Which receptor is sensitive to changes in muscle length and the rate of that change?

• A. Muscle spindles
• B. Golgi tendon organs
• C. Muscle fibers
• D. Joint receptors

Answer: (A)

Muscle spindles are proprioceptors that monitor how stretched a muscle is and how fast that length is changing. They lie in parallel with the muscle fibers, so when the muscle lengthens, the spindle is stretched and its sensory nerves fire more rapidly. The dynamic (Ia) fibers mainly signal the speed of length change, while static (II) fibers convey information about the actual length. This input helps the nervous system regulate muscle tone and the stretch reflex to prevent overextension. Golgi tendon organs, in contrast, sense tendon tension, not length, and joint receptors sense joint position and movement. So the receptor tuned to both how long the muscle is and how quickly that length is changing is the muscle spindle.