h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I noticed more than once that if my hand is cold and I hit it against something it hurts more than if my hand were warm. Why is this? I've wondered about this for some time and never came accross a satisfactory answer...

I get this wonderful mental image of scientific hand-hitting trials.

I know exactly the phenomenon you mean, but not the answer, sadly.

**waits patiently for a biologist**

B

I remember this as a child in Canada, banging an almost frozen foot against something and it hurt like f**kity! Much more than if I had banged a nicely warm foot against something.

Something to do with circulation?

*also patiently awaits further knowledge*


az

I'm now waiting to get cold and bang my foot just so I can exclaim 'That hurt like F**kity' very loudly. Nice.

Also eagerly anticipating an explanation.

I, too, await explanation.

Oh, I'm waiting to catch a train. Am I in the wrong place? And why are you all putting your feet in the fridge and then kicking things?

Because there's nothing on telly. Personally, I went with the freezer. The fridge is just too slow.

Running out of variations of 'awaiting an explanation'. Someone tell us, please.

PLEASE.

Before my foot falls off.

Because it's just [i]so[/i] much fun!

Innit marvellous. I've just got over my Glastonbury trenchfoot, and now I've gone and got bleedin' frostbite. I used to like that little toe.

Where'd I put the hacksaw...

Ooh, that was a bit grim wasn't it. Sorry.

I *think* it is because the receptors for heat and pain signals from nerve endings in the brain are two distinct things, but below a certain temperature (around 10oC) the signal from the nerve endings that sense cold are interpreted by the brain as a pain signal. Thus a double whammy of pain signals, ergo it hurts more

I hope you're right, because that's a remarkably concise and logical explanation! I can actually get my head 'round it.

allrighty, where's the good doctor Z?

I'd settle for bad doctor 2legs, even.

B

That's dangerous talk. Have you no fear?

Brief exposure to cold can certainly make touch receptors hypersensitive - I remember after wading a very cold river to an island, walking through scratchy undergrowth to the next river channel with bare legs was certainly an intense sensory experience, but I'm not sure I'd actually qualify it as *pain*, just very close to sensory overload.

I presume an extended exposure makes the skin nerves numb by either direct further cooling, and/or by shutting off the blood supply

There are basically three kinds of sensory receptors in the skin:
* heat receptors
* cold recptors
those are often transmembrane proteins that get activated within various temperature ranges. They can be seen as the endings of sensory nerves. The nerve endings get activated and send a signal to the dorsal root ganglion, a hub in the back of the spinal cord. One signal goes up to the brain telling it´s warm, cold etc. Another is sent to the ventral root ganglion in the front of your spinal cord, and then via the motor nerves to your muscles that will make you drag away your hand from a hot saucepan for instance.

Some heat receptors also emmitts a pain signal if stiumalated with temperature over approximately 43C or capsaicin, the "hot" in chili peppers. The same is true with some cold receptors that respond to cold temperatures with pain.


* pain receptors, so called nociceptors, which are nerves that responds for acute pain or for chronic pain. The thin fibers that are responsible for the acute pain is also sensitive for cold, whereas the thicker fibres that are involved in chronic or slow pain also responds to heat.

We did a experiment to determine the ratio of heat receptors to cold ones in the skin on the back of our hands. Very simple, all you need is a not very sharp pencil (put in fridge for a few minutes), a paper clip and a cup of tea. If you slowly move the cold pencil point over your skin you can feel it when you come across a cold receptor. Dip the paper clip in the tea to warm it up, then do the same as before to identify the hot receptors. You have lots more hot than cold receptors Then drink the while you contemplate your results.