Dear Science,

When I take a shower with cold feet, it feels like the hot water is burning them. Is it? Is this bad for my feet?

Cold Feet

So long as the water doesn't feel too hot elsewhere on your body, your feet are safe—although Science suspects you need a bath mat, pronto.

Why does it hurt? Because the neural circuitry connecting your body to your brain is indistinguishable from that box of Christmas lights you never bothered to untangle before plugging in. Signals from your hot and cold sensors travel on the same neural circuitry—the ventral spinal cord pathways—as from your pain sensors. Smart work there, intelligent designer.

The human body has both cold and hot sensors—neither works like a thermometer you'd use to take your temperature. Your sensors tend to be more focused on detecting rapid changes—down or up, respectively—in temperature. You know how this works. When you jump into a warm shower, it feels shockingly hot. After a minute or so, even though the water is packing the same heat, it just feels warm. The sensors have adapted to the new temperature, with the hot and cold sensors settling down to the new normal. Hop out of the shower—another quick change—and the cold sensors go crazy. Brrr.

Just before the shower, your feet are probably on a really cold floor. When the hot water hits, the rapid change in temperature causes the heat sensors in your feet to freak out and signal like mad—so much so that the signal is probably spilling over to the part of the circuitry in your spinal cord normally used for the pain sensors. So, when the signal makes it up to your brain, it's hot and ouch when it should just be ahh. For a similar reason, this is why cold showers hurt—with the cold sensors signaling so strongly, thanks to the quick change downward in temperature, that the firing spills over to the pain-sensor side of the track.

How do we know all this? Lots of earnest and patient dissection by Hensel and Kenshalo in 1969 isolated the warm and cold receptors. The receptors were pinned and hooked up to amplifiers so the weak electrical signals generated by each could be read. Anything from 30O C to 45O C (where heat stops and barbecue begins) caused the warm sensors to fire off electrical impulses, with sudden shifts upward causing a cacophony of electrical pulses.

Science loves the hot shower in the morning more than anything else—craving the jolt to the temperature and pain systems caused by that first burst of hot water on the body. Civilization is worth saving for that alone.

Sensitively Yours,

Science

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