Anatomy and Physiology of Male Erectile Function

© Springer-Verlag Berlin Heidelberg 2015
Vincenzo Mirone (ed.)Clinical Uro-Andrology10.1007/978-3-662-45018-5_1

1. Anatomy and Physiology of Male Erectile Function

E. Wespes 

Urologic Department, CHU de Charleroi, Charleroi, 6000, Belgium



E. Wespes

The knowledge of penile anatomy is clearly very important for the physiology of erection. This latter is still much more important because it allows to discover new therapeutic modalities.

During these last years, many researches have been performed to improve knowledge on this fundamental function for the quality of life and the reproductive human system.

According to the guidelines, physical examination in patients with erectile dysfunction is limited but must cover different vascular or endocrine area.

Several radiological investigations have been created to study erectile function in practice.

This chapter develops the different aspects of these problems.

1.1 Penile Anatomy

The human penis is a unique structure composed of multiple fascial layers that surround three cylinders of erectile sinusoids, including a pair of corpora cavernosa and a single corpus spongiosum. The erectile tissue is housed within the paired corpora cavernosa which are enveloped by a membrane: the tunica albuginea. This envelope also covers the corpus spongiosum, but there it is thinner (Shetty and Farah 1999; Lue 2000; Gratzke et al. 2010).

The tunica albuginea of the corpora cavernosa is the essential fibro-skeleton necessary for rigidity during sexual intercourse. The tunica is composed of elastic fibers that form an irregular, latticed network on which the collagen fibers rest. The microstructure provides strength and allows the tissue to return to its baseline configuration after moderate stretch. In erectile dysfunction, decrease of the elastic fibers or modification of the ratio between the different types of collagen has been observed (Hsu et al. 1992).

It is a bilayered structure in which the inner layer is arranged circumferentially and in which fibers of the outer layer are arrayed longitudinally. The inner layer completely contains and, together with the intracavernosal pillars, supports the sinusoids (Hsu et al. 1992).

At the pendulous portion of the penis, the median septum is incomplete. This septum and the tunica albuginea together facilitate penile rigidity when erect. The corpora have the ability to engorge with blood to generate the penile rigidity necessary for vaginal penetration.

Within the tunica albuginea is a conglomeration of sinusoids, which are larger in the center and smaller at the periphery. The interconnected surrounded sinusoids are separated by smooth muscle trabeculae and by elastic fibers, collagen, and loose areolar tissue covered by endothelial cells (Shetty and Farah 1999; Lue 2000; Gratzke et al. 2010). These cells protect the sinusoid spaces but also secrete a great number of neurotransmitters that will provoke contraction or relaxation of intracavernous smooth musculature (Saenz de Tejada et al. 1989b). In erectile dysfunction, the number of these cells is not decreased, but their function could be seriously affected (Sattar et al. 1995b). Alterations of the neurotransmitters are at the origin of absence of smooth muscle cells relaxation or overcontraction (Saenz de Tejada et al. 1989b; Maher et al. 1996; Gu et al. 1984).

It has been suggested that there are gap junctions in the membrane of the adjacent muscle cells to explain the synchronous activity in the cavernous tissue of patients with normal erection, while there is a relatively sparse neuronal innervation of the cavernous smooth muscles (Campos de Calvalho et al. 1993).

More proximally, towards the perineum, the corporal bodies diverge bilaterally to form the crura of the penis. Each crus is anchored to the pubic arch at the level of the ischial tuberosity, where it is surrounded by the fibers of the ischiocavernosus muscles.

The corpus spongiosum is a single cylindrical tube that lies just ventral to the paired cavernosal bodies. It surrounds the urethra in its pendulous and bulbar portions. The distal portion of the spongiosum expands and covers the distal portion of the corporal bodies; this is the glans of the penis (Shetty and Farah 1999; Lue 2000; Gratzke et al. 2010).

Superficial to the tunica albuginea is Buck’s fascia, a layer of tissue that encircles both the cavernosa and the spongiosum. It is immediately superficial to the deep dorsal vein of the penis, the paired dorsal arteries of the penis, and branches of the dorsal nerves of the penis, all of which directly overlie the tunica. Buck’s fascia covers the spongiosum and the crura at the penile base, helping to fix these structures to the pelvic bones and the inferior fascia of the perineal membrane (Hsu et al. 1992).

The bulbospongiosus and ischiocavernosus musculature are superficial to Buck’s fascia. More superficial is the areolar dartos fascia, or Colles’ fascia, which additionally invests the perineum and the scrotal contents, extending to form the scrotal septum and the median raphe of the ventral penis, the scrotum, and the perineum.

Two ligaments suspend the pendulous penis from the anterior abdominal tissues and the pubis. The suspensory ligament is the more inferiorly located of the two structures and is a thickening of Colles’ fascia.

