Chapter 13. Male Reproductive System

Big Picture

The male reproductive system primarily consists of the paired testes and the penis. In addition, accessory sex glands contribute to seminal fluid. The male reproductive system matures during adolescence and remains active for the remainder of the lifespan of the male.

Male Genital Organs and Glands

The following genital organs and glands comprise the male reproductive system (Figure 13-1A):

• Testes. The primary male sex organ. The testes produce sperm and sex hormones (e.g., testosterone) and are located within the scrotum.
• Epididymis. A convoluted duct that sits on the superior pole of each testis. Sperm are stored in the epididymis during the maturation process.
• Ductus deferens. A thick-walled tube in the spermatic cord that transports sperm from the epididymis to the ejaculatory ducts in the prostate gland. The ductus deferens traverses the superficial inguinal ring, coursing through the inguinal canal, and enters the pelvis through the deep inguinal ring lateral to the inferior epigastric artery. En route to the ejaculatory duct, the ductus deferens crosses the medial side of the umbilical artery and the obturator neurovascular structures. Sympathetic nerves from the inferior hypogastric plexus cause peristaltic contractions in the thick smooth muscle wall and propel sperm during ejaculation.
• Ejaculatory ducts. Formed by the union of the ductus deferens and ducts from the seminal vesicles. The ejaculatory ducts open into the prostatic urethra.
• Seminal vesicles. Lobular glands located on the base of the bladder. During emission and ejaculation, the seminal vesicles empty their secretions (e.g., fructose, citric acid, prostaglandins, and fibrinogen) into the ejaculatory duct, along with sperm from the ductus deferens. Seminal vesicle secretions add substantially to the volume of semen.
• Prostate gland. Composed of five lobes, all surrounding the prostatic urethra. The prostate gland is located superior to the pelvic diaphragm and anterior to the rectum. The prostate gland secretes a milky fluid that contributes to the bulk of the semen.

Deep Perineal Space

The deep perineal space (pouch) is the region within the urogenital diaphragm (Figure 13-1D). In males, this space contains the bulbourethral glands and the internal urethral sphincter. The bulbourethral glands secrete mucus that flows through the urethra during sexual intercourse to aid in lubrication. Parasympathetic nerves from the inferior hypogastric plexus innervate the bulbourethral glands.

• Membranous urethra. The portion of the urethra that courses through the deep perineal space.

Superficial Perineal Space

The superficial perineal space (pouch) is the region inferior to the urogenital diaphragm. It is enclosed by the superficial perineal (Colles') fascia. The superficial perineal space contains (Figure 13-1A–D):

• Deep (Buck's) fascia of the penis. Continuous with the external spermatic fascia and deep perineal fascia. The deep dorsal vein of the penis is inside this fascial layer.

Crura of the Penis

Each crus of the penis is composed of erectile tissue that is continuous with the paired corpus cavernosa of the penis. Erectile tissue consists of large, cavernous venous sinusoids (spaces like those found in a sponge) that normally are somewhat void of blood. However, during sexual arousal, the corpus cavernosa fill with blood, causing the penis to become erect (Figure 13-1B and C).

• Corpus cavernosa. The crura of the penis are separate along the ischiopubic ramus. However, at the pubic symphysis, the corpora cavernosa course along the dorsum of the penis, adjacent to each other. The deep arteries of the penis (branches of the internal pudendal artery) course within the center of the corpus cavernosa, providing blood that is necessary for an erection.
• Ischiocavernosus muscle. The ischiocavernosus muscles are voluntary skeletal muscles that surround the crura of the penis. The ischiocavernosus muscle helps to stabilize an erect penis and compresses the crus of the penis to impede venous blood return to maintain an erection. This muscle is innervated by the perineal nerve (branch of the pudendal nerve).

Bulb of the Penis

The bulb of the penis is the origin of the corpus spongiosum. The bulb of the penis is composed of erectile tissue and is continuous with the corpus spongiosum of the penis.

• Corpus spongiosum. The ventrally located erectile tissue that surrounds the spongy urethra, which transports urine and semen.
• Glans penis. The terminal part of the corpus spongiosum. A fold of skin called the prepuce (foreskin) covers the glans penis. The frenulum of the prepuce is a median ventral fold passing from the deep surface of the prepuce.

• Bulbospongiosus muscle. Contributes to erection and ejaculation. This muscle also helps expel the final drops of urine during micturition. The bulbospongiosus muscle is innervated by the perineal nerve (branch of the pudendal nerve).
• Tunica albuginea. A thin layer of connective tissue that surrounds the corpora cavernosa and corpus spongiosum. The tunica albuginea is denser around the corpora cavernosa and inhibits blood return during an erection. The tunica albuginea is more elastic around the corpus spongiosum and enables semen to pass through the urethra during ejaculation.

Big Picture

The male sex act begins with sexual stimulation. Somatic sensory nerves relay this information to the central nervous system. Parasympathetic impulses from the S2–S4 levels of the spinal cord cause blood to flow into the erectile tissue of the penis, resulting in penile erection. Sympathetic impulses from the T10–L2 spinal cord levels cause seminal fluids to mix with the sperm in the urethra in a process called emission. Ejaculation is the expulsion of the semen from the penis, which is caused by sympathetic innervation as well.

Erection

The corpora cavernosa and corpus spongiosum are bodies of erectile tissue. Erectile tissue is made of cavernous sinusoids.

Penile erection is initiated by parasympathetic innervation. The impulses originate in the S2–S4 levels of the spinal cord and travel through the pelvic splanchnic nerves to the central arteries of the corpora cavernosa and corpus spongiosum (erectile tissue) (Figure 13-2A). The parasympathetic innervation relaxes the smooth muscle in arteries. As a result, blood fills the spaces of the erectile tissue, causing the erectile bodies to enlarge. Because the erectile tissue is surrounded by the deep penile (Buck's) fascia: pressure within the sinusoids increases, causing expansion of the erectile tissue so that the penis becomes elongated and rigid, forming an erection.

Emission and Ejaculation

Emission and ejaculation are the climax of the male sex act; both processes result through sympathetic innervation. Impulses originating in the T10–L2 spinal cord levels travel through the lumbar splanchnic nerves and through the hypogastric plexus to the male reproductive organs and glands (Figure 13-2B).

• Emission. Begins with the contraction of the ductus deferens by moving sperm into the urethra, where contractions of the seminal vesicles and the prostate and bulbourethral glands secrete seminal fluid into the urethra and force the sperm forward. All of the fluids mix in the urethra, forming semen. This process is known as emission.
• Ejaculation. Sympathetic impulses cause rhythmic contraction of smooth muscle within the ductus deferens and urethra. In addition, the pudendal nerves cause contraction of the ischiocavernosus and bulbocavernosus muscles, which compress the bases of the penile erectile tissue. These effects together cause wavelike increases in pressure in the erectile tissue of the penis, the genital ducts, and the urethra, which ejaculate the semen from the urethra to the exterior.