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• You can have an erection by thinking about sex, or seeing or hearing something that's sexually exciting. Your brain sends messages to the nerves in your penis. The nerves tell the penis to get stiff.
• You can also have an erection because someone or something is touching your penis, even without sexual thoughts. This is called a reflex erection. It doesn't come from your brain. Nerves in your penis sense the touching and send a message to other nerves in your spinal cord. These nerves then tell the penis to become erect.
• Often, you get excited by thinking about sex and someone touching you sexually at the same time.

When your nerves give the signal to your penis to trigger an erection, they release a chemical called nitric oxide.

• Nitric oxide makes the blood vessels in your penis open wider. So, more blood can flow through the blood vessels into your penis.
• Nitric oxide also helps the spaces in the spongy columns open up and fill with blood. This makes the columns firm and gives you an erection.

Nitric oxide works by forming another chemical called cyclic GMP.

• Cyclic GMP makes the walls of your blood vessels stay wide open. And it keeps the spaces in the spongy columns open.
• Once sex is over, you stop making cyclic GMP. The spaces start to close up and squeeze blood out of the spongy columns. So your penis gets soft again.
• Some of the drugs that treat erection problems work by increasing the amount of cyclic GMP.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.