Withdrawal Reflex: Why We Pull Away from Heat Instantly

The immediate hand withdrawal from a hot object is a spinal reflex arc—a hardwired protective circuit that bypasses the brain for speed.

The Reflex Arc (5 Steps)

1. Receptor: Thermoreceptors (TRPV1 channels) in skin detect noxious heat (>43°C/109°F).
2. Sensory neuron: Action potential travels via Aδ fibers (myelinated, fast) to spinal cord dorsal horn.
3. Interneuron: In spinal cord (gray matter), synapses with inhibitory and excitatory interneurons.
4. Motor neuron: Excitatory signal → alpha motor neuron → efferent signal via ventral root.
5. Effector: Flexor muscles (biceps, brachialis) contract → hand pulls away. Simultaneously: inhibitory interneuron relaxes extensor (triceps) — reciprocal inhibition.

Key Features

• Latency: ~0.5–1 second (spinal only). Brain perceives pain after withdrawal.
• No conscious decision: Occurs in decerebrate animals, spinal cord injury patients (below lesion).
• Protective: Limits tissue damage—burn depth correlates with contact time.
• Polysynaptic: Involves interneurons (vs. monosynaptic stretch reflex).

Clinical Relevance

• Absent/reflex delay: Peripheral neuropathy, spinal cord lesion, sedation, congenital insensitivity to pain.
• Testing: Part of neurological exam (superficial reflexes).
• Pain perception: Separate pathway (spinothalamic tract → thalamus → cortex). Reflex ≠ pain feeling.

Summary: Heat → skin receptors → spinal cord → motor neurons → muscle contraction → withdrawal. All before you "feel" the pain. Evolution's solution to thermal injury.