What might constitute a sixth sense? Perhaps, it involves possessing a second sight or superhuman abilities. A classic example of this would be Spider-Man and his ‘spidey-sense’ — an instinctual warning system that alerts him to imminent danger. Enhancing his reflexes and agility, his sixth sense enables him to evade threats with precision. 

Turns out Spider-Man is not the sole bearer of a ‘spidey sense’. While we may not be scaling walls anytime soon, we too possess a special sense that unconsciously guides our movements. It might sound peculiar, but knowing your arm is indeed your own arm involves a unique form of sensory processing. Considered by neuroscientists as our own ‘sixth sense’, proprioception is our own way of helping the brain to understand the position of our body and limbs in space (Sherrington, 1907).

Consider a typical scenario: your first sip of coffee in the morning. Eyes shut, you savour your latte before the day begins. Such a simple act, yet impossible without proprioception. With closed eyes, how do you know where your mouth is? How do you gauge the position of your arm to ensure the coffee cup reaches your lips? Proprioception seamlessly transmits information about muscle tension, joint position, and force to the brain, making drinking your coffee an automatic and coordinated process.

Proprioception operates on principles akin to those guiding our other senses. Specialised cells, known as receptors, are found in each sensory organ and receive information from the environment. Receptors in your eyes capture visual information, while those in your ears detect auditory stimuli. This sensory information is transduced through signals to the central nervous system – through the spinal cord and to the brain – where it’s integrated and processed to determine an appropriate response.

Analogously, proprioceptive information is mediated by proprioceptors, a unique type of receptors located in your muscles and joints (Proske & Gandevia, 2012). Unlike our other senses, proprioception does not rely on input from the external environment. Rather, it provides feedback to the brain about what the body itself is doing. Changes in muscle tension and the position of our joints are relayed to the brain, ensuring awareness of the body’s whereabouts at any given moment. 

One implication of this ‘internal’ feedback loop is that proprioception never turns ‘off’. When you cover your ears, you experience silence. If you hold your nose, you can block out the smell. Yet even when still, in motion, or unconscious, your brain continuously receives proprioceptive input. 

Imagine this in the context of going to bed each night. What exactly prevents you from falling out of bed, once asleep? While most senses are subdued when sleeping, proprioception remains active, informing the brain about the slightest changes in the position of the body. This ensures a perpetual awareness of our body in space – and luckily for us, stops us from rolling out of bed (Proske & Gandevia, 2012). 

It can be hard to appreciate what our proprioceptive system allows us to do, given its unconscious nature and integration with our other senses. Rare neurological disorders affecting proprioception highlight just how critical this sense is in our daily lives. 

The case of Ian Waterman – now known as ‘the man who lost his body – offers profound insights into the significance of proprioception (McNeill et al., 2009). Following a fever in 1971 at age 19, a subsequent auto-immune reaction destroyed all his sensory neurons from the neck down–a condition termed ‘neuronopathy’. Despite retaining his intact motor functions, Waterman lost all proprioceptive abilities, rendering him unaware of his body's position in space.

Although the viral infection’s initial effect was that of immobility, this loss was not due to paralysis. Rather, it was Waterman’s lack of control over his body that inhibited his ability to move. Sitting, walking, and manipulating objects became impossible tasks as a result of the absence of any proprioceptive feedback from the body.

Remarkably, Waterman has been able to teach himself precise strategies to walk and function with a degree of normality (Swain, 2017). Yet, all movement requires concerted planning and relies entirely on vision to compensate for the unconscious proprioceptive processing.  In the absence of any light, Waterman is unable to see his limbs, thus restricting his ability to move.

An understanding of the molecular mechanisms underlying proprioception remains somewhat of a mystery compared to that of our other senses. However, recent genetic advancements are paving the way for the development of novel therapies aimed at neurological and musculoskeletal disorders (Woo et al., 2015).

A study involving two young patients with unique neurological disorders affecting their body awareness revealed a mutation in their PIEZO2 gene (Chesler et al., 2016). Both individuals experienced significant challenges with balance and movement, coupled with progressive scoliosis and deformities in the hips, fingers, and feet.

The PIEZO2 gene typically encodes a type of mechanosensitive protein in cells, responsible for generating electrical signals in response to alterations in cell shape (Coste et al., 2010). Mutations to this gene prevent signal generation and render the neurons incapable of detecting limb or body movement. These findings firmly establish PIEZO2 as a critical gene for facilitating proprioception in humans, a sense that is crucial for bodily awareness.

PIEZO2 mutations have also been implicated in genetic musculoskeletal disorders (Coste et al., 2010). Joint problems and scoliosis experienced by the patients in a study suggest that proprioception may also indirectly guide skeletal development. These insights into the role of the PIEZO2 gene in proprioception and musculoskeletal development open up promising avenues for understanding and treating neurological and musculoskeletal disorders.

It’s more than fitting to regard proprioception as our sixth sense. The capacity of our nervous system to seamlessly process vast amounts of information from our joints and muscles, all without any conscious effort on our part, is truly remarkable. So, the next time you have that eyes-shut first sip of coffee, give yourself a pat on the back. With your sixth sense at play, you’re clearly a superhero!

References

Chesler, A. T., Szczot, M., Bharucha-Goebel, D., Čeko, M., Donkervoort, S., Laubacher, C., Hayes, L. H., Alter, K., Zampieri, C., Stanley, C., Innes, A. M., Mah, J. K., Grosmann, C. M., Bradley, N., Nguyen, D., Foley, A. R., Le Pichon, C. E., & Bönnemann, C. G. (2016). The Role of PIEZO2 in Human Mechanosensation. N Engl J Med, 375(14), 1355-1364. https://doi.org/10.1056/NEJMoa1602812

Coste, B., Mathur, J., Schmidt, M., Earley, T. J., Ranade, S., Petrus, M. J., Dubin, A. E., & Patapoutian, A. (2010). Piezo1 and Piezo2 are essential components of distinct mechanically activated cation channels. Science, 330(6000), 55-60. 

McNeill, D., Quaeghebeur, L., & Duncan, S. (2009). IW - “The Man Who Lost His Body”. In (pp. 519-543). https://doi.org/10.1007/978-90-481-2646-0_27

Proske, U., & Gandevia, S. C. (2012). The Proprioceptive Senses: Their Roles in Signaling Body Shape, Body Position and Movement, and Muscle Force. Physiological Reviews, 92(4), 1651-1697. https://doi.org/10.1152/physrev.00048.2011

Sherrington, C. S. (1907). On the proprio-ceptive system, especially in its reflex aspect. Brain, 29(4), 467-482. 

Swain, K. (2017). The phenomenology of touch. The Lancet Neurology, 16(2), 114. https://doi.org/10.1016/S1474-4422(16)30389-1

Woo, S. H., Lukacs, V., de Nooij, J. C., Zaytseva, D., Criddle, C. R., Francisco, A., Jessell, T. M., Wilkinson, K. A., & Patapoutian, A. (2015). Piezo2 is the principal mechanotransduction channel for proprioception. Nature Neuroscience, 18(12), 1756-1762. https://doi.org/10.1038/nn.4162