While walking barefoot, a person steps on a sharp object with the right foot. The right leg flexes at the hip and knee while the left leg extends to support body weight. The circuitry responsible for the left limb response is best described as:
- A Monosynaptic activation of left limb extensor motor neurons by Ia afferents
- B Ib afferent input from Golgi tendon organs directly exciting contralateral extensors
- C Polysynaptic commissural interneurons exciting contralateral extensor motor neurons while flexors are inhibited ✓
- D Descending reticulospinal drive triggered by collateral activation of the spinothalamic tract
Correct answer: C. Polysynaptic commissural interneurons exciting contralateral extensor motor neurons while flexors are inhibited
Explanation
The crossed extensor reflex is polysynaptic. Pain afferents entering the right dorsal horn activate interneurons that cross the midline through the anterior commissure and excite alpha motor neurons of contralateral extensors while inhibiting contralateral flexors, maintaining posture during withdrawal. The monosynaptic Ia arc described in option A mediates only the ipsilateral stretch reflex and involves no commissural relay, so it cannot produce the contralateral response.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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