A person steps barefoot on a sharp tack with the right foot. The right leg flexes and withdraws while the left leg simultaneously extends to bear body weight. The circuitry underlying extension of the left leg is:

• A Monosynaptic excitation of left extensor motor neurons by right-sided Ia afferents
• B Direct activation of the left leg by the crossed corticospinal tract
• C Polysynaptic commissural interneurons crossing the midline to excite contralateral extensor motor neurons ✓
• D Recurrent inhibition of left flexor motor neurons by Renshaw cells

Explanation

The withdrawal reflex is polysynaptic. On the stimulated side, nociceptive afferents excite flexor motor neurons via interneurons and inhibit extensors. Collaterals cross the midline through commissural interneurons to excite contralateral extensor motor neurons and inhibit contralateral flexors, producing the crossed extensor reflex that supports the body during withdrawal. It is entirely spinal and does not involve the corticospinal tract, and monosynaptic connections exist only in the stretch reflex.

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

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