By Emily Sheng
Have you ever taken a shower after a freezing winter day and set it to the hottest temperature possible? Did it feel warm as expected, or did it end up feeling cold? The human body has a powerful way of interpreting the conditions of our environment through the somatosensory nervous system. A nervous system is a collection of special cells called neurons that form nerves and allow different parts of your body to communicate with each other. Because of how the nervous system is wired, we sometimes feel cold while taking very hot showers.
So how exactly do these neurons detect the temperature of water? Neurons close to the skin contain a thermoreceptor that is able to detect the temperature of the surrounding skin. For example, if you wash your hands with cold water, the thermoreceptors inside your skin will send signals to the brain telling the brain that it has detected cold temperatures. This general process of your neurons communicating with each other to send information to and from the brain requires chemicals called neurotransmitters and electrical signals called action potentials.
Specifically, there are two types of thermoreceptors in the human body. There are warm receptors that respond to temperatures between 86˚F to 104˚F, and cold receptors that respond to temperatures between 68˚F to 86˚F (while this might not seem cold at first glance, for comparison the human body is around 98.6˚F). These receptors respond by increasing how often they fire electrical signals when temperatures decrease for cold receptors and when temperatures increase for warm receptors. However, some cold receptors also respond to temperatures greater than 113˚F, a painfully hot temperature. Because these cold receptors do double duty, this can cause really hot water to feel cold. This phenomenon where your body perceives a cold sensation above 113˚F is called paradoxical cold.
Another interesting fact about these thermoreceptors is that some not only respond to temperature, but they also respond to certain chemicals. For example, when you are brushing your teeth with minty toothpaste or eating a candy cane, your mouth feels cold even though the temperature hasn’t changed. This experience is caused by a specific type of thermoreceptor called TRPM8 that responds to cold temperatures below77˚F and natural chemicals found in mint like menthol. Similarly, when you eat a spicy bowl of noodles or hot salsa, your warm thermoreceptors also can get activated. In chili peppers, there is a chemical called capsaicin that is even made into spices like chili powder and paprika to make food feel hotter temperature wise!
One unique condition about these thermoreceptors is that they are most active right after the temperature change first occurs. This is exemplified when you first jump into the swimming pool on a hot summer day and the water feels really cold, but after some time has passed, the water feels comfortable. This also happens when you take your first bite of spicy fried chicken or first dip of hot sauce. Initially after a temperature change, warm and cold receptors are the most sensitive, so they increase the rate at which they fire electrical signals to tell the brain of the new temperature.
So, the next time you take a really hot shower or suck on a peppermint candy, remember that how your brain perceives the temperature of the environment may not be completely accurate!
Sources:
Stanfield, C. L. (2016). Principles of human physiology (6th ed.). Pearson.
https://www.smithsonianmag.com/science-nature/why-does-very-hot-water-sometimes-feel-cold-180953532/
