Why do stinging nettles sting?

We put this question to Chris Smith...

Chris - The answer is that stinging nettles have - all over their leaves - lots of tiny needles. If you look at the leaves and stems of a stinging nettle closely, or use a magnifying glass, you'll see these tiny little things sticking out of the leaf surface, and they are hollow. They're made of silica, the same stuff that sand is made from. Inside them is a liquid and that liquid is histamine and another liquid called serotonin, and some other chemicals that the plant makes.

Those little straws of glass or sand are very, very fragile. When you touch or brush past the leaf or the stem, they get into your skin, scratch the skin and then they discharge or release some of histamine and the serotonin that's inside them into the surface layers of your skin.

Those two chemicals are really irritant. They actually stimulate your nervous system and they stimulate the nerves that signal pain and itch in the skin. This causes an inflammatory reaction and that's why you get a little bump where the sting was.

The reason the stinging nettles do it, of course, is to warn you off of treading on them, or eating them, in the future because if you get a painful contact with a stinging nettle, you know to watch out for them in the future and avoid them. So they've evolved to do that as a means of defence!

Kat - Does that make sense to you, Amelia?

Amelia - Yes.

Kat - Did you do anything like rub a dock leaf on your stings?

Amelia - No.

Kat - Did you just style it out and deal with the pain?

Amelia - Yes.

Kat - I was going to ask the rest of the people in the studio, do people believe the dock leaf thing because you meant to rub dock leaves on stings? Do you think that helps? Is there any science behind that?

Ginny - I have done it before. Whether it's actually any better than rubbing any old leaf because we know if you've hurt yourself, rubbing it makes you feel better. And that's because you're stimulating all the other nerves that are going on, you're stimulating your touch senses and that kind of overpowers that feeling of pain. That's why if you bumped your knee, you give it a rub, but I don't actually know if there's anything in the chemistry of dock leaves that makes them better.

Kat - I feel I reckon it's probably the rubbing.

克里斯,我认为你是对的。有一些called the gate theory of pain that Melzack and Wall came up with about 40 or 50 years ago. And, just as you say, Ginny, that when you stimulate large, low-threshold mechanoreceptors - which in plain English are the nerve cells that you stimulate when you brush skin gently - when you look at what they do in the spinal cord, as well as telling your spinal cord, "my skin is being brushed gently," they turn off the cells in the spinal cord that signal pain. And therefore, that's why, when you experience pain, you rub something better. You're effectively using your own in-built anaesthetic system to turn it off.

Kat - Absolutely fascinating. So, thanks very much for your question, Amelia.