Stepping on a sharp object with the right foot causes rapid right leg flexion while the left leg extends to support body weight. The crossed extension of the left limb depends on which arrangement?

• A Monosynaptic excitation of left extensor motor neurons by right Ia afferents
• B Corticospinal tract activation of the contralateral supporting limb
• C Direct activation of left side Golgi tendon organs by the nociceptive input
• D Commissural interneurons exciting contralateral extensor motor neurons ✓

Explanation

The flexor withdrawal reflex is polysynaptic. Collaterals of interneurons cross the midline through commissural interneurons, which excite alpha motor neurons of contralateral extensors and inhibit contralateral flexors, providing postural support during withdrawal. The reflex is spinal and does not require the corticospinal tract, and no monosynaptic connection exists between afferents of one limb and motor neurons of the other.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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