4.5.2 The human nervous system

Why this matters

Like the endocrine system, the nervous system exists to coordinate the body — but it does so using electrical impulses travelling along specialised cells called neurones, rather than chemicals carried in the blood. Receptors all over the body detect stimuli (light, sound, temperature, pressure, chemicals). They convert those stimuli into electrical impulses which travel along sensory neurones to the central nervous system (CNS) — the brain and spinal cord. The CNS acts as the coordination centre: it processes the information and decides what to do. Motor neurones then carry impulses back out to effectors, which are either muscles (which contract) or glands (which secrete substances such as hormones). For most responses, the brain is involved and the response is consciously controlled — pulling out a chair, picking up a pen. But for some life-or-death situations — touching something hot, dust hitting the eye, food entering the back of the throat — speed matters more than control. Reflex actions short-circuit the brain entirely: the impulse goes only as far as the spinal cord, where a single relay neurone passes it directly to the motor neurone. The result is a response that is automatic, rapid and unconscious — which protects the body from damage. Synapses, the tiny gaps between neurones, are the one place where the signal is briefly chemical rather than electrical — vesicles release a neurotransmitter that diffuses across the cleft and triggers a fresh impulse in the next neurone.

How to learn this topic

Build on what you already know

• Cells have specialised structures (from 4.1) — neurones have a long axon and dendrites.
• The body is organised into systems (from 4.2) — the nervous system is one of two coordination systems.
• Basic idea of homeostasis (introduced earlier in 4.5).
• Comparison with the endocrine system from 4.5.3 — chemical vs electrical, slow vs fast.

1. Define stimulus, receptor, coordinator, effector and response — the five stages of any nervous-system response.
2. Split the nervous system into the CNS (brain + spinal cord) and the peripheral nerves.
3. Introduce the three neurone types — sensory, relay and motor — and what each carries and where it lives.
4. Build the reflex arc step by step: stimulus → receptor → sensory → relay → motor → effector → response.
5. Contrast a reflex (no brain) with a conscious response (brain involved, slower).
6. Explain synapses — how a chemical neurotransmitter bridges the gap between neurones.
7. (Higher tier) Map the regions of the brain — cerebral cortex, cerebellum, medulla — and how they are studied.
8. Practise four-mark answers using the exact AQA marking phrases.

Key terms

nervous system

The body's fast, electrical coordination system, made of neurones — including the central nervous system and peripheral nerves. (Contrast with the endocrine system: nervous is electrical/fast/short-lived; hormonal is chemical/slow/long-lasting.)

central nervous system (CNS)

The brain and spinal cord. Acts as the coordination centre that receives information from receptors and sends instructions to effectors.

neurone

A long, thin cell that carries electrical impulses. Three types: sensory, relay and motor. (Spell 'neurone' carefully — 'neuron' is also accepted but be consistent.)

sensory neurone

A neurone that carries impulses from a receptor to the CNS. Cell body sits in the middle of the cell. (Always say 'from a receptor to the CNS' — direction matters in mark schemes.)

relay neurone

A short neurone inside the CNS (especially the spinal cord) that connects a sensory neurone to a motor neurone. (Relay neurones live in the spinal cord — examiners want this location named in reflex-arc answers.)

motor neurone

A neurone that carries impulses from the CNS to an effector (muscle or gland). Has a cell body at one end with many dendrites and a long axon. (Always pair motor neurone with an effector in your answer.)

receptor

A specialised cell that detects a stimulus (e.g. light in the retina, pressure in the skin) and converts it into a nerve impulse. (Receptors DETECT stimuli — that's the marking phrase. Don't say 'feel' or 'sense'.)

effector

A muscle (which contracts) or a gland (which secretes) — the part of the body that actually carries out a response. (Two and only two kinds: muscles and glands. Worth a mark to name both.)

synapse

The tiny gap between two neurones. The electrical impulse is briefly converted to a chemical signal — a neurotransmitter — that diffuses across the gap. (Higher-tier students should mention vesicles, diffusion and receptors on the postsynaptic membrane.)

neurotransmitter

The chemical released from vesicles on the presynaptic side of a synapse. It diffuses across the synaptic cleft and binds to receptors on the next neurone, triggering a new impulse.

reflex action

An automatic and rapid response that does not involve the conscious part of the brain. Protects the body from damage. (Marking-phrase trio: 'rapid', 'automatic', 'does not involve the brain / conscious thought'.)

reflex arc

The pathway of neurones that produces a reflex: stimulus → receptor → sensory neurone → relay neurone in spinal cord → motor neurone → effector → response. (Memorise the order. Every word is a potential mark.)

cerebral cortex

The wrinkled outer layer of the brain. Controls consciousness, intelligence, memory and language. (Higher tier.)

cerebellum

The region at the back of the brain that controls balance and coordination of muscular movement. (Higher tier.)

medulla

The region at the base of the brain that controls unconscious activities such as heart rate and breathing. (Higher tier.)

