Proprioception, the True Sixth Sense

There really is a sixth sense: it’s called proprioception. It is the sense of position and movement. It is produced by nerves in our connective tissues (ligaments, bone, fascia), our 300-or-so muscles and skin.1

Without proprioception, you couldn’t stand up (standing up is actually shockingly complicated). You couldn’t so much as scratch your nose, because you wouldn’t be able to find it.

Proprioception is a major sensory component of “coordination,” the ability to move precisely. There’s more to it, of course (like balance), but proprioception is the big one.

What does proprioception “feel” like?

It’s like asking a fish what water feels like. We can’t know what proprioception feels like, because we cannot shut it off.

You can’t truly know what proprioception feels like until it’s gone. Fortunately, this only happens to a few people who get an extravagantly rare neurological condition that destroys proprioception (polyneuropathy of the neuraxis). Oliver Sacks describes a case of it in his book, The Man Who Mistook His Wife for a Hat:

She continues to feel the loss of proprioception, that her body is dead, not-real, not-hers … She can find no words for this state, and can only use analogies derived from other senses … .

Proprioception’s greatest hits

This is an extremely high-quality slow-motion video of a cheetah running (well, five of them over three days actually). It is an amazing demonstration of proprioception at work, as the vital prerequiste for complex coordinated movement. Or lack of movement. Note how level the head stays — that is one exquisitely fine-tuned and effective motion control system! Cheetahs probably keep their gaze steady to help the hunt, so vision is a no doubt a major part of the equation here: but it’s mostly proprioception that keeps track of where all the body parts are.

Just fascinating. And beautiful. And cute too! All at once.

Complex and subtle

Proprioception is a large sense. It produces a tremendous amount of data, as much or more than all the other senses combined. So although it is a silent sense, it is a very important one. Just knowing that it exists is significant self-knowledge, good “owner’s manual” stuff.

The nerves that generate proprioception are embedded in the tissues of our musculoskeletal system: in muscles, tendons, ligaments, joint capsules and cartilage. They send information to the brain about how much tension or pressure is being applied to them, and how quickly it’s changing. The brain uses this information to figure out:

• how hard your quadriceps are contracting
• how bent or twisted your knee is
• how long a step you’ve taken
• the size of something held in your arms by their position
• the effort needed to lift a glass of water without throwing it into your face

But proprioception is even crazier than that!

You might expect the brain to be able to figure out the position of the eye based on what you are looking at. But that’s not how it works.

You actually know the direction and focus of your gaze because you know the position of your eyeball, and the effort it took to change the shape of your lens. Without those nerves in the muscles of the eyeballs, we would be able to see, but we wouldn’t know where any of it was.

Try to imagine that! You would, in effect, be virtually blind without proprioception — even if your eyeball was still perfectly functional otherwise.

Proprioception in rehab, physical therapy, and massage

An understanding of proprioception is routinely applied in a variety of physical therapies. One familiar example is the relaxing effect of vibration. This probably works because of a simple principle I call “proprioceptive confusion.”

If you shake the body randomly and rhythmically, the brain gets a deluge of strange, meaningless proprioceptive noise. The nervous system effectively “gives up” and stops resisting the movement, providing you with some significant muscular relaxation. This is rather vividly demonstrated by an well-documented effect on flexibility: vibrating muscles temporarily but significantly increases their maximum extensibility.234

Another basic example is “proprioceptive training,” which is just a fancy way of talking about training for coordination. Unsurprisingly, practicing a variety of precisely coordinated movements makes people better at moving, and to some extent prevents injuries.5

Proprioception is also responsible for a lot of the (delicious) feeling of sensory novelty that we experience when we are massaged. Although stroking sensations on the skin are certainly significant, most of us also crave the deeper sensations of our proprioceptive nerve endings being stimulated for us — the unfamiliar movements of joints, the pressures on our muscles in places we can’t reach ourselves, the incongruously effortless stretches of tendons and ligaments that are normally only stimulated by intense exercise.

Without proprioception, every massage would feel like just a skin massage: a poorer sensory experience.

How does proprioception work?

