Withdrawal Reflex vs. Patellar Reflex: Effector Muscles and Pathways

Correction: The withdrawal reflex and patellar reflex are distinct; the effector muscles differ.

Withdrawal Reflex (Nociceptive Flexion Reflex)

• Stimulus: Painful (noxious) to skin.
• Response: Flexion of ipsilateral limb (withdrawal), extension of contralateral limb (crossed extension).
• Effector Muscles: Flexors of the stimulated limb (e.g., hamstrings, iliopsoas, biceps femoris for leg; biceps, brachialis for arm).
• Pathway: Nociceptor → dorsal horn interneurons → α-motor neurons to flexors (ipsilateral) + extensors (contralateral). Polysynaptic.

Patellar Reflex (Knee-Jerk, Stretch Reflex)

• Stimulus: Tendon tap → muscle spindle stretch.
• Response: Quadriceps contraction → knee extension.
• Effector Muscle: Quadriceps femoris (rectus femoris, vastus lateralis/medialis/intermedius).
• Pathway: Ia afferent → α-motor neuron (monosynaptic).

Clinical Use

Withdrawal reflex tests nociceptive pathways and spinal cord integrity (e.g., in coma). Patellar reflex tests L2–L4 segments and monosynaptic arc.