English: A.Stimulus

B.Sensory Neurons C.Spinal Column D.Interneuron E.Motor Neuron F.Muscle G.Bone

In a reflex arc, an action potential never travels to the brain for processing and so results in a much quicker reaction. When a stimulus (pain) is encountered, the signal from that stimulus will travel up the sensory neuron to the spinal column. There, it will likely pass through a short interneuron before continuing down a motor neuron to the origin of the signal. Then, a contraction of the muscles is triggered.