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The withdrawal reflex

Pulling away from a hot stove or a tack. Pain excites the motor neurons of the withdrawing muscle and, through an inhibitory interneuron, quiets the opposing muscle so the movement is faster.

Updated 2026-10-022 sources

Depth:

Why it matters

Heat and pain from a hot stove make the arm withdraw through a connection in the spinal cord that leads to contraction of the biceps brachii [1].

What it is

The withdrawal reflex is a spinal reflex: a sensory neuron that detects a harmful stimulus connects in the cord to the motor neurons of the muscles that move the body part away [1]. Pain is transduced by nociceptors, which respond to stimuli beyond a threshold [2].

How it works

  1. The sensory neuron excites the motor neurons that make the biceps brachii contract [1].
  2. As the biceps contracts, the antagonistic triceps brachii needs to relax; skeletal muscle does not actively relax, so its motor neuron must be inhibited [1].
  3. An interneuron with its cell body in the dorsal horn receives a synapse from the sensory neuron and inhibits the motor neuron of the triceps, hyperpolarising it [1].
Dorsal hornVentral hornRootsArmPain receptorhandSensory axondorsal rootInterneurondorsal hornBiceps motor neuronTriceps motor neuronBiceps brachiicontractsTriceps brachiirelaxes
Withdrawal reflex (hot stove)Pain from the hand excites the biceps motor neuron, so the elbow flexes; at the same time an interneuron in the dorsal horn inhibits the triceps motor neuron, so the opposing muscle relaxes.
  • Excitatory synapse (filled arrowhead)
  • Inhibitory synapse (bar)
  • Modulatory (open circle)
  • Signal or data flow, not a synapse (dashed)
  • Midline crossing (decussation)
Figure 1. Withdrawal reflex (hot stove). Pain from the hand excites the biceps motor neuron, so the elbow flexes; at the same time an interneuron in the dorsal horn inhibits the triceps motor neuron, so the opposing muscle relaxes.[1] Schematic
  • Skeletal muscle does not actively relax: its motor neuron has to be inhibited, which here happens through the interneuron.[1]

Simplified: The muscles are drawn near the midline only to make room; everything here is on one side of the body.

Text description of this diagram
  1. Pain from the hand excites the biceps motor neuron, so the elbow flexes; at the same time an interneuron in the dorsal horn inhibits the triceps motor neuron, so the opposing muscle relaxes.
  2. Pain receptor (left side, Arm) to Sensory axon (left side, Roots); signal or data flow (not a synapse).
  3. Sensory axon (left side, Roots) to Biceps motor neuron (left side, Ventral horn); excitatory synapse.
  4. Sensory axon (left side, Roots) to Interneuron (left side, Dorsal horn); excitatory synapse; stimulates the interneuron.
  5. Interneuron (left side, Dorsal horn) to Triceps motor neuron (left side, Ventral horn); inhibitory synapse; hyperpolarises.
  6. Biceps motor neuron (left side, Ventral horn) to Biceps brachii (midline, Arm); excitatory synapse; neuromuscular junction.
  7. Triceps motor neuron (left side, Ventral horn) to Triceps brachii (midline, Arm); signal or data flow (not a synapse); quieted: fewer signals.

Stepping on a tack works the same way in the leg: the nociceptors activate the motor neurons of tibialis anterior, dorsiflexing the foot, and an inhibitory interneuron activated by a collateral of the nociceptor fibre inhibits the motor neurons of gastrocnemius and soleus to cancel plantar flexion [1]. Here the inhibition matters more than the contraction, because continued plantar flexion would push the foot further onto the tack [1].

Why it is built this way

Without the antagonistic contraction, withdrawal from the hot stove is faster and keeps further tissue damage from occurring [1].

Common misconceptions

Misconception: Muscles relax by actively pulling the other way.

Skeletal muscle does not actively relax; its motor neuron has to quiet down, which in this reflex is done by an inhibitory interneuron [1].

Check yourself

Where is the inhibitory interneuron's cell body?

In the dorsal horn of the spinal cord [1].

Why does the reflex inhibit the triceps?

So that the antagonist does not oppose the biceps, making withdrawal faster [1].

Read next

References

  1. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 14.3 Motor Responses. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses
  2. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 14.1 Sensory Perception. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/14-1-sensory-perception

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