Here’s a challenge: Close your eyes and then try touching your nose with your finger.
Did you do it? Even without using any of the five famous senses—sight, hearing, taste, touch, and smell—you most likely found your nose without any trouble. This ability is known as proprioception, or what the biologist Seung-Hyun Woo at the Scripps Institute describes as “the sense of your body parts’ position.” It’s the reason you can switch from the gas pedal to the brake without looking at your feet, or bring popcorn to your mouth without taking your eyes off the movie screen.
Often considered a sixth sense, proprioception is much less understood than the other five—researchers have identified molecules related to taste and smell, for example, but research on proprioception has lagged behind.
But it’s finally catching up. In a study recently published in the journal Nature, Woo and her colleagues from Scripps, Columbia University, and San Jose State University identified the key molecule that governs proprioception: the protein Piezo2, found in the membranes of special nerve cells in our muscles and tendons called proprioceptors.
When we move, our muscles and tendons get stretched, which puts tension on the proprioceptors’ membranes, explained Jorg Grandl, a Duke University neurobiologist unaffiliated with the study. That tension distorts each Piezo2 protein and causes a tunnel to open in its center. Small activating particles rush through the tunnel into the proprioceptor cell, causing it to fire an electrical impulse up the limb, through the spinal cord, to the brain. The whole process happens in a matter of milliseconds.