Close your eyes and touch your nose. If everything is working properly, this should be easy because your brain can sense your body, as well as its position and movement through space.
This is called proprioception. But how does this “sixth sense” work — and what happens when it clashes with other senses?
We’re all familiar with the five standard senses, which include vision, hearing, smell, taste and touch. The idea that there are only five of them has been rooted in our minds since the time of Aristotle, who explicitly rejected the idea of a sixth sense. But for centuries scientists have seriously entertained the idea of a sixth sense that allows us to perceive our bodies.
There remains a lot of debate about whether this sense, which later became known as proprioception, can be considered an additional sense alongside the five standard ones. After all, the five senses all allow us to experience the outside world, whereas proprioception allows us to understand our physical place within that world.
Sixth sense or not, proprioception is recognized as being vital to our daily experiences and something that contributes to our overall body ownership. As Nature’s Allison Abbot says: “Without it, our brains are lost.”
What Is Proprioception?
Let’s start with what proprioception is not.
Proprioception is not the vestibular system — the master controller of our balance and spatial orientation. However, the vestibular system does contribute to the guidance of our bodily movements.
And though the two terms are often used interchangeably, proprioception is not kinesthesia. Like proprioception, kinesthesia involves the senses of limb position and movement, but scientists typically view the focus of these two as being quite different.
That is, kinesthesia is behavioral in nature, and it places an emphasis on the body’s motions, as well as incorporates routine or habitual behaviors to improve movements. Both hand-eye coordination and muscle memory involve kinesthesia — the more you perform certain actions, such as during sports, the better at them you will become. Comparatively, proprioception has more to do with body position, and focuses on the cognitive awareness of the body in space.
Importantly, the proprioceptive system is often considered to include both the vestibular and kinesthetic systems.
According to a 2012 review on the topic, proprioception includes “the senses of position and movement of our limbs and trunk, the sense of effort, the sense of force, and the sense of heaviness.” Proprioception uses receptors located in the skin, muscles and joints to build the internal sense of our bodies.
Surprisingly, the study of proprioception began hundreds of years ago. In 1826, neurologist Charles Bell questioned what functions the muscles had, other than to contract under the command of motor nerves.
He concluded that they must provide our brains with the position of the body and limbs when the brain has no other way of knowing these things (such as if you can’t see your limbs). In 1887, neurologist Henry Charlton Bastian proposed “kinesthesis” to replace the common terms “muscular sense” and “sense of force.” To Bastian, kinesthesia, then, was the perception of the position and movements of the limbs, as well as their degrees of resistance and weight.
Almost 20 years later, in 1906, neurophysiologist Charles Scott Sherrington coined the terms proprioception, interoception and exteroception. Proprioception, he said, was an awareness of the body, which stems from sensory receptors — proprioceptors — in the muscles, tendons and joints. Interoceptors and exteroceptors provide the brain with information about the internal organs and external world, respectively.
Originally experts believed we figured out the positions of our limbs from our muscles. But in the 20th century they ascribed this to joints instead, based primarily on studies on the stretch receptors in the knee joints of cats. Today the view has flipped again, with most physiologists believing that the principle proprioceptors are housed in the muscles.
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