If you focus on the chair that you’re sitting on, you can consciously feel the stiffness of its surface, the way your legs are positioned, and maybe the way your fingers are hovered over the mouse or keyboard. This awareness of the location of our limbs relative to each other, known as proprioception, is constantly in the back of our minds and is used in our daily interactions.
Without this “sixth sense,” everyday tasks such as walking with regular strides and reaching for objects, as well as more complex ones such as doing gymnastic flips, become very difficult. Our body uses proprioception in these activities by processing the angle of our arms and legs, the sensation of hot and cold, and other information from our movements. Although this sense is closely related to touch, it involves not only our perception of the environment but also its relation to ourselves.
We are able to unconsciously process information from proprioception through our nervous system, including the brain, spinal cord, and proprioceptors, which are sensory neurons. These proprioceptors are cells that can respond to stress and tension through a specific ion channel, Piezo2. For example, if pressure is applied to a skin cell, the Piezo2 ion channels in the related neurons will open, allowing ions to transfer inside and outside of the cell.
The ion channels are able to react in this way because of their structure as mechanically activated ion channels. Located in the cell membrane, they are connected to the cell’s cytoskeleton and surrounding structure through tethers. These attachments stretch in response to forces, such as pressing the tip of your nose with your finger, which causes the ion channels to open. The resulting movement of ions allows input from the outside world, (for example, touching your nose with your eyes closed), to be translated into a flow of chemicals interpreted by the body.
When there is a mutation affecting the Piezo2 ion channel in humans, the ion channels are hyperactive and, after opening, have delayed inactivation. As a result, this loss of sensitivity to the cell’s surroundings affects proprioception, leading to impaired bone growth and decreased awareness of joint position. In one NIH study, two patients with the mutation had similar affected walking gaits and progressive scoliosis. Although they were able to use the relative location of a pencil to form letters when their eyes were open, they could not control the movement of the pencil based on touch alone once they put on a blindfold.
The patients had otherwise healthy nervous systems and for certain activities, such as writing, had developed workarounds. For example, they based the movement of their hands on visual cues rather than on the hand’s relative location to the body due to their lack of proprioception. The flexibility of the nervous system in adjusting to this mutation could be a potential factor in treating other conditions with affected proprioception, including traumatic brain injuries and Parkinson’s disease.
Using proprioception, we not only have an awareness of ourselves in our surroundings but in our internal structures too, as seen in the patients with scoliosis. Even if this sixth sense may not allow us to see through walls or around buildings, it does allow us to respond to our constantly changing environment — and that’s pretty sense-ational.