Tuesday, March 14, 2017

The Physiology of Your “Sense of Self”

Quick! Name all of your senses!

Now, close your eyes and wave your arms over your head. Which of those senses are helping you know where your arms are in space?

The answer is the often-forgotten sense of proprioception. Proprioception (derived from the Latin for “sense of self”) is an animal’s sense of its body’s position in space. We have several different specialized receptor cells that all detect a change in body position in different ways.

Grays muscle picture by Mikael Haggstrom
at Wikimedia Commons.
If you raise your arms over your head as if you are going to grab a pull-up bar, then some muscles in your back (like your trapezius muscles), shoulders (like your deltoids and rotator cuff muscles), and arms (like your triceps) will contract. Muscles are all connected with tendons to the bones they pull on. When a muscle contracts, its tendons are stretched. Specialized proprioceptor cells called Golgi tendon organs merge with tendons and detect when their corresponding muscle is being stretched. Together, they inform the brain about muscle tension in muscles all across the body.

Grays muscle picture by Mikael Haggstrom
at Wikimedia Commons.

However, while some muscles will contract during your movement, other muscles in your chest (like your pecs) and arms (like your biceps) will stretch. Each muscle contains muscle spindles, another kind of specialized proprioceptor cell. Muscle spindles are wrapped around individual muscle fibers within the muscles. They send signals to the brain to let it know when the muscle is stretched and by how much.

Joint receptors are specialized proprioceptor cells located between bones in the capsular tissue of joints. When the angle of a joint changes, the bones and tissues put pressure on the joint receptor, causing it to send a signal to the brain. Your brain collects information from all of your Golgi tendon organs, muscle spindles and joint receptors to know the angle of each joint and the tension and length of each muscle in your body, and thus, your body’s position in space.

gif by Extremistpullup at Wikimedia Commons.
Some animals, and some individuals, are better at this than others. This guy should be pretty proud of his proprioceptive abilities (and strength). But then again, let’s see him try this:

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