Your penis is not a muscle, and 6 other things you probably didn’t know about erections

Half the population get them, yet many of us are not aware of the science behind how erections work.

Think of it like a reflex — one you have very little control over.

Erections have long been the subject of questionable jokes, half-truths and straight-up lies. Despite a plethora of sex-positive information and the easing of some taboos, most people still don’t know all that much about how erections work.

Maybe it’s a hangover from our collective Victorian prudishness. Maybe it’s because it wasn’t on the syllabus at school. Or maybe — most likely — it’s because when you scratch the surface, the humble erection can be a complex and mind-boggling thing.

If you want to know all the ins-and-outs — the fascinating physiology of a fully functioning penis — you can read Chapter 1 of our Book of Erections. But for now, here are 7 genuinely interesting things you might not know about how your erection works.

1. Your penis is not a muscle

With nicknames like the ‘love muscle’, it’s no wonder that most of us probably assume the penis is just that. That’s why, when we talk about erections, we tend to assume it’s the same as tensing any other muscle. After all, your biceps become hard when you squeeze them. Why would this be any different?

Contrary to popular belief, the penis isn’t a muscle. But that’s not to say it doesn’t contain a unique muscular structure. When you’re aroused, the smooth muscles in the arteries actually relax, opening them up and allowing the penis to fill with blood and a hard erection to materialise.

2. An erection is like a reflex

We’ve used the bicep analogy already, and it’s a good one to demonstrate how unique the penis is. Tense your bicep, and it will get hard. Tense your penis? Not going to happen.

That’s because an erection is like a reflex. In the same way that your leg will kick involuntarily with the tap of a hammer below the knee, your penis will become involuntarily hard in reaction to arousing stimuli. This can be touch, sight or sound. Whatever it is, it’ll spring your parasympathetic nervous system (PSNS) into action, causing a reflex-like response that tells your penis it’s time to get to work.

3. Your ‘fight or flight’ response can stop an erection dead in its tracks

Back in caveman times, our fight or flight response was a valuable tool to prepare us if we were about to be mauled by a saber-toothed tiger. Nowadays, it’s the feeling you get when you bump into a large angry man at a bar, or when your boss asks you into his office for a ‘chat’.

At first glance, it doesn’t seem to have much to do with erections. Yet the sympathetic nervous system (SNS), which controls this response, is responsible for pumping up your heart rate and pumping blood to the organs required for immediate survival. The penis, sadly, is not one of these organs. The result? Stress and anxiety in the bedroom can leave you struggling to get or maintain an erection.

4. Relaxation is key to a harder penis

Seems counter-intuitive, we know. Yet, as discussed above, the penis isn’t a muscle, and it definitely isn’t something that copes well with undue stress and anxiety.

While the SNS controls the fight-or-flight response, it’s the parasympathetic nervous system (PSNS) which brings about your erection. The PSNS is yin to the SNS’s yang: while the latter is activated when you’re anxious, the former kicks in while you’re relaxed, dilating the arteries in the penis and encouraging blood flow in.

That’s why a spot of mindfulness before sex or a Spotify ‘Slow Jamz’ playlist might just help get you in the mood.

5. There’s a molecule whose whole job is to help your erection run smoothly

As Jennifer Aniston would no doubt say: here comes the science bit, concentrate… When your penis becomes erect, it’s thanks to a signalling molecule called cGMP.

We could tell you how cGMP instructs the muscle proteins to modulate in structure in more detail (as we’ve done in the Book of Erections) but for now, you simply need to know it makes the muscles relax. Think of it as the guy on the door with the clipboard who tells them to step back while he allows the blood in.

6. Your penis needs to trap blood to stay erect

When your erection subsides, it’s because there’s a change of shift. The cGMP is broken down by an enzyme called PDE5, which is a bit like the guy who turns up to tell you the party’s over. The blood leaves the penis, and you’re left soft again.

The interesting part is that this is where ED drugs like, sildenafil, come in. They inhibit PDE5 before it has time to do its job, allowing you to stay harder for longer.

7. Testosterone helps erections - but no one knows why

Studies have shown that low testosterone plays a role in erectile dysfunction. The problem is, no-one knows exactly why.

That’s not to say there aren’t theories. Some think it’s because this key male hormone is involved in the healthy growth and functioning of the nerves involved in erection, while others say it’s due to its role in the production of nitric oxide, and thus the activity of PDE5.

Either way, it's best to keep your testosterone at healthy levels. Not only does it regulate libido (which can have an impact on your erections), but it also plays a number of important roles, including regulating sperm production, muscle size and strength, and bone growth and strength.

The numan take

Your erection isn’t a muscle, it’s more like a reflex. An involuntary response to erotic stimuli: a gentle touch, a sweet nothing in your ear, a ‘special’ movie on your laptop or the power of your imagination...

If you want to know more about how erections work, take a look at our comprehensive Book of Erections.