Are you one of those people who thinks hot water is ice cold?

Some of you will not know what this headline means. Others will have stuck your hand (or your whole body) under a shower and felt as though you were in an ice bath for a millisecond before you realized you've scalded yourself. For some people, sufficiently hot water will, for a moment, feel ice cold.

If you're reading this at a desk, you probably have a pen handy. Take the pen in one hand, make a fist with the other hand, and press the ball against the stretch of skin between your knuckles. Pick different spots to press the pen tip against. The metal will feel cold at some points. At others you'll feel a strange radiating sense of warmth, as if you've just hit the spot with a laser pointer. You've just mapped out some of the nerve endings on your skin. These nerves feel different sensations, but they all go up towards the brain using the spinothalamic tract. (Incidentally, if you poked too hard and hurt yourself, the pain sensations will use the same path.)

For the most part the nerves can handle anything you throw at them. The nerves that detect cold let the brain know when you're touching something cold, but ignore sensations of heat. The nerves that detect heat will remain dormant when exposed to cold. Neither is overworked or overwhelmed in your everyday life - until you stick your hand under something really really hot. When you stick your hand into something incredibly hot there's something of stampede that part of your nervous system. Every nerve gets swept up in it, and so when the signal reaches the brain, many people will feel a strange sensation of extreme cold before, or even while, they feel extreme heat.

There's some debate as to whether we feel the cold before the heat - which may mean that the signals from the cold sensors reach the brain first - or if they arrive at the same time and we just remember the cold as coming first. The nervous system pulls this kind of thing on us all the time. We withdraw our hands from the scalding water as quickly as we do because of the reflex arc. Often, when the nerves detect some immediately sharp pain, the signal will only get as far as the spinal cord before the motor neurons take over and force the body to jump away from the pain. We remember the pain happening first, but our reaction time is too quick for the brain to have processed what happened.

Paradoxical cold, as the sensation of cold caused by heat is called, is balanced by paradoxical heat. Sometimes we feel extreme cold as a burning sensation before the nerves sort themselves out and let us know we're feeling cold. People suffering from hypothermia also feel a sensation of paradoxical heat, which is why many hypothermic people are found without clothes on. I often feel either sensation as a strange simultaneous hot-cold mix. Have you felt paradoxical cold or heat?

[Via Ben Biggs, Mental Floss, Naked Scientists.]