Our sensations are transmitted from our skin through different neurons that are fine-tuned for specific sensations, such as hot or cold, touch, or pain. Which sensation or sensations a neuron transmits is determined by the type of protein receptors it expresses. And while there are several types of receptors that respond to heat, it turns out that mammals have one main receptor for cold detection called TRPM8, part of the transient receptor potential (TRP; pronounced “trip”) family of proteins that act as molecular thermo-detectors.
To study how cold is processed by the nervous system, the scientists used a toxin to kill all TRPM8-containing neurons in the mice. To test how the absence of the neurons affected their ability to detect cold, the mice were placed on a surface that ranged from 0 degrees Celsius at one end to 50 degrees at the other (32 to 122 degrees Fahrenheit) where they were allowed to move freely. Normal mice with all their neurons, not liking it too hot or too cold but just right, would tend to end up at an area where the temperature was a cozy 30 degrees Celsius (86 degrees Fahrenheit). The mice without TRPM8 neurons, however, avoided the hot area of the surface but did not avoid the colder areas, even areas near freezing where the surface should have been painful. Without the proper neurons, the mice couldn’t detect the cold. In another set of experiments, the scientists showed that cold no longer relieved pain in the TRPM8-lacking mice. Importantly, the mice’s ability to sense touch, heat and body position remained normal.The study was published in the February 13 issue of the Journal of Neuroscience. Led by David McKemy, associate professor of neurobiology at the University of Southern California Dornsife College of Letters, Arts and Sciences, the study was motivated by a desire to disentangle how different types of sensory information – hot, cold, pain – are processed by the neurons under our skin and then later by higher centers in the brain. McKemy hopes the research will help develop more nuanced pain treatments. “The problem with pain drugs now is that they typically just reduce inflammation, which is just one potential cause of pain,” he said in a press release, “or they knock out all sensation, which often is not desirable. One of our goals is to pave the way for medications that address the pain directly in a way that does not leave the patients completely numb.”
It’s probably up in the brain’s higher processing centers where the explanation for bizarre cases like Wim Hof and Lynne Cox exist. Hof, aka “The Ice Man,” is a Dutch man with an extraordinary ability to control his body’s normal responses with his mind. It allowed him to trek up Mount Everest wearing only shorts. “It was easy,” he says. And Cox, who crossed the English Channel in record time and, in 1987, became the first person to swim across the Bering Strait through near freezing waters (less than 4 degrees Celsius, 38 degrees Fahrenheit). Scientists continue to study these X-men-like phenoms, but with few new insights. Scientists can only guess that there’s some strange wiring in Hof’s brain that explains his ability to control, not only his own body temperature but his autonomic nervous and immune systems as well.
Pain is a good thing, it tells us when we’re likely to be injured and so helps us to avoid it. Chronic pain, however, serves no useful purpose and can drastically reduce a person’s quality of life. By studying pain at the extremities, as in the current study, and the higher brain centers of people with extraordinary abilities to suppress pain, we maximize the chances of finding pain relief for everyone.