Curriculum fit: GCSE Biology and Combined Science (the human nervous system). US: high school life science (structure and function: nervous system and homeostasis).
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What Is It?
The nervous system lets an organism react to changes in its surroundings. A change is called a stimulus. It is detected by a receptor, and the response is carried out by an effector, which is a muscle or a gland.
Information travels as electrical impulses along neurones. The brain and spinal cord form the central nervous system (CNS), which coordinates the response.
Key Formulas and Ideas
- Pathway: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.
- A reflex is rapid, automatic and does not involve conscious thought.
- Neurones are linked at a synapse, where chemical transmitters cross a tiny gap.
Worked Examples
Reflex Pathway
Describe what happens when your hand touches a hot object.
- Pain receptors in the skin detect the stimulus.
- A sensory neurone carries the impulse to the spinal cord.
- A relay neurone passes it across synapses to a motor neurone.
- The motor neurone sends the impulse to a muscle in the arm (the effector), which contracts.
- The hand is pulled away.
✓ Answer The hand is quickly withdrawn, before the brain feels the pain
Reflex Versus Voluntary
Give two differences between a reflex action and a voluntary action.
- Reflexes are automatic and very fast.
- Voluntary actions involve conscious decisions by the brain.
✓ Answer Reflex: automatic and fast. Voluntary: conscious and slower
Reaction Time
Describe how you could measure reaction time using a ruler.
- One person holds a ruler vertically; the other has their hand ready at the zero mark.
- The ruler is dropped without warning and caught.
- Record the distance fallen and compare it with a reaction time conversion.
✓ Answer Distance fallen shows reaction time
Common Mistakes to Avoid
- Saying the brain controls reflexes. They are coordinated by the spinal cord.
- Mixing up sensory and motor neurones.
- Forgetting the synapse.
- Calling the muscle a receptor.
Practice Questions
Frequently Asked Questions
Why are reflexes useful?
They protect the body by responding quickly, such as pulling away from danger.
What is the difference between a nerve and a neurone?
A neurone is a single nerve cell. A nerve is a bundle of many neurones.
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Reviewed by StudyMorrow editorial review on 5 Nov 2026.