If you are in the dark and want to move your arm from one place to another, how does your brain know where the arm is to start with (your eyes won't be able to give you information if it's dark!) and when to stop moving it because it has got to the desired location?
Great question! The brain's ability to know where one's arms and legs are located in three-dimensional space, without having to look at them, is known as "conscious proprioception" or, in laymen's terms, "limb position sense."
First, we need a little background information. Muscles, tendons, ligaments, joint capsules and the deeper layers of your skin contain an abundance of specialized sensory receptors known as "proprioceptors." Proprioceptors function as "stretch detectors" or "tension-detectors." When you move an arm or a leg, some of the proprioceptors that supply the tissues in that limb are stretched, put under tension, or otherwise mechanically distorted. This causes the sensory nerve fibers that supply these proprioceptors to be stimulated, and changes the frequency at which electrical impulses are sent to the brain. The brain "interprets" the frequency code of the electrical signals and uses this information to determine and adjust (without using the eyes) the direction, velocity, and duration of limb movement.
When your body is "at rest" (e.g., sitting quietly in a chair or lying in bed in the dark), your muscles remain in a state of partial contraction, known as "muscle tone." Without muscle tone, our bodies would become totally flaccid. This state of partial muscle contraction causes the proprioceptors in the muscles, tendons, ligaments and joint capsules to continuously "fire," albeit at a different frequency than they would if the limb were actively moving. This continuous stream of electrical impulses in the absence of overt movement keeps the brain informed of the static position of your body parts, even though you do not consciously "feel" these sensations in the traditional sense.
The phenomenon of "limb position sense" is closely related to the phenomenon of "self perception." Recognition of body part position occurs mainly in the parietal lobe of the brain. During infancy and childhood, the regular influx of sensory impulses from proprioceptors in muscles, tendons, ligaments, joints, and skin stimulates the parietal lobe of the brain to lay down a pattern of memory traces that develop into ones concept of "body image" or "body scheme. " Thus, as you lie quietly in bed in total darkness, your brain "knows" (in the absence of visual input) that your left arm is resting at your side, that your right arm is flexed at the elbow so that your hand is lying on your chest, and that your left leg is dangling from the side of the bed suspended about 6 inches from the floor.
By extension, should you decide to move your arm in darkness to a new position, your brain will be able to calculate, in the absence of vision, the correct sequence and forces of muscle contractions necessary to execute this action. As the muscles in your arm begins to contract in the appropriate sequence and amplitude (under the influence of learned "motor programs" in the brain) proprioceptors in the muscles, tendons, and joints alter their rates of firing. The brain uses this new information from the proprioceptors to continuously monitor moment-by-moment changes (in microseconds!) in limb position, velocity, and direction of movement, without having to look at the limb. The brain (especially the cerebellum) uses this "sensory feedback" from the limb proprioceptors to make any necessary adjustments in speed, direction, or force of contraction, to initiate the braking process near the end of the desired movement, and to bring the limb smoothly to its new location without overshooting or undershooting.