In the stretch reflex, what structure is responsible for sensing stretch?

The muscle spindle is the correct answer because it is a sensory receptor located within the belly of skeletal muscles that responds to changes in muscle length, specifically stretch. When a muscle is stretched, the muscle spindles detect this elongation and send signals to the central nervous system. This mechanism is critical for maintaining muscle tone and protecting the muscle from excessive stretching, as it triggers the stretch reflex, leading to a contraction of the muscle being stretched.

The tendon organ, or Golgi tendon organ, is another type of proprioceptor but it primarily senses tension within the muscle tendon, not stretch. It helps prevent muscle damage by inhibiting muscle contraction when excessive tension is detected. The joint capsule contains various receptors that can sense position and movement of joints but does not specifically detect muscle stretch. Thus, the muscle spindle is uniquely suited for this function within the stretch reflex.