Flexor withdrawal with crossed extension is achieved through which organizational principles?

• A. Convergence, divergence
• B. Monosynaptic connectivity
• C. Cortex-dominated control
• D. Purely ipsilateral inhibition

Answer: (A)

Flexor withdrawal with crossed extension is produced by a spinal network that takes a single nociceptive input and coordinates actions in both legs. This relies on convergence, where many sensory fibers feed into interneurons that generate the motor response, and divergence, where those same interneurons influence multiple motor pools. The result is an ipsilateral flexor withdrawal to pull the stimulated limb away, plus crossed extension on the opposite side to stabilize and support weight. This is a polysynaptic spinal circuit with commissural (cross-midline) connections, not a simple monosynaptic pathway, and it involves more than just ipsilateral inhibition. Cortex-driven control isn’t required for the reflex to occur, though the cortex can modulate it. So convergence and divergence best explain how this reflex is organized.