Which mechanoreceptor senses changes in length?

• A. Tendons
• B. Proprioception
• C. Muscle spindles
• D. Sarcomeres

Answer: (C)

Muscle spindles are stretch-sensitive mechanoreceptors embedded in skeletal muscle that detect changes in muscle length. When the muscle lengthenens, the spindle is stretched, activating sensory endings that send signals to the spinal cord and brain about how much and how quickly the muscle is lengthening. This information helps regulate muscle contraction and coordinate movement, including triggering a stretch reflex that resists further lengthening to protect the joint. Tendons, through Golgi tendon organs, sense muscle tension rather than length. Proprioception refers to the overall sense of body position, not a specific receptor, and sarcomeres are the contractile units inside muscle fibers, not receptors.