Unwanted erections are something we may not talk about, but we've all experienced them. When I was a teenager I had a hair-trigger dick. I would ride the bus to school every morning and I don't know whether it was the vibrations of the bus or the fact that Suzie (with conspicuous nipples protruding through the stretched fabric of her tank top) brushed my leg on the way to her seat, but by the end of the ride I had to get off the bus with my books strategically held in front of me. I wasn't the only one either. I wonder, as everybody now seems to tote a backpack, how young men manage these days. Do they employ the hands-in-pocket trick of making sure one's knuckles protrude farther than the profile of one's stiffy? Or do they just let it all go?
The unwanted erection is a de rigeur comic plot device in teen movies; running a close second behind the perennially popular guy-getting-hit-in-the-balls-and-pulling-a-funny-face gag. In the funniest scene from the movie Diner, Mickey Rourke makes a bet with his friends that a woman he's taking to the movies will have her hand on his dick before the night is through. During the movie he inserts his penis through the bottom of the popcorn box and tells his date to dig in. When she inevitably finds his penis she shrieks and runs from the theater. Mickey chases after her and explains that sitting next to her got him all worked up. He opened his fly to "give it a little air" and it "burst" through the bottom of the tub. Her eyes grow wide. It burst through? Mickey nods sheepishly and the girl totally buys it because she knows that dicks really do have a mind of their own.
The post-teen years are a little less eventful stiffywise, and as we age, the opposite problem occurs and we don't get erections when we want. A minority of older men don't get erections at all. It's not that the desire isn't there; it's just that the magic that once worked so unfailingly simply cannot be summoned. Why is this? What actually goes on down there?
Interestingly, the human male is one of the few mammals (along with horses, rabbits, kangaroos and hyenas) not to have a penile bone (called a baculum) to help support the stiffness and shape of an erection. Even our closest relatives, the bonobo chimps, have one. In fact, some scientists speculate our lack of baculus may come down to a single mutation on the 21st chromosome that swept through the humane genome. Why we lost the bone in our boner is anybody's guess, but it doesn't look like it's coming back anytime soon.
The mechanics of an erection are actually pretty simple. Arterial blood flows into the spongy tissue of the penis while the veins are closed off. The resulting pressure causes the penis to stiffen and rise. It's like hydraulics, but the simplicity ends here. All the other forces involved in the creation and maintenance of an erection rely on such a precarious series of events that when cognizant of them one is amazed that hard-ons manage to occur at all.
What triggers an erection depends on what kind of erection it is. Most erections that we're aware of arise from some form of stimulation. Take my late teenage bus ride, for example. Vibrations on the bus make micro-movements that excite the nerves on the surface of my penis. These nerves send a flow of "feels nice" signals to my thalamus. When Suzie brushes by me the visual, tactile and (most likely) olfactory sensations trigger thoughts of what Suzie meant by brushing past me like that. These in turn generate thoughts of Suzie naked and by that point it's pretty much game-over. These thoughts result in a cascade of electrical signals that are routed through a part of the brain called the hypothalamus (specifically the medial preoptic area and the paraventricular nucleus, for those of you who simply must know these things). The hypothalamus in turn releases a cocktail of neurotransmitters which result in a flood of signals down through the parasympathetic nerve system to a secondary relay station located at the base of the spine. From here, the signals pass by way of peripheral nerves radiating out and into the penis. At the nerve terminus, neurotransmitters are released that cause the smooth muscle of the blood vessels in the penis to relax. As it does, the blood vessels dilate allowing blood to rush in at 60 times the normal rate.
The idea that erections are created by the relaxation of muscle tissue is counter-intuitive. Most muscles harden when contracted, and right up until the early 80s there was fierce debate as to whether erections were created by the relaxation of arteries or the contraction of veins. The debate ended in 1983 when a neuropharmacologist from England named Giles Brindley injected his penis with a powerful muscle relaxant called phenoxybenzamine resulting in an erection. According to rumor, Dr. Brindley announced his discovery during a lecture on the pharmacology of erectile function by pulling down his pants and revealing a solid erection to his audience. Word has it he walked up and down the aisles of the lecture hall inviting people to inspect its firmness. Such daring sans-pants educational antics ensured a place in pharmacological folklore for Dr. Brindley.
But the question remained, what kept the blood in the penis? Why aren't the veins dilated as well? The answer is that as the erection chambers fill, they press against small veins which normally drain the penis of blood. These veins get squeezed shut against the outer layers of the erection chambers, trapping the blood inside.
The erection is also mediated by the nerves of the sympathetic nervous system. These release neurotransmitters that contract the arterial blood vessels, cutting-off flow. It is the rivalry between these two nervous systems that determines the extent and duration of an erection. In periods of repose, or when you're thinking of sex with grandma, it is the signals from the sympathetic system that are dominant.
An erection requires an almost constant cascade of signals through the parasympathetic system. This is where tactile stimulation comes in. During sex the sensation of friction sends a steady stream of signals to the brain which, along with erotic thoughts (I mean, hello, you're having sex!), excites the parasympathetic system which keeps the blood vessels dilated.
The exception to all the above is the nocturnal erection. The jury is still out on what causes the ol' Morning Glory, but it seems that higher brain functions are not involved. Remember to tell your girlfriend this after she stops pummeling you because you admitted you were dreaming about your ex. There is a thought that such erections are caused by fluctuations in the blood levels of testosterone and DHT but like I said, no one really knows. It's likely that the nighttime woody is just the body's way of conducting a system check.
Any breakdown in any component of this complex system (neural, hormonal, vascular, psychological) can result in erectile dysfunction. And while this can be devastating both to the guy who can't get it up when he wants or the guy who popped a stiffy in his Speedos just as his Mom came in, it helps to remember that none of this is your fault. Because neither the parasympathetic and sympathetic systems are subject to conscious control, you cannot will yourself to have an erection anymore than you can will yourself not to. All you can control are the thoughts and sensations that lead to the excitation of one nervous system or the other.
What's cool about this is how thought in and of itself can provoke a physical response. Next time your girlfriend complains that you never convert thought into action, picture her naked and then tell her to "check it out, baby."
