Which proprioceptive structure determines muscle length?

Muscle spindles are specialized sensory receptors located within the belly of muscles that play a crucial role in detecting changes in muscle length and the rate of that change. These spindle fibers are arranged parallel to the muscle fibers they monitor and contain intrafusal muscle fibers. When a muscle is stretched, the muscle spindles are also stretched, leading to an increase in the firing rate of the sensory neurons associated with them. This information is sent to the central nervous system, which uses it to coordinate muscle activity and maintain posture, balance, and movement.

In contrast, Golgi tendon organs primarily sense muscle tension rather than length, joint receptors provide information about joint position and movement, and neuromuscular junctions are the sites where motor neurons communicate with muscle fibers to trigger contraction. Hence, for determining muscle length specifically, muscle spindles are the key proprioceptive structures involved.