Erection
From Encyclopedia of Sex and Sexuality
A man’s penis is said to become erect when it increases in size, girth, and firmness, changing from its usually flaccid (soft or yielding) state to one of rigidity. The penis’s erection is a complex process that depends on the coordination and integration of psychological, neurological, endocrinal (hormonal), and vascular mechanisms. These mechanisms have only been understood since the late 1980s and there are still steps in the phenomenon that remain a mystery.
An erection can be caused either by psychogenic stimuli (erotic thoughts) or by reflexes which occur when sensory receptors on the penile skin and glands are activated (touched). However, the penis’s corpora cavernosa (vessels containing caverns that become engorged with blood during erection) must also be healthy if an erection is to occur and be maintained.
Within the penis a dense fascia, the tunica albuginea, surrounds the corpora cavernosa. Arteries supplying blood to the penis run deep in the middle of each corpus and lead to tiny caverns. When the penis is flaccid, the arteries are constricted and the caverns they connect with are squeezed by surrounding muscle tissue. When aroused, the arteries dilate and the smooth muscle tissue surrounding the caverns relaxes.
The expansion of these caverns not only serves to store blood and increase the size and girth of the penis, but also compresses the small exit veins (located at the periphery of the corpus cavernosum) against the thick and relatively inelastic tunica albuginea. This is the mechanism by which blood is trapped within the penis to enlarge and stiffen it. A substance called nitric oxide is released by cells lining the caverns and blood vessels as well as from the nerve endings, and serves as an activator of muscle relaxation.
If muscle relaxation fails to occur, not only is resistance high to blood flowing into the penis, but resistance is low to blood flowing out, because veins draining blood from the penis remain open. It is this nitric oxide-mediated relaxation that is currently thought to be impaired in men suffering from impotence related to diabetes mellitus, arteriosclerosis and high cholesterol levels. More than 80 percent of the men treated obtain a firm erection with a small and painless injection of a vasodilator substance directly into one of the corpora cavernosa. Erectile dysfunction, or impotence, is a major problem of adult men in the United States. An estimated 10 million men suffer from erection problems and it is likely that this number will grow as the population ages. Treatment options include the use of an external vacuum tumescence device, self-injections, or penile prostheses. In young patients with a history of pelvic trauma, vascular reconstruction is also performed but with much less success. Currently, painless penile injections with prostaglandin E when erection is desired seem to be the treatment of choice by most men suffering from impotence.
