What you'll learn
The nervous system lets you detect changes around you and respond quickly, from catching a ball to pulling your hand away from something hot. For AQA GCSE Biology you need to understand the parts of the nervous system, the three types of neurone, how a nerve impulse travels from a receptor to an effector, and how a reflex arc produces a fast, automatic response. This guide explains the central and peripheral nervous systems, the role of synapses, and the exact pathway of a reflex. By the end you should be able to describe the reflex arc in order, explain why reflexes are fast and automatic, and use the correct term for each stage.
Key terms and definitions
Nervous system — The system that detects stimuli and coordinates the body's responses using electrical impulses.
Central nervous system (CNS) — The brain and spinal cord, which coordinate responses.
Neurone — A nerve cell that carries electrical impulses.
Receptor — A cell or organ that detects a stimulus, such as light, heat or pressure.
Effector — A muscle or gland that carries out a response.
Stimulus — A change in the environment that is detected by a receptor.
Synapse — The tiny gap between two neurones, where the impulse is passed on by chemicals.
Reflex — A fast, automatic response that does not involve conscious thought.
Core concepts
The parts of the nervous system
The nervous system has two main parts. The central nervous system (CNS) is made up of the brain and spinal cord. The rest of the nervous system — the nerves connecting the CNS to the body — is the peripheral nervous system. Information travels as electrical impulses along neurones. The basic sequence of any response is:
stimulus → receptor → coordinator (CNS) → effector → response
A receptor detects the stimulus, the impulse travels to the CNS, which coordinates a response, and an effector (a muscle or gland) carries it out.
The three types of neurone
Impulses are carried by three types of neurone:
- Sensory neurone — carries impulses from a receptor to the CNS.
- Relay neurone — found within the CNS; carries impulses from a sensory neurone to a motor neurone.
- Motor neurone — carries impulses from the CNS to an effector.
Learning the order — sensory, relay, motor — is essential, because the reflex arc uses all three in that sequence.
Synapses
Where two neurones meet, there is a tiny gap called a synapse. The electrical impulse cannot jump the gap directly. Instead, when the impulse reaches the end of one neurone, it causes the release of a chemical (neurotransmitter), which diffuses across the gap and starts a new impulse in the next neurone. This is why synapses slow the impulse down slightly, and it is a favourite exam point.
What a reflex is
A reflex is a fast, automatic response that protects the body from harm, such as pulling your hand away from a hot object or blinking when something approaches your eye. Reflexes do not involve conscious thought — the brain is not needed to produce the response, which is why they are so quick. This speed can prevent injury.
The reflex arc
The pathway an impulse takes in a reflex is called the reflex arc. Using the example of touching a hot object, the stages in order are:
- A receptor in the skin detects the stimulus (heat).
- An impulse travels along a sensory neurone to the spinal cord (CNS).
- In the CNS, the impulse passes across a synapse to a relay neurone.
- The impulse passes across another synapse to a motor neurone.
- The motor neurone carries the impulse to an effector (a muscle in the arm).
- The effector contracts, pulling the hand away — the response.
Because the impulse goes through the spinal cord and not the conscious part of the brain, the response happens automatically and very quickly.
Why reflexes are fast
Reflexes are fast for two reasons: they do not involve the conscious brain making a decision, and the pathway is short and direct. The relay neurone in the spinal cord connects the sensory and motor neurones without waiting for the brain, so the response happens before you have even thought about it. This is important because reflexes protect the body from danger.
Coordinated response versus reflex
It helps to compare a reflex with a normal, conscious response. If you decide to pick up a pen, information travels to the conscious part of your brain, which processes it, makes a decision, and sends impulses to your muscles. This takes longer because the brain is involved in thinking about the action. In a reflex, the impulse takes a shortcut through the spinal cord and misses out the conscious decision entirely, which is why a reflex is quicker. The trade-off is that you have no conscious control over a reflex — it happens automatically whether you want it to or not, which is exactly what makes it good for protection.
The nervous system and homeostasis
The nervous system is one of the ways the body keeps its internal conditions stable, which is called homeostasis. Receptors detect changes, the CNS coordinates a response, and effectors bring conditions back towards normal. Reflexes are part of this: for example, the pupil reflex automatically adjusts the size of the pupil to protect the eye from bright light. Seeing reflexes as part of the body's wider control systems helps you connect this topic to others in the course.
Worked examples
Example 1: Ordering the reflex arc
Put these in the correct order for a reflex: motor neurone, receptor, relay neurone, sensory neurone, effector. The correct order is: receptor, sensory neurone, relay neurone, motor neurone, effector. This is the path an impulse takes from detecting the stimulus to producing the response.
Example 2: Explaining why a reflex is fast
Explain why a reflex response is faster than a normal response. A reflex does not involve the conscious brain making a decision. The impulse travels through a short pathway in the spinal cord via a relay neurone, so the response happens automatically and quickly, which helps protect the body from harm.
Example 3: The role of the synapse
Describe how an impulse crosses a synapse. When the impulse reaches the end of a neurone, it triggers the release of a chemical. This chemical diffuses across the gap and binds to the next neurone, starting a new electrical impulse. The impulse itself does not jump the gap.
Example 4: Naming the effector
A person blinks when dust flies towards their eye. Identify the receptor, the effector and the response. The receptor is in the eye (detecting the dust or movement). The effector is a muscle in the eyelid. The response is the eyelid closing (blinking) to protect the eye.
Common mistakes and how to avoid them
The most common error is getting the order of neurones wrong. It is always sensory → relay → motor. Mixing these up breaks the whole pathway, so learn the order firmly.
Students often say the impulse "jumps" across the synapse. It does not — a chemical is released and diffuses across the gap to start a new impulse. Always describe the chemical.
Another mistake is including the brain in a reflex. Most reflex arcs go through the spinal cord, not the conscious brain, which is exactly why they are fast and automatic. Referring to the brain making a decision loses marks.
Be careful to distinguish a receptor from an effector. A receptor detects the stimulus; an effector (a muscle or gland) carries out the response.
Finally, do not confuse a stimulus with a response. The stimulus is the change detected (such as heat); the response is what the body does (such as pulling away).
Exam technique for "The nervous system and reflex arcs"
Reflex arc questions very often ask you to describe the pathway in order, so practise writing "receptor, sensory neurone, relay neurone, motor neurone, effector" until it is automatic, and be ready to add "synapse" between the neurones.
When explaining why reflexes are fast, give both reasons: no conscious thought is involved and the pathway is short and direct through the spinal cord. Linking the speed to protecting the body from harm often earns an extra mark.
For synapse questions, always mention the release of a chemical that diffuses across the gap. Diagrams of the reflex arc are common — be ready to label the receptor, the three neurones, the synapses, the spinal cord and the effector. Use the correct term at every stage rather than everyday words.
Quick revision summary
- The CNS is the brain and spinal cord; the general pathway is stimulus → receptor → CNS → effector → response.
- Three neurones carry impulses: sensory (to CNS), relay (within CNS), motor (to effector).
- At a synapse, a chemical is released and diffuses across the gap to start a new impulse.
- A reflex is a fast, automatic response that does not involve conscious thought and protects the body.
- The reflex arc: receptor → sensory neurone → relay neurone → motor neurone → effector, with synapses between the neurones.
- Reflexes are fast because they bypass the conscious brain and use a short, direct pathway.