Production and mixing of sperm with seminal contents. During emission, contraction of the vas deferens coincides with contraction of the muscular coats of the two accessory sex glands, the seminal vesicles (right and left) and the prostate gland (which surrounds the prostatic urethra). At this point, sperm become mixed with all the components of semen. The seminal vesicles secrete approximately 60% of the volume. These glands are the primary source of fructose, a critical nutrient for sperm. The seminal vesicles also secrete semenogelins, which induce coagulation of semen immediately after ejaculation. The alkaline secretions of the prostate, which make up about 30% of the volume, are high in citrate, zinc, spermine, and acid phosphatase. Prostate-specific antigen (PSA) is a serine protease that liquefies coagulated semen after a few minutes. PSA can be detected in blood under conditions of prostatic infection, benign prostatic hypertrophy, and prostatic carcinoma and is currently used as one indicator of prostatic health. The predominant buffers in semen are phosphate and bicarbonate. A third accessory gland, the bulbourethral glands (also called Cowper’s glands), empty into the penile urethra in response to sexual excitement before emission and ejaculation. This secretion is high in mucus, which lubricates, cleanses, and buffers the urethra. Average sperm counts are between 60 and 100 million/mL semen. Men with sperm counts below 20 million/mL, less than 50% motile sperm, or less than 60% normally conformed sperm are usually infertile.
4.
Erection, penetration, and ejaculation. Emission and ejaculation occur during coitus in response to a reflex arc that involves sensory stimulation from the penis (via the pudendal nerve) followed by sympathetic motor stimulation to the smooth muscle of the male tract and somatic motor stimulation to the musculature associated with the base of the penis. However, for sexual intercourse to occur in the first place, the man has to achieve and maintain an erection of the penis. The penis has evolved as an intromittent organ designed to separate the walls of the vagina, pass through the potential space of the vaginal lumen, and deposit semen at the distal end of the vaginal lumen near the cervix. This process of internal insemination can be performed only if the penis is stiffened from the process of erection. Erection is a neurovascular event. The penis is composed of three erectile bodies: two corpora cavernosa and one corpus spongiosum (Fig. 44.12A ). The penile urethra runs through the corpus spongiosum. These three bodies are composed of erectile tissue—an anastomosing network of potential cavernous vascular spaces lined with continuous endothelia within a loose connective tissue support. During the flaccid state, blood flow to the cavernous spaces is minimal (see
Fig. 44.12A). This is due
CHAPTER 44 The Male and Female Reproductive Systems of the vasculature (called the helicine arteries) and shunting of blood flow away from the cavernous spaces. In response to sexual arousal the parasympathetic cavernous nerves innervating the vascular smooth muscle of the helicine arteries release nitric oxide (NO). NO activates guanylyl cyclase, thereby increasing cyclic guanosine monophosphate (cGMP), which decreases intracellular [Ca++] and causes muscular relaxation (see
Fig. 44.12B ). Vasodilation allows blood to flow into the cavernous spaces to induce engorgement and erection. It also presses on veins in the penis and reduces venous drainage (see
Fig. 44.12B