During passive knee extension testing, resistance suddenly gives way when the quadriceps tension reaches a high level, and the muscle relaxes. Which receptor initiates this protective response?

• A Muscle spindle secondary endings in intrafusal fibers
• B Free nerve endings in the joint capsule
• C Pacinian corpuscles in the periosteum
• D Golgi tendon organs in series with extrafusal fibers ✓

Explanation

Golgi tendon organs lie in series with extrafusal muscle fibers at the musculotendinous junction and sense tension rather than length. Their Ib afferents excite inhibitory interneurons that suppress homonymous alpha motor neurons, producing autogenic inhibition, also called the inverse stretch reflex or lengthening reaction. Muscle spindles are arranged in parallel and detect length changes, mediating the opposite, excitatory stretch reflex. Joint and periosteal receptors contribute proprioception but do not trigger this reflex arc.

Reference: Ganong's Review of Medical Physiology, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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