Proprioception

Proprioception is the sense of where one's own body parts are and how they're moving, generated internally and updated continuously without requiring vision to confirm it. Close your eyes and you can still touch your nose, describe whether your elbow is bent, and catch yourself from a stumble — all proprioceptive, all running on internal sensors rather than sight.

Two receptor types do most of the work. Muscle spindles, embedded within skeletal muscle fibers, detect changes in muscle length and the speed of that change, reporting stretch. Golgi tendon organs, at the muscle-tendon junction, detect tension the muscle is generating rather than its length, providing a check against overload — the reflex that makes an overstrained muscle suddenly relax is this sensor intervening. Joint receptors and skin stretch receptors around joints supply a further layer, especially at the extremes of a joint's range of motion.

Proprioception is precisely what field sobriety tests probe when they ask someone to close their eyes and touch their nose or walk a straight line: alcohol degrades the integration of proprioceptive signal with balance and coordination well before it obviously impairs strength, so those tests are measuring a specific breakdown, not general clumsiness.

Phantom limb sensation is proprioception's most striking failure mode: after amputation, the brain's body map — built and continuously updated from a lifetime of proprioceptive input — persists and can still generate a vivid, located sense of the missing limb's position, sometimes in a fixed, uncomfortable posture the person cannot voluntarily move, because there is no longer proprioceptive feedback available to update that map. This is one reason mirror-box therapy works: it supplies substitute visual feedback the missing proprioception no longer can.