The reflex arc

A reflex arc diagram shows the path a nerve impulse takes during a reflex: from a receptor, along a sensory neurone to a relay neurone in the spinal cord, then along a motor neurone to an effector, such as a muscle. Because the impulse skips the thinking part of the brain, the response is fast and automatic. Pulling your hand away from a hot pan is a reflex.

The reflex arc diagram

This is the general pathway AQA gives for the nervous system, stimulus → receptor → coordinator → effector → response, with the spinal cord as the coordinator. Between each pair of neurones there is a tiny gap called a synapse, which the impulse crosses chemically.

The parts of a reflex arc

AQA's GCSE Biology specification asks students to explain how the parts of a reflex arc relate to their function, naming the sensory neurone, synapse, relay neurone and motor neurone, and to explain why reflex actions are important.

Test yourself on the order, then press Mark to check it in the course.

How a synapse works

1. An impulse arrives at the end of the first neurone.
2. It triggers the release of chemicals called neurotransmitters into the gap.
3. They diffuse across the gap and bind to receptors on the next neurone.
4. This starts a new impulse in the next neurone.

Synapses make sure impulses travel in one direction only, because only the end of the first neurone releases neurotransmitter.

Why reflexes are important

Reflexes protect the body. They are automatic and rapid, as AQA puts it, so the response happens before you have time to think. Examples include blinking when something comes near your eye, coughing when something irritates your airway, and your pupils narrowing in bright light. You only notice the pain afterwards, once impulses reach the conscious part of the brain.

Reflex actions and reaction time

AQA's required practical 7 investigates a factor that affects human reaction time, often using the ruler drop test. A reaction to a sound or a sight is not a reflex: it goes through the conscious brain, so it is slower. The practical also gives practice in turning reaction time data into graphs and back.

Common mistakes

• Getting the order wrong. It is always receptor, sensory, relay, motor, effector: the impulse travels in, across and out.
• Saying the brain decides. In a spinal reflex the relay neurone is in the spinal cord, not the conscious brain.
• Calling the effector a neurone. It is a muscle or a gland.
• Saying impulses jump across a synapse electrically. A chemical neurotransmitter diffuses across.

To test the nervous system under time pressure, try today's GCSE Biology Higher short timed paper. At A-level, receptors such as the Pacinian corpuscle show how the impulse starts.

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