A reflex action is an automatic, rapid response to a stimulus that does not wait for the brain to decide. The pathway it follows is called the reflex arc.

The impulse begins at the receptor (which detects the stimulus), travels along a sensory neuron to the spinal cord, where it is relayed to a motor neuron, and finally reaches the effector muscle, which responds. This gives the order: Receptor → Spinal cord → Motor neuron → Muscle.

The route through the brain (Receptor → Spinal cord → Brain → Muscle, or Receptor → Brain → Spinal cord → Muscle) is wrong for a reflex, because involving the brain would slow the response; the spinal cord handles reflexes to save time.

The path Receptor → Motor neuron → Spinal cord → Muscle is wrong because the motor neuron carries the signal away from the spinal cord to the muscle; it cannot come before the spinal cord.

Hence, the correct reflex arc is Receptor → Spinal cord → Motor neuron → Muscle.