Which structure is responsible for detecting muscle length?

The correct answer is the muscle spindle, as it is specifically designed to detect changes in muscle length and the rate of that change. Muscle spindles are sensory receptors located within the belly of muscles. They consist of specialized muscle fibers called intrafusal fibers, which are surrounded by a connective tissue capsule.

When a muscle stretches, the muscle spindles are also stretched, activating their sensory neurons. This activation sends signals to the central nervous system, informing it about the degree of stretch and helping to regulate muscle contraction and maintain posture. This feedback is crucial for proprioception, allowing the body to know the position of limbs and movement without visual cues.

Other options, such as Golgi tendon organs, which sense changes in muscle tension, and motor neurons, which transmit signals to muscles to induce contraction, serve different functions and do not directly measure muscle length. The neuromuscular junction is the site where motor neurons connect with muscle fibers and facilitate muscle contraction but is not involved in detecting muscle length. Thus, the muscle spindle is the accurate structure responsible for detecting muscle length.