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A reflex action is a protective, automatic and rapid response to a stimulus.

It follows the same general sequence as a normal reaction but often it does not involve the brain. Instead, the spinal cord coordinates the response so the impulse can reach the effector quickly.

The nerve pathway followed by a reflex action is called a reflex arc. The process is:

stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response

A withdrawal reflex can occur when you accidentally touch something hot.

1. Receptor cells in the skin detect a stimulus (the heat from the candle).
2. Sensory neurone sends impulses to relay neurone in the spinal cord.
3. The relay neurone connects to the motor neurone (and also sends a message to the brain).
4. Motor neurone sends impulses to effector.
5. Effector produces a response (muscle contracts to move hand away).

The pupil reflex

The amount of light entering the eye is controlled by a reflex action. The size of the pupil changes in response to bright or dim light. This is controlled by the muscles of the iris.

This reaction is:

• protective - it prevents harmful bright light from damaging the receptor cells at the back of the eye
• involuntary - you don’t have to think about it, it happens automatically
• rapid – it happens very quickly

Try it - in a bright room, ask someone to close their eyes for 30 seconds. When they open their eyes, observe how quickly their pupils react.

Impulse pathway - Higher tier only

Once the stimulus is detected the following process occurs.

1. A receptor cell passes an electrical impulse along the sensory neurone.
2. The sensory neurone reaches the coordinator, the CNS.
3. The sensory neurone connects to a relay neurone, via a synapse, inside the CNS.
4. The relay neurone connects to a motor neurone, via a synapse, inside the CNS.
5. The motor neurone leaves the CNS and the impulse is carried to the effector.