Anatomy, Abdomen and Pelvis, Penis

Structure and Function

The 2 main functions of the penis are intercourse and urination. For intercourse, the penis must be erect. Erection occurs via parasympathetic innervation which allows engorgement of the corporal bodies. The bulbospongiosus and ischiocavernosus muscle compress the veins to prevent them from draining blood from the corpora cavernosa. The erectile tissue of the cavernosum becomes firm and enlarged when the erection occurs. The emission phase is due to the sympathetic response and occurs when semen exits the vas deferens to the ejaculatory ducts where fluid from the seminal glands are added. They then continue onto the prostatic urethra where the prostatic fluid is added. The ejaculation phase is due to the sympathetic response and is when semen exits from the urethra through the external urethral orifice. After ejaculation, the penis becomes flaccid again due to the sympathetic stimulation and constriction of the coiled helicine arteries and relaxation of the bulbospongiosus and ischiocavernosus muscles allowing blood to drain from the cavernous spaces.[5][6][7]

The other function of the penis is to drain urine from the body. Urine starts draining from the bladder into the bladder neck where the internal urethral sphincter will relax to allow urine to pass via involuntary control of the parasympathetic system. Urine will then pass into the prostatic urethra and the membranous urethra. The external sphincter will relax to allow urine to pass from the membranous urethra into the spongy penile urethra, then into the navicular fossa and out of the urethra through the external urethral orifice.

Embryology

The male penis is derived in the fetus from the genital tubercle. The genital tubercle begins to develop into the phallus under the influence of androgens from the testis. The elongation process draws the urethral folds forward to form the lateral walls of the urethral groove. The epithelial lining, which is from the groove, develops into the urethral plate. The penile urethra is created when both the urethral folds enclose the urethral plate. This process usually occurs towards the end of month three. The urethra's most distal part forms at the end of month 4 and terminates at the external urethral meatus. The 2 urethral folds come together and fuse; this part will eventually become the spongy body. Then, above the spongy body, there will be 2 cavernous bodies that arise, which will eventually become the corpora cavernosa.

Blood Supply and Lymphatics

The branches of the internal pudendal arteries are the main suppliers of blood to the penis. The dorsal arteries of the penis run along the dorsal vein between the corpora cavernosa to supply the fibrous tissue surrounding the corpora cavernosa, corpus spongiosum, spongy urethra, and penile skin. The deep arteries of the penis run in the center of each corpus cavernosum to supply the erectile tissue of the penis. The helicine arteries of the penis are branches of the deep arteries of the penis that are coiled when the penis is flaccid. The arteries of the bulb of the penis supply the bulbous part of the corpus spongiosum, urethra, and bulbourethral gland. The superficial and deep branches of the external pudendal arteries supply the penile skin. The deep dorsal vein of the penis receives blood from the venous plexus, which receives blood drained from the cavernous spaces. The superficial dorsal vein drains blood from the skin and subcutaneous tissue of the penis. The skin of the penis and all of the perineum, excluding the glans penis, drain lymph to the superficial inguinal nodes. The lymphatics of the intermediate and proximal parts of the urethra and cavernous bodies drain into the internal iliac lymph nodes. The distal spongy urethra and glans penis drain to the deep inguinal nodes.

Nerves

The penis innervation is from spinal cord segments of the S2 through S4. These cord segments pass through the pelvic splanchnic nerves. The spinal ganglia of S2 through S4 pass through the pudendal nerves. The dorsal nerve provides sympathetic plus sensory innervation to the penis. The pudendal nerve branches to become the dorsal nerve of the penis. The nerve runs next to the dorsal artery and also supplies the skin and glans penis. The glans penis has a copious amount of sensory nerve endings. The root of the penis is innervated by the ilioinguinal nerve. The cavernous nerves have parasympathetic fibers that innervate the helicine arteries, which supply erectile tissue.