In 2021, David Julius and Ardem Patapoutian shared the Nobel for Medicine or Physiology for their discovery of proteins that transduce some basic stimuli into nerve impulses. It’s all dizzyingly complex, of course, but it can be oversimplified down to this: those proteins enable us to detect heat, cold, and physical force.

And force is the key to proprioception.

These discoveries are only about ten to twenty years old, and have been followed by many more like them, the dawn of a much more detailed science of sensation.

How do physical stimuli turn into sensations? How exactly do we turn what’s around us into nerve impulses? “Transduction” is one of my favourite concepts in biology: the conversion of forces into electricity. And how do we “transduce” physical forces into nerve impulses?

It’s all about the proteins

The most specific explanation possible for most biological marvels is proteins. One way or another, it’s all about the proteins. But how does a protein help us feel anything? Proteins are mind-bendingly complex little nanomachines, effectively black boxes with intricate clockwork-like guts we can barely comprehend. But we know some things about what they react to and how.

The Nobel proteins are “receptors,” embedded in cell membranes with their heads and antennae sticking out, and their tails on the inside. They are “pores” in the cell of the skin that can open or close. When exposed to the right stimulus, they warp and wiggle in an instant, opening a channel for a flood of ions (atoms with a charge). This why you can also call them “ion gates,” which sounds very sci-fi. If enough ion gates open at once, they triggers a nerve impulse — a signal about something happening around the cell, like “heat!” or “mechanical deformation!” or “Klingons on the starboard bow!”

Guido4, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

Poking cells for science

Papoutian’s team earned half of the Nobel for their work identify the proteins that transduce mechanical deformation. Charmingly, they started out by identifying natural cells that were sensitive to touch by poking them, microscopically. I am not joking. They literally poked cells with a microscopic stick until they found ones that responded with a little “spark” of electricity.

They finally found that spark, and then they “just” had to identify the spark plug: the protein that was reacting to the poke. They found two: Piezo1 and Piezo2 were announced in 2010.6

Piezo1 and Piezo2 use the force

Piezo1 and Piezo2 both react to the flexing of the cell membranes in which they are thickly embedded, like rhinestones on an Elvis impersonator. The deformation of the cell membrane is a microcosm of a thumb pressing into your skin, which bends the cell membranes of the nerve cells in the area, opening millions of Piezo1 and Piezo2 proteins, so that kajillions of ions can pass through them, triggering nerve impulses.

This is scientific reductionism at its most dazzling, revealing the nano-scale engines that power familiar human experiences, the answer at the end of a chain of child’s “but why” questions.

The thumb bends the skin, flexing the membranes of every cell in the area. Proteins embedded in the cell membranes, Piezo1 & Piezo2, transduce the flexion into nerve impulses.

But they also power un-familiar human experiences! What we consciously feel thanks to the Piezo proteins is the tip of a mighty iceberg of subtle sensation. The biological power and utility of these engines is breathtaking.

Proprioception… and beyond!

Proprioception generally refers to the sense of position and movement, and the piezo proteins are critical for this. But the Piezo proteins transduce mechanical deformation that is also critical for detecting all touching.

And that’s still not the full repertoire. Propioception is largely unconscious, but the piezo proteins are used for an even larger category of even more unconscious information: every scrap of data about the pressure inside your vessels and viscera, the pressure on the walls of blood vessels, bladders, lungs… everything. This basic method is used in every tube and container in human biology. You know that your bladder is full because Piezo proteins respond to the stretch on the cells of the bladder wall.

Early in this article I gave an example of someone crippled by a missing sense of proprioception, due to a failure of brain-level processing of sensory data. But if all your Piezo receptors vanished — the source of vastly more data about the state of your body — it would probably kill you in seconds or minutes. That’s how much your physiology depends on knowing when tissues are stretching, flexing, twisting, and compressing. It’s not proprioception, technically, but it is closely related.

