Answer:
Reflex action/reflex arc/reflex (allow "involuntary") The stimulus (heat) is detected by temperature receptors in the skin. This causes impulses to travel along a sensory neurone to the spinal cord, due to chemical changes across the synapses. Here they go along a relay neurone, and then are sent via a motor neurone to the effector (an arm muscle in this case). The muscle contracts, moving the hand away and preventing further harm.