A person steps on a sharp object with the right foot. The right leg flexes at hip and knee while the left leg suddenly extends to support body weight. The contralateral limb extension is produced by:

• A Collateral branches of Ib afferents crossing to the opposite anterior horn
• B Commissural interneurons that excite contralateral extensor motor neurons ✓
• C Descending reticulospinal input activating antigravity muscles bilaterally
• D Gamma motor neuron activation on the unsupported side

Explanation

The flexor (withdrawal) reflex includes a crossed extensor component: nociceptive afferents activate commissural interneurons that cross the midline and excite extensor alpha motor neurons of the opposite limb while flexors there are inhibited, providing postural support. This makes the reflex polysynaptic and multisegmental. Option C is incorrect because the response is spinally mediated and present even after complete cord transection above the lumbosacral segments.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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