A patient steps on a sharp object with the right foot while standing. The right leg flexes rapidly at the hip and knee while the left leg extends stiffly to support body weight. The neural circuit responsible for the left limb extension is:

• A Monosynaptic excitation of contralateral extensor motor neurons by Ia afferents
• B Ib afferent activation of contralateral flexor motor neurons
• C Crossed extensor reflex mediated by commissural interneurons in the spinal cord ✓
• D Descending reticulospinal facilitation of antigravity muscles triggered by the spinothalamic relay

Explanation

The flexor withdrawal reflex is polysynaptic. Collateral branches of nociceptive afferents activate commissural interneurons that cross the midline and excite alpha motor neurons supplying contralateral extensors, producing the crossed extensor component that maintains postural support. Monosynaptic Ia connections belong to the stretch reflex, which involves only the ipsilateral agonist. Ib afferents from Golgi tendon organs mediate autogenic inhibition. This circuit is entirely spinal and does not require supraspinal relay, though it is modified by descending input.

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

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Written and medically reviewed by the StethoPrep medical team.