An example of muscle spindle function is a protective reflex during stretching where the muscle contracts to resist stretch. Which description fits this?

• A. A reflex that relaxes the muscle
• B. Voluntary contraction
• C. Numbness
• D. Reflex that resists stretch by contracting

Answer: (D)

Muscle spindles mediate the stretch reflex, a protective response that resists muscle lengthening by contracting the same muscle. When the muscle is stretched, Ia afferents fire and monosynaptically drive the alpha motor neurons to contract, producing a quick, automatic contraction that opposes the stretch to prevent injury. This contrasts with a relaxation reflex, voluntary contraction, or numbness, which are not the automatic protective response here. The Golgi tendon organ, by contrast, helps limit force rather than produce the immediate contraction seen with the stretch reflex. So the description that fits is a reflex that resists stretch by contracting.