1.2 Penile Arterial Inflow

The internal pudendal arteries, the final branches of the anterior trunk of the internal iliac artery, give the penile arteries after the vessels pass the urogenital diaphragm. The penile arteries travel along the medial aspects of the inferior pubic rami and supply the corpora and the glans via branching vessels (Breza et al. 1989; Droupy et al. 1997; Huguet et al. 1981). An accessory internal pudendal artery may arise from the obturator, inferior vesical or superior vesical (Droupy et al. 1999). The bulbourethral artery supplies the bulb of the urethra, the corpus spongiosum, and the glans penis. It may arise from cavernous dorsal or accessory pudendal arteries, and this may be at risk during radical pelvic surgery (Polascik and Walsh 1995). The urethral artery commonly arises as a separate branch from the penile artery, but many arise from the artery to the bulb, the cavernous, or the dorsal artery (Breza et al. 1989; Droupy et al. 1997; Huguet et al. 1981). It runs on the ventral surface of the corpus spongiosum beneath the tunica albuginea. The cavernous arteries run centrally within the erectile bodies, closer the septum, while sending out various helicine branches that empty into the lacunar spaces. Most of these open directly into the sinusoids bounded by trabeculae, but a few helicine arteries terminate in capillaries that supply the trabeculae. There are very important to maintain a good function of the muscular component. These arteries have a tortuous configuration to accommodate for elongation during erection.

Occasionally the cavernosal arteries divide into two or three branches inside the corpus cavernosum. There are communications between both cavernosal arteries in some instances; connections with dorsal arteries are also present (Droupy et al. 1997).

The glans is supplied by the last branches of the penile arteries, the dorsal arteries. These give off circumflex vessels that communicate with the urethral arteries and send numerous perforating vessels to the skin (Breza et al. 1989; Droupy et al. 1997; Huguet et al. 1981).

The supply of freshly perfused, highly oxygenated blood is required to support the enhanced cellular metabolism associated with erection (with oxygen reduction the smooth muscles are reduced and the intracavernous structures are more fibrotic), and there is a local homoeostatic role as oxygen alters the synthesis of local erectogenic vasodilator substances (Moreland 1998; Sattar et al. 1995b; Kim et al. 1993).

Three main vascular systems supply the anterior perineal region and scrotum via smaller tributaries: the femoral system, which provides the anterior scrotal arteries, via the external pudendal arteries; the internal iliac system, which provides the posterior scrotal arteries via the superficial perineal vessels; and the external iliac system, which provides the cremasteric arteries via the inferior deep epigastric arteries. The external pudendal arteries of the femoral system give off arterial branches at the spermatic cord (Tauber et al. 2003).

From the internal iliac system, the internal pudendal arteries send superficial perineal vessels lateral to the bulbospongiosus and superficial to the superficial perineal membrane. The internal posterior scrotal arteries branch off between the external spermatic fascia and the dartos and travel lateral to the scrotal raphe to supply the dorsal scrotal septum, the posteromedial spermatic-scrotal fascia, and the perineal fat (Tauber et al. 2003; Jordan 2002).

The testicular arteries branch off of the aorta and travel downwards through the spermatic cords to supply the blood to the testes and the upper portions of the epididymis. The testes themselves are invested in a series of distinct tissue layers. From superficial to deep, these are the scrotal skin, dartos, external spermatic fascia, cremasteric muscles, internal spermatic fascia, and the tunica vaginalis, the last of which only provides a testicular covering anterolaterally. Each testis also has an outer tunica albuginea that invaginates into the posterior aspect of the testicle to form the mediastinum testis, which sends out fibrous septa that divide the testicle into numerous lobules (Tauber et al. 2003).

The arteries to the vasa originate from the inferior vesicle or the internal iliac arteries and run posteriorly within the internal spermatic fascia along with the vasa or the ducts of the testes. The cremasteric vessels originate from the inferior epigastric arteries at the internal ring of the inguinal canal and travel with the genital branches of the genitofemoral nerves between the spermatic fascial layers (Tauber et al. 2003; Jordan 2002).

1.3 Penile Venous Outflow

Penile venous return from the pendulous penis occurs through the deep and superficial dorsal veins of the penis, whereas the proximal crura drain through the cavernous and crural veins (Aboseif et al. 1989; Wespes et al. 1987). The endothelial-lined lacunar spaces of the corpora cavernosa are drained by small venules that form a subalbugineal plexus. These subalbugineal venules coalesce to form emissary veins that penetrate the tunica albuginea and open directly within the deep dorsal vein or through the circumflex system (Hsieh et al. 2012). More often than not one deep dorsal vein exists. Valves and polsters have been identified in the lumen of the deep dorsal vein. The deep dorsal and cavernous veins terminate in Santorini’s vesicoprostatic plexus and into the internal pudendal veins. The skin, the prepuce, and the glans are drained by the superficial dorsal veins that communicate with the external pudendal vein and/or the saphenous vein. The bulb is drained by the bulbar veins which drain into the prostatic plexus. The superficial and deep venous systems are interrelated by multiple anastomoses. Thus, the venous system of the penis communicates with the internal iliac vein, spermatic venous plexus, and saphenous vein.

