Sudden stretching of a contracted quadriceps activates Golgi tendon organ Ib afferents. The immediate spinal reflex effect is:

• A Contraction of the quadriceps via monosynaptic excitation of alpha motor neurons
• B Relaxation of the quadriceps with contraction of its antagonist via disynaptic inhibition ✓
• C Contraction of both quadriceps and hamstrings simultaneously
• D No reflex effect because Ib afferents ascend without spinal connections

Explanation

Golgi tendon organs lie in series with muscle fibres and respond to tension. Their Ib afferents excite inhibitory interneurons in the spinal cord that suppress the homonymous alpha motor neurons and excite antagonists, producing autogenic inhibition, the inverse myotatic reflex. This protects against excessive force and contributes to clasp-knife release in upper motor neuron lesions. Option A describes the muscle spindle Ia stretch reflex, which acts in series with fibres and senses length instead.

Reference: Ganong Review of Medical Physiology, 26th ed.

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