You trudge across the sodium lit street toward the front door, footsteps echoing off the adjacent houses—it’s been a long day. Plodding up the stairs, you enter the bathroom and turn on the shower. Finally, a time to relax and unwind. But when the steaming water first hits your skin, you're jolted by a sharp, icy-cold sensation, accompanied by searing pain. Why does that hot water feel so cold?
The human body senses temperature changes through specialized nerve endings called thermoreceptors, located just beneath the skin. These receptors are distributed throughout the body and are constantly transmitting temperature information to the brain. A decrease in temperature activates cold receptors, and an increase activates warm receptors. Thermoreceptors can also respond to specific chemicals. For example, menthol activates cold receptors, which explains the chilling sensation you might feel after brushing your teeth or using an analgesic cream. Capsaicin, a chemical found in chili peppers, has been shown to activate warm receptors, causing the familiar red-hot burning and sweating reaction that accompanies a spicy meal.
Cold receptors primarily react to temperatures ranging from 68 to 86˚F, while warm receptors are activated between 86˚F and 104˚F. At extreme temperatures—below 60˚F and beyond 113˚F—the temperature signal is accompanied by a sensation of pain. Weirdly, researchers have discovered that at temperatures greater than 113˚F, some cold receptors can also fire. This phenomenon, known as paradoxical cold, has puzzled scientists for decades. No one is quite sure why the effect happens, since it doesn't seem to offer an evolutionary or adaptive benefit, says Barry Green, director of the John B. Pierce Laboratory and professor of surgery at Yale University School of Medicine. Today researchers are considering a wide array of interpretations of the strange sensation.
The majority of scientists support the theory that paradoxical cold is a malfunction of the thermoreceptor system. Evidence suggests that pain receptors that respond to potentially harmful heat levels coexist on the same sensory fibers as cold thermoreceptors, says Lynette Jones, a senior research scientist at MIT. So when the nerve fiber sends a signal to the brain, it can sometimes be misinterpreted as a sensation of extreme cold. Paradoxical cold is the “strange operation of a system under unusual stimulation conditions,” she says.
It's also possible that cold receptors can do double duty, says Green. Based on his research, he thinks cold receptors can be recruited to help the brain sense potentially harmful temperatures at both hot and cold extremes. So instead of considering the input from cold and warm receptors separately, the brain integrates them.
“The brain is a highly economical computational machine. It is using all the information it can to make as quick and accurate a judgment as possible,” says Green. “There is an array of receptors that comes into play, and I believe it is the total readout that the brain is using.” This theory is supported by the fact that there are far more cold receptors beneath the skin than warm ones, and the signals from cold receptors actually travel to the brain up to ten times faster than signals from warm receptors. That suggests cold receptors could provide additional pain signaling when you encounter dangerous temperatures.
However, paradoxical cold only activates a subset of cold receptors, and your body temperature at the time determines whether you feel it. Having a higher internal body temperature lowers your threshold for sensing cold, so the warmer you are, the greater the chance of experiencing a paradoxical cold response.
Scientists have also confirmed the equally puzzling existence of paradoxical heat, in which even a relatively mild cold blast produces a hot sensation. Until sufficient research is found to tip the balance toward a particular theory, the actual workings of paradoxical sensations will remain a topic of heated debate in the scientific community.