withdrawal reflex

**Reflex arc (polysynaptic, ipsilateral + contralateral):** - Receptor: nociceptors (free nerve endings) in skin, muscle, or joints. - Afferent: Aδ (fast, sharp pain) and C fibers (slow, dull pain), plus some Aβ fibers. They enter the dorsal horn. - Integration: one or more excitatory interneurons in the spinal cord. Because there is more than one synapse, the reflex is polysynaptic and has a longer latency than the monosynaptic stretch reflex. - Efferent: alpha motor neurons to the ipsilateral flexors (excited) and extensors (inhibited via inhibitory interneurons, i.e., reciprocal inhibition). This pulls the limb away from the stimulus. - Crossed extensor reflex: collaterals cross the midline to excite contralateral extensors and inhibit contralateral flexors. This supports body weight while the injured limb is lifted.

**Key properties:** - Divergence: one afferent input activates multiple spinal segments, so the response involves several muscle groups (e.g., hip, knee, and ankle flexion). - Irradiation/recruitment: a stronger stimulus recruits more muscles and spreads to more segments. - After-discharge: reverberating circuits keep the response going after the stimulus stops. - Graded response: the intensity and extent of the reflex scale with stimulus strength. - Protective and stereotyped: the response is fast, involuntary, and does not require the cortex. Pain perception comes afterward, as the signal ascends via the spinothalamic tract. - Supraspinal modulation: descending pathways (e.g., corticospinal and reticulospinal) modulate it. Loss of descending inhibition (upper motor neuron lesion) can exaggerate it.

**Comparison with the stretch (myotatic) reflex:** - The stretch reflex is monosynaptic, uses Ia afferents from muscle spindles, and contracts the same muscle (e.g., patellar reflex, L2–L4). - The withdrawal reflex is polysynaptic, uses nociceptive afferents, and contracts flexors while relaxing extensors. - The Golgi tendon (inverse myotatic) reflex is disynaptic. Ib afferents cause relaxation of the muscle.

**Clinical significance:** - Spinal shock: after acute spinal cord transection, all reflexes below the lesion are lost (areflexia, flaccid paralysis). Over days to weeks, they return as hyperreflexia and spasticity. - Babinski sign: a pathologic withdrawal-type (flexor) response of the toe (dorsiflexion of the great toe with fanning) to plantar stroking. It indicates an upper motor neuron lesion in adults and is normal in infants under about 1 year because of incomplete corticospinal myelination. - Flexor spasms and mass reflex: patients with chronic spinal cord injury may have exaggerated flexor withdrawal spasms in response to minor stimuli. - Tabes dorsalis, syringomyelia, and peripheral neuropathy: a loss of afferent input (e.g., the loss of pain and temperature sensation in a "cape-like" distribution in syringomyelia from damage to the anterior white commissure) impairs the reflex and leads to unnoticed injury. - Lower motor neuron or peripheral nerve lesions abolish the efferent limb, so the reflex is absent. - Brain death and coma assessment: preserved spinal reflexes (including withdrawal) can occur in brain death (Lazarus sign), so they do not indicate brain function. - Exam use: a limb that withdraws to a noxious stimulus shows an intact spinal arc. It does not prove conscious perception. Withdrawal with grimacing or vocalization implies cortical processing.

**High-yield pearls:** - Polysynaptic + ipsilateral flexion + contralateral extension = withdrawal (flexor) reflex with crossed extension. - Pain afferents: Aδ (fast, myelinated) and C (slow, unmyelinated). - Cortex is not needed for the reflex itself.