Muscles

The bulbospongiosus muscles cross the pelvic outlet, intersecting with the superficial transverse perineal muscles. These structures support the perineal body and pelvic viscera. The bulbospongiosus muscles compress the bulb of the penis and the corpus spongiosum, which can aid in emptying the spongy urethra. The anterior fibers of the muscle surround the proximal part of the body of the penis. These fibers can increase pressure on the erectile tissue at the root of the penis during erections. The bulbospongiosus muscle can compress the deep dorsal veins to prevent drainage of the cavernous spaces and help promote erection of the penis. The root of the penis contains the crura, which are encircled by the ischiocavernosus muscle. The muscle can pump blood into the distal portions of the corpora cavernosa and help maintain erections. The muscle can also lead to compression of the deep dorsal vein of the penis as it leaves the crus of the penis and restrict venous outflow to help with erections.

Physiologic Variants

Hypospadias is a fairly common congenital variant. The external urethral orifice opens on the ventral surface of the glans penis or penile shaft. Hypospadias results from failure of the urogenital folds to fuse correctly. Consequently, urine exits through the ectopic urethral opening rather than the external urethral meatus. Epispadias occurs less commonly and results from a urethral meatus located on the dorsal surface of the penis. Epispadias is most commonly associated with bladder exstrophy. Phimosis occurs when the prepuce fits too tightly over the glans and cannot be retracted. Micropenis is defined as penile length 2.5 SDs less than the mean, measured along the dorsal surface from the pubis to the tip of the stretched penis. Micropenis is thought to result from insufficient androgen stimulation during the fetal period. Chordee is another fairly common congenital anomaly. Chordee is a ventral curvature of the penis that develops during the fetal period. This curvature is most notable during an erection. Some patients with chordee may also have hypospadias, although this association is inconsistent. Chordee is usually not painful and may be treated with a minor surgical procedure.[8][9][10]

Clinical Significance

Erectile dysfunction, the inability to achieve or maintain an erection, is the most common penile disorder. Erectile dysfunction may result from psychological stress, organic disease, or a combination of both. Risk factors associated with erectile dysfunction include obesity, smoking, atherosclerotic disease, physical inactivity, obstructive sleep apnea, diabetes, medications, and many other causes. Antidepressants and antihypertensive medications are common medication classes associated with erectile dysfunction. The most common cause of erectile dysfunction is vascular disease. Therefore, first-line treatment is usually a phosphodiesterase type 5 inhibitor, such as sildenafil, vardenafil, or tadalafil. These medications increase nitric oxide–induced vasodilation by increasing cyclic guanosine monophosphate levels. Phosphodiesterase type 5 inhibitors are contraindicated in patients taking nitrates because the vasodilatory effects of both medication classes can lead to severe hypotension. Lifestyle modifications are usually indicated for the treatment of erectile dysfunction as well. Weight loss, physical activity, and smoking cessation can improve erectile function. If initial therapy fails, second-line treatments include vacuum-assisted mechanical devices, penile injections with alprostadil, or penile prostheses.

Peyronie disease is an abnormal curvature of the penis that is usually benign but may be associated with painful erections or erectile dysfunction. The condition can resolve spontaneously, become stable, or worsen. The underlying pathogenesis of Peyronie disease is not known. However, evidence suggests repeated microtrauma to the penis leads to fibrin deposition from microvascular injury. The fibrous plaques contain excessive collagen, causing focal inelasticity and leading to penile curvature. Several conditions have been associated with Peyronie disease, such as Paget disease and Dupuytren contracture. The diagnosis of Peyronie disease is usually clinical, although imaging may be helpful. Current medical treatment options for Peyronie disease include pentoxifylline, vitamin E, potassium para-aminobenzoate, colchicine, tamoxifen, and carnitine. Pentoxifylline is usually the first-line therapy. Intralesional drug therapy includes verapamil, interferon alfa-2b, and collagenase. Other treatments include penile traction therapy, iontophoresis, extracorporeal shockwave therapy, surgical straightening procedures, penile prosthesis placement, and radiation.