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Proprioceptors help the body recognize, activate, and coordinate its various parts in relation to its other parts and the environment. For example, being able to touch your nose with your finger while your eyes are closed is due to your proprioceptive sense. Walking without watching where each foot lands is also due to functional proprioceptors. Tying your shoes, finding your house keys, and unlocking the door without looking are all possible with the assistance of proprioceptors as well. Proprioception also governs the sense of owning your body, the sense that your limbs belong to you. This sense usually remains even when a limb is non-functional.
At a basic level, muscle proprioceptors typically function through muscle spindles and Golgi tension organs. Muscle spindles recognize and monitor muscle length. The Golgi tension organs keep track of muscle tension. These proprioceptors send data about muscle tone and joint angle to the central nervous system. The brain then integrates this internal sensory information and makes large and small adjustments to movement, posture, balance, and angle.
Some proprioceptors seem to be fully functioning from birth while others appear to increase in functionality over time. Even small babies typically have a self-righting reflex, cocking their heads to achieve a horizontal angle when tilted. On the other hand, infants appear to have a limited sense of self. They seem unaware that their feet belong to them and usually cannot maneuver their hands to grasp their toes easily, for example. This sense of self seems to develop over time as the child’s proprioceptive sense matures. Proponents of Tai Chi and yoga claim that these exercises increase the proprioceptive sensitivity.
The proprioceptive sense sometimes goes wrong. In his book, The Man Who Mistook His Wife for a Hat, neurologist Oliver Sachs describes two cases of non-functional proprioceptors. In the chapter, “The Disembodied Lady,” a young woman loses her proprioceptive sense entirely. Her brain receives no feedback from the proprioceptors so cannot direct body movement. She is unable to sit, control her hands, or walk without watching and consciously directing her every move. In another case, a man loses the sense of owning one of his legs. This man wakes up each night to find a strange leg in his bed and accuses the nurses of putting an amputated limb in with him as a cruel joke. This rare phenomenon can sometimes occur because of a viral infection or brain injury. It can also occur in reverse, leading the mind to detect a body part that is no longer present. When the brain perceives an amputated limb to itch, for example, a condition called phantom limb, it may be due to a malfunctioning proprioceptive system.
So phantom pain is part of the proprioception system. That's interesting.
I wonder if my propensity to extreme motion sickness has anything to do with my proprioceptor system. I can get carsick at the drop of a hat, and it's awful. I don't have it when I drive, thank goodness, but I cannot ride in the back seat of a car, period, and I also cannot read or anything like that when I'm riding. Even thirty seconds of reading will start making me feel queasy.
I know motion sickness has something to with balance and horizon and so forth, but it would make sense that the proprioceptors might also play a role.
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