. The normal volume of semen is 3 mL with 20 to 50 million sperm per milliliter, normal is considered more than 20 million sperm per milliliter. The seminal vesicles contribute about 75% of the semen volume. Their secretion contains fructose (the principal substrate for glycolysis of ejaculated sperm), ascorbic acid, and prostaglandins. In fact, prostaglandin concentrations are high and were first discovered in semen but were mistakenly considered the product of the prostate. Seminal vesicle secretions are also responsible for coagulation of the semen seconds after ejaculation. Prostate gland secretions (~0.5 mL) include fibrinolysin, which is responsible for liquefaction of the coagulated semen 15 to 30 minutes after ejaculation, releasing sperm.
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