Which of the following structures detects rapid movement and initiates the stretch reflex?

Which of the following structures detects rapid movement and initiates the stretch reflex?

• A. GTO
• B. muscle spindle
• C. extrafusal muscle fiber
• D. Pacinian corpuscle

Answer: (B)

The muscle spindle is the sensor that detects rapid movement and triggers the stretch reflex. It sits inside the muscle as intrafusal fibers with sensory endings (Ia and II). When the muscle is stretched quickly, the Ia afferents fire more rapidly, sending a signal to the spinal cord. This signal directly (monosynaptically) activates alpha motor neurons to contract the same muscle, resisting the stretch and restoring length. This rapid, automatic response helps maintain posture and protect the muscle from potential damage from sudden stretching. The Golgi tendon organ, by contrast, senses tension in the tendon and helps regulate force to prevent damage. Pacinian corpuscles are pressure and vibration receptors in the skin and joint tissues, not the primary detectors of muscle stretch. Extrafusal muscle fibers are the actual contractile fibers that produce force, not sensors.

The muscle spindle is the sensor that detects rapid movement and triggers the stretch reflex. It sits inside the muscle as intrafusal fibers with sensory endings (Ia and II). When the muscle is stretched quickly, the Ia afferents fire more rapidly, sending a signal to the spinal cord. This signal directly (monosynaptically) activates alpha motor neurons to contract the same muscle, resisting the stretch and restoring length. This rapid, automatic response helps maintain posture and protect the muscle from potential damage from sudden stretching.

The Golgi tendon organ, by contrast, senses tension in the tendon and helps regulate force to prevent damage. Pacinian corpuscles are pressure and vibration receptors in the skin and joint tissues, not the primary detectors of muscle stretch. Extrafusal muscle fibers are the actual contractile fibers that produce force, not sensors.