Smooth muscle plays a key role in every erection. HowStuffWorks.com

The first real breakthrough in the treatment of erectile dysfunction came in 1983. Prior to that time, it was thought that erectile dysfunction -- the inability to achieve an erection -- was primarily mental.

That concept came crashing down at the 1983 American Urological Association meeting in Las Vegas when Dr. Giles Brindley injected his penis with the drug phentolamine. Following the injection, Brindley appeared on stage and dropped his pants to display one of the first drug-induced erections to the incredulous audience of urologists.



What did the phentolamine do? It relaxed a muscle.

Inside the body there are several kinds of muscle:

Skeletal muscle is what we see at the Olympics -- bulging biceps, and so on.

Cardiac muscle powers the heart

Smooth muscle can be found in things like blood vessels, the intestines and the stomach, and it usually acts involuntarily.

Smooth muscle plays a key role in every erection, and phentolamine is a drug that relaxes smooth muscle.

The reason why an injection of phentolamine produced an erection was especially interesting in 1983 because no one had really thought about it before. Here's what happened:

The arteries of a limp penis are constricted, and they keep blood from entering the corpora cavernosa.

Brindley's injection relaxed the smooth muscle in the artery walls inside his penis, causing them to open up.

Blood surged into the corpora cavernosa, and the blood pressure inflated his penis, giving him an instant erection.

Starting in the mid-1980s, it became common for men with erectile dysfunction to inject smooth-muscle-relaxing drugs as a treatment for the problem.

Viagra makes the process a whole lot easier by doing the same kind of thing with a pill instead of an injection. Another advantage of Viagra over an injection of phentolamine is that Viagra only causes an erection when the man is sexually aroused. Phentolamine, by contrast, causes an immediate and uncontrolled erection.

How can a pill work only on the smooth muscle in the penis and not the entire body, and only when the man is aroused? The answers to these questions begin with an understanding of how blood flow works in the body, so let's start there.