The reflex arc

A reflex action is a quick, automatic response to a stimulus, made without thinking.

The reflex arc

The path an impulse takes in a reflex action is the reflex arc.

Receptor → Sensory neuron → Relay neuron in the spinal cord → Motor neuron → Effector

Figure to be added

Figure — a reflex arc: a hand touching a hot object, the impulse passing along a sensory neuron into the spinal cord, across a relay neuron and out along a motor neuron to the arm muscle.

The impulse does not need to reach the brain before the muscle acts, so the response is very fast — and it protects the body from harm.