Proprioception

We are often told that we have five senses, but that’s not really correct. What about your sense of balance? Even though balance is created in the inner ear, it has nothing to do with hearing. There are more than five senses, and proprioception is a sense that we do not often think about or consider a perception.

vitruvian-manThe word “Proprioception” come from Latin proprius, meaning “one’s own”, “individual”, and capio, capere, to take or grasp, is the sense of the relative position of neighboring parts of the body and strength of effort being employed in movement.

In medicine, it is defined as “the ability to sense stimuli arising within the body regarding position, motion, and equilibrium.” Proprioception lets you know where your body is, how it is oriented, and how it is moving.

To be clear, proprioception depends in part on touch/feeling, but it also may include your sense of balance for equilibrium/positioning. But it also includes information from other parts of the body.

This “body orientation” is also “provided by proprioceptors (muscle spindles) in skeletal striated muscles and tendons (Golgi tendon organ) and the fibrous capsules in joints. It is distinguished from exteroception, by which one perceives the outside world, and interception, by which one perceives pain, hunger, etc., and the movement of internal organs.”

“The brain integrates information from proprioception and from the vestibular system into its overall sense of body position, movement, and acceleration. The word kinesthesia or kinæsthesia (kinesthetic sense) strictly means movement sense, but has been used inconsistently to refer either to proprioception alone or to the brain’s integration of proprioceptive and vestibular inputs.”

To put it simply, we know where are body is and how to control it. We can touch our nose if we have our eyes closed, we can walk fairly well if we are blindfolded, we know our body position when we wake up in the morning, and often even when we are asleep, even sleep walking. And we even know our body position when we are weightless or when a limb is numb (or asleep). Think also about repetitive movement like typing or assembly line work – you can usually tell the moment our hands or fingers are “off” target even a little.

Now, we don’t really think about most of our senses until we lose one, and it is possible to lose proprioception from contracting Multiple Sclerosis, a spinal cord injury or tumor, or even a virus, just as a man named Ian did. his story is demonstrated in Andrew Dawson’s 2011 TEDMED talk. There’s also a documentary about Ian called The Man who lost his body.

It is interesting to note that Ian was not paralyzed, but nerve damage kept him from feeling his body and required that he look at his body to determine and control what it was doing.

One more interesting bit of evidence that suggests that proprioception is much more than just feeling: Phantom Limb syndrome – the sensation experienced by someone who has had a limb amputated, but still feels that the limb is present. A 2009 study suggest that this syndrome may be caused by proprioceptive memory. The brain still perceiving the lost limb is a perfect example of our minds constructing reality.

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None, with the possible exception of virtual reality in the future.

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Balance – Equilibrioception

Every step we take dances on the edge of disaster – one miscalculation at the moment when we are shifting weight from one foot to the other, and we fall. Balance is a sixth sense and a crossmodal perception. It has nothing to do with hearing, but clearly begins with sensors in the inner ear. We call it Equilibrioception.

Our sense of balance depends on the integration of three sensory systems:

  • Vision: seeing helps us determine our body’s position in reference to the world (gravity). Note: some blind people have issues with balance
  • Proprioception: (see related post) uses the skeletal systems (the muscles and joints and their sensors) to determine the position of the body
  • Vestibular system: The section of the inner ear composed of semicircular canal system, which indicate rotational movements; and the otoliths, which indicate linear acceleration.

The vestibular apparatus (shown below) includes the utricle, saccule, and three semicircular canals (Anterior, Horizontal, and Posterior). The utricle and saccule detect gravity (information in a vertical orientation) and linear movement. As we move our heads fluid moves through the canals and tells us the relative position of our head and its movement. The otoliths act as a kind of accelerometer, helping determine the speed of the body or heads movement. The vestibular system then sends signals to the neural structures that control eye movements, and to the muscles that keep an animal upright.

Vestibular_System.jpg

Observation
Human balance perception is not quite terrestrial, that is, we certainly have some perceptual systems or strategies that other land animals do not. For example: a human can stand in a bus holding onto a pole and have little or no issues with balance, but a horse standing in a horse trailer has significant problems with the movement of the vehicle, even though it has four legs (four points of stability). We almost certainly owe this extra bit of balance expertise to our ancestors of the trees.

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Virtual reality comes to mind again. It should be noted that professional level flight simulators use hydraulic mechanisms to provide a sense of pitch and acceleration by moving entire simulator rooms. Without some feedback of this sort, virtual reality will always be semi-virtual.

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