About Paul Ingraham

I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of ScienceBasedMedicine.org for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter., or subscribe:

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Related Reading

This article is part of the Biological Literacy series — fun explorations of how the human body works, what I think of as “owner’s manual stuff.” Here are ten of the most popular articles on this theme:

• The Unstretchables — Eleven muscles you can’t actually stretch hard (but wish you could)
• When To Worry About Shortness of Breath … and When Not To — Three minor causes of a scary symptom that might be treatable
• Micro Muscles and the Dance of the Sarcomeres — A mental picture of muscle knot physiology helps to explain four familiar features of muscle pain
• Why Do We Get Sick? — The curious and tangled connections between pain, poor health, and the lives we lead
• Organ Health Does Not Depend on Spinal Nerves! — One of the key selling points for chiropractic care is the anatomically impossible premise that your spinal nerve roots are important to your general health
• Does Fascia Matter? — A detailed critical analysis of the clinical relevance of fascia science and fascia properties
• A Painful Biological Glitch that Causes Pointless Inflammation — How an evolutionary wrong turn led to a biological glitch that condemned the animal kingdom — you included — to much louder, longer pain
• You Might Just Be Weird — The clinical significance of normal — and not so normal — anatomical variations
• 38 Surprising Causes of Pain — Trying to understand pain when there is no obvious explanation
• Tissue Provocation Therapies in Musculoskeletal Medicine — Can healing be forced? The laws of tissue adaptation & therapies like Prolotherapy & Graston Technique
• Toxins, Schmoxins! — The idea of “toxins” is used to scare people into buying snake oil

Notes

1. Proske U, Gandevia SC. The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force. Physiol Rev. 2012 Oct;92(4):1651–97. PubMed 23073629 ❐
2. Issurin VB, Liebermann DG, Tenenbaum G. Effect of vibratory stimulation training on maximal force and flexibility. J Sports Sci. 1994 Dec;12(6):561–6. PubMed 7853452 ❐

   In this 1994 experiment, as described by Sands et al., gymnasts “used a vibrating ring suspended by a cable, in which the foot of the subject was placed while they stretched forward over the raised leg, targeting the hamstrings. The resulting increase in ROM was astonishing. These researchers demonstrated that vibration could enhance flexibility.” The results were replicated by Sands et al. in 2006, and Kinser et al. in 2008.

3. Sands WA, McNeal JR, Stone MH, Russell EM, Jemni M. Flexibility enhancement with vibration: Acute and long-term. Med Sci Sports Exerc. 2006 Apr;38(4):720–5. PubMed 16679989 ❐

   This experiment replicated the results of an intriguing 1994 experiment by Issurin et al. Ten highly trained gymnasts did forward splits with or without vibration. They stretched to the point of discomfort for 4 minutes, alternating between each leg, 10 seconds of stretching at a time. Flexibility immediately after stretching with vibration was dramatically greater; the long-term results were less striking.

4. Kinser AM, Ramsey MW, O’Bryant HS, et al. Vibration and stretching effects on flexibility and explosive strength in young gymnasts. Med Sci Sports Exerc. 2008 Jan;40(1):133–40. PubMed 18091012 ❐

   Replicates the findings of both Issurin and Sands — “simultaneous vibration and stretching may greatly increase flexibility, while not altering explosive strength.”

5. Riva D, Bianchi R, Rocca F, Mamo C. Proprioceptive Training and Injury Prevention in a Professional Men's Basketball Team: A Six-Year Prospective Study. J Strength Cond Res. 2016 Feb;30(2):461–75. PubMed 26203850 ❐

   This particularly long-term study followed a men’s basketball team for six years, tracking their injury rates in response to “classic proprioceptive [coordination] exercises” — which seemed to clearly reduce ankle sprains, “by 81% from the first to the third biennium.” The results actually showed reductions in knee sprains and back pain as well, but those results were much less statistically certain.

   For whatever that’s worth, the athletes also got really good at the proprioceptive control exercises themselves. 😉

6. Coste B, Mathur J, Schmidt M, et al. Piezo1 and Piezo2 are essential components of distinct mechanically activated cation channels. Science. 2010 Oct;330(6000):55–60. PubMed 20813920 ❐ PainSci Bibliography 52196 ❐