Withdrawal reflex arc
Pulling away from something harmful
Heat and pain from a hot stove pull the arm back through a connection in the spinal cord: the biceps brachii contracts, and at the same time an interneuron in the dorsal horn inhibits the motor neuron of the triceps brachii, so the opposing muscle relaxes and the withdrawal is faster.
Drawn for the left side of the body. Schematic route through the model’s landmarks; the pulse speed is illustrative.
Route
Step by step
- Pain receptor in the skin · served side
- Sensory axon enters the cord · served side
- Biceps motor neuron excited · served side · synapse
- Dorsal-horn interneuron inhibits the triceps motor neuron · served side · synapse
- Biceps contracts, triceps relaxes · served side
Schematic Filled circles: synapses. X: the pathway crosses the midline. Numbers match the list.
The route enters at C6 (the thumb’s key sensory point) and leaves at C5, the segment the ISNCSCI examination uses for the elbow flexors; real reflex connections spread over several segments.
What the sources say
“The example described at the beginning of the chapter involved heat and pain sensations from a hot stove causing withdrawal of the arm through a connection in the spinal cord that leads to contraction of the biceps brachii.” Betts et al. 2022, §14.3
“As the biceps brachii contracts, the antagonistic triceps brachii needs to relax.” Betts et al. 2022, §14.3
“In the hot-stove withdrawal reflex, this occurs through an interneuron in the spinal cord.” Betts et al. 2022, §14.3
“In response to this stimulation from the sensory neuron, the interneuron then inhibits the motor neuron that controls the triceps brachii.” Betts et al. 2022, §14.3
“Without the antagonistic contraction, withdrawal from the hot stove is faster and keeps further tissue damage from occurring.” Betts et al. 2022, §14.3
Sources: Betts et al. 2022, §14.3; Rupp et al. 2021