The relationship of the vasculature to the fibro-skeleton is interesting, and the difference of the penile venous and arterial paths is substantial. The veins traverse an oblique path between the outer layers of the tunica albuginea, whereas the arteries take more direct route. This design is facilitating penile erection in that the venous vasculature is susceptible to being compressed (Aboseif et al. 1989; Wespes et al. 1987; Hsieh et al. 2012).

The pampiniform plexuses are collections of veins deep to the internal spermatic layers that drain the testes, eventually consolidating into testicular veins. These plexuses comprise the bulk of the cord (Hsieh et al. 2012).

1.4 Neuroanatomy of the Penis

1.4.1 Peripheral Pathways

The innervation of the penis is both autonomic (sympathetic and parasympathetic) and somatic (sensory and motor) (Steers 1994; Lepor et al. 1985; Halata and Munger 1986; Lue et al. 1984). From the neurons in the spinal cord and peripheral ganglia, the sympathetic and parasympathetic nerves merge to form the cavernous nerves, which enter the corpora cavernosa and corpus spongiosum to effect the neurovascular events during erection and detumescence.

The somatic nerves are responsible for sensation and contraction of the bulbocavernosus and ischiocavernosus muscles (Steers 1994).

1.4.2 Autonomic Pathways

The sympathetic pathway originates from the eleventh thoracic to the second lumbar spinal segments and passes via the white rami to the sympathetic chain ganglia. Some fibers then travel via the lumbar splanchnic nerves to the inferior mesenteric and superior hypogastric plexuses, from which fibers travel in the hypogastric nerves to the pelvic plexus (Steers 1994; Lepor et al. 1985).

Adrenergic nerve fibers and receptors have been demonstrated in the cavernous trabeculae and surrounding the cavernous arteries, and norepinephrine has generally been accepted as the principal neurotransmitter to control penile flaccidity and detumescence. Receptor-binding studies have shown the number of α-adrenoceptors to be ten times higher than the number of β-adrenoreceptors (Levin and Wein 1980).

The parasympathetic pathway arises from neurons in the intermediolateral cell columns of the second, third, and fourth sacral spinal cord segments. The preganglionic fibers pass in the pelvic nerves to the pelvic plexus, where they are joined by the sympathetic nerves from the superior hypogastric plexus (Chuang and Steers 1999).

The cavernous nerves and branches of the pelvic plexus innervate the penis.

Acetylcholine is required for ganglionic transmission (by nicotinic receptors) and vascular smooth muscle relaxation (by muscarinic receptors). Cholinergic nerves have been demonstrated within the human cavernous smooth muscle and surrounding penile arteries, and ultrastructural examination has also identified terminals containing cholinergic vesicles in the same area. It has been suggested that acetylcholine inhibits the sympathetic nerves and stimulates the release of NO from endothelial cells (Chuang and Steers 1999).

Nitric oxide is synthesized from endogenous l-arginine by NO synthase (NOS) located in the vascular endothelium. Nitric oxide may be synthesized and released as a neurotransmitter by the nonadrenergic/noncholinergic (NANC) neurons after their excitation by either electrical or chemical stimulation (Azadzoi et al. 1992; Saenz de Tejada et al. 1988).

NO released from NANC neurons increases the production of cyclic guanosine monophosphate (cGMP), which in turn relaxes the cavernous smooth muscle (Burnett et al. 1992).

NO diffuses locally into adjacent smooth muscle cells of the corpus cavernosum and binds with its physiologic receptor, soluble guanylyl cyclase. The enzyme catalyzes the conversion of guanosine triphosphate (GTP) to 3′,5′-cyclic guanosine monophosphate (cGMP). This cyclic nucleotide then serves as a second-messenger function by activating protein kinase G, alternatively known as cGMP-dependent protein kinase I (cGKI), which in turn exerts actions involving ion channels and contractile regulatory proteins that regulate the contractile state of corporal smooth muscle (Bush et al. 1992; Hurt et al. 2002).

Other neurotransmitters like vasoactive intestinal peptide or prostaglandin E (PGE-1) interact with different muscular receptors (Shetty and Farah 1999; Lue 2000; Gratzke et al. 2010). They increase the level of adenosine cyclic phosphate (cAMP) and also decrease the intracellular Ca2+ and produce relaxation of smooth muscle.

The decay in cytosolic calcium concentration provokes relaxation of the smooth muscle, resulting in dilation of arterial vessels and increased blood flow into the sinusoids of the corpora cavernosa.

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Jun 30, 2017 | Posted by in UROLOGY | Comments Off on Anatomy and Physiology of Male Erectile Function
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