In November I wrote about the senses, in particular the ones beyond the usual 5. This post focusses on the group of senses that tells us the position and movements of our body, called proprioception (pro-PREE-o-SEP-SHUN).

Proprioception is not a new discovery. The term proprio-ceptive has been used by physiologists since the early 1900s1, and the idea that the brain senses the body’s movements has been around even longer2. So what exactly is proprioception for?

Overall, proprioception is the body’s sense of its own movement and position, as well as the force and effort applied to movement. This is quite different to the usual 5 senses. Vision, taste, hearing, smell and touch are all for sensing things outside the body (called exteroception). Proprioception is also different from sense of pain, hunger, and others that sense things occuring inside the body (interoception).

What is proprioception for?

If you close your eyes, you can still touch your nose with your finger. You can still clap your hands with your eyes closed. In fact, when it comes to knowing where our own body is, we are very good that that when our eyes are closed, because we have proprioception.

Proprioception is also important for controlling our movements. Many of the reflexes that help us to stand, walk, and move around depend upon the receptors that sense the position and movement our our body.

Without this critical group of senses, you would be unable to move easily around the world and would lose track of your body when you closed your eyes3.

Sensing joint position and movement

You might expect that the brain gets information about the movement and position of the body from sensors (called receptors) in the joints, and you would be right. However, the joint receptors are not actually very good at signalling joint position because they are not sensitive to different directions of movement. The joints do assist with proprioception, and are probably important for telling the brain how far a particular joint can move4.

Muscle receptors are best

The main receptors for sensing the position and movement of the body are in the muscles5. They are called muscle spindles, and they come in different types. Some are sensitive to the length of the muscle, and some are sensitive to the speed at which the muscle moves. Because the muscles are use to control joint position, by knowing the length of the muscles the brain can work out the position of a joint. Furthermore, because there are muscles pulling in different directions to control each joint,the brain can also use muscle spindles to work out the direction of a movement.

Skin stretch is also important

Another contributor to our sense of position and movement is the skin. There a several different types of receptor in the skin, but the ones involved in sensing movement of the joints are the ones that detect slow stretches of the skin. This is because the skin around a joint is stretched when it moves, so the brain can get some information about joint position. However, because of the way skin is stretched during movement, these receptors are not as accurate as muscle spindles for sensing joint position and movement. Skin receptors are thought to be more important in the finger joints. This is because the muscles that control the fingers are located back in the arm, so physiologists think that the extra information from skin receptors is more important6.

Measuring proprioception

So how do we measure the sense of body position and movement? Physiologists do it in laboratories with specialised equipment that accuracly measures joint position and movement. They may temporarily take the sense away, or use an illusion to disrupt the sense. It’s a little bit like taking things apart to see how it works. You may have seen some illusions that use vision.

Movement illusions

There are some also some illusions that affect proprioception. Muscle spindles are very sensitive to vibration of 80 – 100 Hz. If you vibrate them at these frequencies they signal the brain that they are getting longer, even through the joint is not moving. This means that they brain thinks the joint is moving when it is not, or think it is moving faster or further than it is.

So for example, if you vibrate the tendon of the triceps muscle (back of the elbow) at 80 Hz (or 4800 rpm), the muscle spindles in the triceps will signal the brain that the muscle is getting longer. The brain interprets this as the elbow flexing. If you have your eyes closed and are already flexing your elbow slowy, you will get an illusion that is has moved much further.

These illusions are difficult to explain in a blog post. You need to experience it yourself, and it can take practice to get it to work. I am planning to make a video in the first quarter of 2019 that will show you how to make some of these illusions work.

In the mean time, look out for events demosntrating these illusions. If you would like a demonstration at your event get in touch, and if you have already experienced this let me know in the comments.

 

Footnotes
  1. C.S. Sherrington first referred to the ‘proprio-ceptive sense’ and ‘proprio-ception’ in his book The Integrative Action of the Nervous System, published in 1906 by Yale University Press (New Haven). In 1900 Sherrington also published a chapter called ‘The muscular sense’ in the Textbook of Physiology, (London, editor Schafer E., pages 1002-1025) which is about proprioception, but does not use that word.
  2. In his Treatise on Physiological Optics (1867; Vol 3, Optical Society of America, Menasha Wisconsin) von Helmholtz referred to a feeling of innervation and an effort of will, which have the same features as the modern ‘sense of effort’. There is other discussion in the literature from that time and before, see Mechanisms of Human Proprioception (Walsh, 2010 UNSW) for more information.
  3. Jonathon Cole’s book Pride and a Daily Marathon (1995, MIT Press), describes the case of a man who lost proprioception below the neck, but learned how to move around again despite that loss.
  4. A key paper supporting this was Characteristics of knee joint receptors in the cat, published by Burgess and Clark (1969) in the Journal of Physiology (v 203, p317-225).
  5. This was controversial for some time, and physiologists really only began to agree in the 1970s. See Mechanisms of Human Proprioception(Walsh, 2010 UNSW) for a detailed review.
  6. This is because illusions of movement can be induced in the fingers by stretching the skin. One paper that demonstrated that these illusions were larger in the fingers was Cutaneous receptors contribute to kinesthesia at the index finger, elbow and knee published by Collins and colleagues in the Journal of Neurophysiology (2005, v94, p1699-1706).
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