Which receptor detects rapid stretch in the stretch reflex?

• A. Muscle spindle
• B. Golgi tendon organ
• C. Free nerve ending
• D. Pacinian corpuscle

Answer: (A)

Rapid stretch in the stretch reflex is detected by the muscle spindle. These receptors sit inside the muscle and include dynamic elements that are especially responsive to how quickly the muscle length changes. When the muscle is stretched suddenly, the Ia afferent fibers from the spindle fire rapidly and send a direct signal to the same muscle’s alpha motor neurons, causing a quick contraction that resists the stretch. This monosynaptic reflex arc is the essence of the myotatic reflex. The Golgi tendon organ responds to tension at the tendon and helps modulate force, not the velocity of length change. Free nerve endings cover general sensations like pain and temperature, not specifically rapid stretch. Pacinian corpuscles detect high-frequency vibration and rapid skin deformation, not the muscle’s length change. So the receptor that detects rapid stretch is the muscle spindle.