Which sensor detects muscle stretch and causes excitation of the agonist and inhibition of the antagonist?

Which sensor detects muscle stretch and causes excitation of the agonist and inhibition of the antagonist?

• A. Golgi tendon organs
• B. Mechanoreceptors
• C. Nociceptors
• D. Muscle spindles

Answer: (D)

Muscle spindles are the stretch detectors inside a muscle. When the muscle lengthens, the intrafusal fibers are stretched and activate sensory afferents that fire to the spinal cord. This input directly excites the alpha motor neurons controlling the same muscle (the agonist), causing it to contract, and it also engages interneurons that inhibit the alpha motor neurons to the opposing muscle (the antagonist), producing reciprocal inhibition. This is the classic stretch (myotatic) reflex that helps maintain posture and reflexively resists being stretched too far. Golgi tendon organs respond to tendon tension and produce inhibition of the agonist with facilitation of the antagonist, not the pattern described here, while mechanoreceptors and nociceptors are broader or relate to different sensations. Therefore, the sensor described is the muscle spindle.

Muscle spindles are the stretch detectors inside a muscle. When the muscle lengthens, the intrafusal fibers are stretched and activate sensory afferents that fire to the spinal cord. This input directly excites the alpha motor neurons controlling the same muscle (the agonist), causing it to contract, and it also engages interneurons that inhibit the alpha motor neurons to the opposing muscle (the antagonist), producing reciprocal inhibition. This is the classic stretch (myotatic) reflex that helps maintain posture and reflexively resists being stretched too far. Golgi tendon organs respond to tendon tension and produce inhibition of the agonist with facilitation of the antagonist, not the pattern described here, while mechanoreceptors and nociceptors are broader or relate to different sensations. Therefore, the sensor described is the muscle spindle.