The Mysterious Sixth Sense of Humans:
Proprioception Bernice Ho (Year 6, Fry)
INTRODUCTION Imagine you have lost senses in your body and are not aware of your body movement, would you be scared? This is an experience that a man named Ian Waterman [1] must deal with for the rest of his life. Due to a rare autoimmune infection that attacked all his sensory neurons below his neck, he was left with the inability to feel the sense of touch and senses from below his neck. Without the feedback that comes with movement, Ian was unable to coordinate his movements in a meaningful way. When he had his eyes shut, he was completely unable to coordinate his muscle movement. Proprioception is sometimes called the “sixth sense,” apart from the well-known five basic senses: vision, hearing, touch, smell and taste. Proprioceptive sensations are a mystery because we are largely unaware of them. This article explains how proprioception works in our body, how important it is to our daily life, and what we can do to improve it. WHAT IS PROPRIOCEPTION? Proprioception is the medical term that describes the ability to sense the orientation of our body in the environment. In other words, it is basically defined as our ability to sense exactly where our body is [2]. It works unconsciously in our body and allows us to move quickly and freely without having to consciously think about where we are in an environment. Let’s try to feel our proprioceptive senses. Close your eyes and use your finger to point to your nose. Is it easy for you? Most of us should easily accomplish this little task. Why? PROPRIOCEPTION. THE SCIENCE BEHIND PROPRIOCEPTION Proprioception involves a complex signalling progress transferring proprioceptive signals from our body parts to the brain. It is a constant feedback loop within our nervous system, telling our brain what position we are in and what forces are acting upon our body at any given point in time. Proprioception is mediated by proprioceptors, which are tiny little sensors located throughout our entire body, especially in the skin, joint and muscle, and mechanosensory neurons. There are multiple types of proprioceptors working in our body (Table 1). The proprioceptive receptors are activated during distinct behaviors and encode distinct types of information such as limb velocity and movement, load on a limb, and limb limits. Thousands and millions of proprioceptive signals are sent to our brains through peripheral and central nervous
Figure 1: Muscle spindles as proprioceptors to transmit the signal of muscle stretch to the brain through the spinal cord. (Modified from Stevenlgourley,“Muscle spindles,” https://www.youtube.com/watch?v=F871bBWS4oY)
22
