FIGURE 22.30 • Photomicrograph of a histologic section of the penis. This photomicrograph shows an H&E–stained specimen of a cross section of the penis near the base of the organ. Note the arrangement of the corpora cavernosa and corpus spongiosum; the latter contains the urethra. 3.
blood, principally derived from the helicine arteries. These arteries dilate during erection (see Folder 22.5) to increase the blood flow to the penis. An arteriovenous (AV) anastomosis exists between the deep artery of the penis and the peripheral venous system (Folder 22.5).
The skin of the penis is thin and loosely attached to the underlying loose connective tissue except at the glans penis, where it is very thin and tightly attached. The skin of the glans is so thin that blood within its large, muscular anastomosing veins that drain the corpus spongiosum may give it a bluish color. There is no adipose tissue in the subcutaneous tissue. There is, however, a thin layer of smooth muscle that is continuous with the dartos layer of the scrotum. In uncircumcised males, the glans is covered with a fold of skin, the prepuce, which resembles a mucous membrane on its inner aspect. Numerous sebaceous glands are present in the skin of the penis just proximal to the glans.
The penis is innervated by somatic, sympathetic, and parasympathetic nerves. Many sensory nerve endings are distributed throughout the tissue of the penis, and sympathetic and parasympathetic visceral motor fibers innervate the smooth muscle of the trabeculae of the tunica albuginea and the blood vessels. Both sensory and visceral motor fibers play essential roles in erectile and ejaculatory responses.
CNS and maintained by complex interactions between vascular and neurologic events. The CNS responds to external or internal stimuli (sensory impulses, perception, desire, etc.) that involve the sympathetic and parasympa-thetic innervation of the penis. Parasympathetic stimulation initiates erection by relaxation of the trabecular smooth muscle cells and dila-tion of the helicine arteries. This leads to expansion of the corpora cavernosa and, to a lesser degree, the corpus spongiosum. Arterial blood accumulates in these erectile tissues by compression of the venules against the nondis-tensible tunica albuginea. This process is referred to as the corporal venoocclusive mechanism. The tunica albuginea also compresses the larger veins that drain blood from the corpora cavernosa so that venous outflow is also blocked, resulting in tumescence, and rigidity of the penis. Two neuromediators, acetylcholine and nitric oxide, are involved in the relaxation of smooth muscle during the initi-ation and maintenance of penile erection. nerve endings and acts primarily on the endothelial cells that line the vascular spaces of the corpora cavernosa. This causes the release of vasoactive intestinal peptide (VIP) and, more importantly, nitric oxide.  Nitric oxide (NO) activates guanylate cyclase in the trabecular smooth muscle cells to produce cyclic guanosine monophosphate (cGMP). cGMP causes the smooth muscle cells to relax. Sympathetic stimulation terminates penile erection by causing contraction of the trabecular smooth muscle cells of the helicine arteries. These events decrease the flow of blood to the corpora cavernosa, reducing blood pressure in the erectile tissue to normal venous pressure. The lower pressure within the corpus cavernosum allows the veins leading from the corpora cavernosa to open and drain the excess blood. Erectile dysfunction (ED) is an inability to achieve and maintain sufficient penile erection to complete satisfactory intercourse. Adequate arterial blood supply is critical for continued next page