Notes

The job of the nervous system

The nervous system lets us react to our surroundings and coordinate our behaviour. It uses electrical impulses travelling along neurones — much faster than the chemical signals used by the endocrine system. Every response, whether reflex or conscious, follows the same five-stage pattern:

stimulus → receptor → coordinator (CNS) → effector → response

• A stimulus is a change in the environment (heat, light, sound, pressure, chemical).
• A receptor is a cell that detects the stimulus and converts it into a nerve impulse.
• The CNS (central nervous system — the brain and spinal cord) acts as the coordination centre.
• An effector is a muscle or a gland — what actually carries out the response.
• The response is the action: a muscle contracts, or a gland secretes.

Receptors and effectors

Receptors are specialised cells that detect particular stimuli:

• light receptors in the retina (eye)
• sound + balance receptors in the inner ear
• temperature, pressure and pain receptors in the skin
• chemical receptors in the tongue (taste) and nose (smell)

Effectors carry out the response. Only two types:

• muscles — contract to move part of the body
• glands — secrete chemicals such as hormones, sweat, saliva or digestive enzymes

The central nervous system (CNS)

The CNS is the brain and spinal cord. It receives information from receptors, processes it, and sends instructions to effectors. The brain is the coordination centre for conscious responses; the spinal cord is a thick column of nerves running down the back, protected by the vertebrae, that coordinates reflex actions and carries signals between brain and body.

Nerves that run between the CNS and the rest of the body make up the peripheral nervous system.

Three types of neurone

Neurones are long, thin cells that carry electrical impulses. Three kinds:

• Sensory neurones carry impulses from a receptor to the CNS. They have a cell body in the middle, with a long dendron carrying the impulse in from the receptor and a long axon carrying it on into the CNS.
• Relay neurones live entirely inside the CNS (usually the spinal cord). They are short, with no myelin, and connect a sensory neurone to a motor neurone.
• Motor neurones carry impulses from the CNS to an effector (a muscle or a gland). They have a cell body with many dendrites at one end and a single long axon stretching out to the effector.

Most sensory and motor neurones are wrapped in a fatty myelin sheath that speeds up the impulse.

Synapses — how impulses cross between neurones

Neurones don't physically touch — there's a tiny gap between them called a synapse (the gap itself is the synaptic cleft). The electrical impulse can't jump across the gap directly, so it's temporarily turned into a chemical signal:

1. The impulse arrives at the end of the presynaptic neurone.
2. Tiny sacs called vesicles release neurotransmitter molecules into the cleft.
3. Neurotransmitter diffuses across the gap.
4. It binds to receptors on the postsynaptic neurone.
5. A new electrical impulse is generated, and travels on.

The whole process is one-way (vesicles are only on the presynaptic side) and takes only a few milliseconds — but each synapse adds a small delay, which is why responses with more synapses (e.g. conscious responses involving the brain) are slower than reflexes.

Reflex actions — the reflex arc

A reflex action is an automatic, rapid response that does not involve the conscious part of the brain. Reflexes protect the body from harm — pulling your hand off a hot pan, blinking when something flies at your eye, your pupils shrinking in bright light.

The path of a reflex is called the reflex arc:

stimulus → receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector → response

Look closely: the impulse travels only as far as the spinal cord. The relay neurone passes the signal straight across to the motor neurone — no trip up to the brain. That's exactly why reflexes are:

• rapid — fewer synapses, no decision-making in the brain
• automatic — no conscious thought required
• protective — they happen before you even feel pain

A classic exam example: you touch a hot object. Pain/temperature receptors in the skin detect the stimulus. An impulse travels along a sensory neurone to the spinal cord. A relay neurone in the spinal cord passes the impulse straight across to a motor neurone. The motor neurone carries the impulse to the effector (a bicep muscle), which contracts — pulling your hand away. The brain only finds out afterwards — which is when you feel the pain.

Reflex vs conscious response

Not every response is a reflex. If you touch something that's just slightly warm — your morning coffee mug — the impulse still goes to the CNS, but the brain processes it consciously, decides whether the temperature is bearable, and tells the muscles what to do. That involves many more synapses (and the conscious cerebral cortex), so it's slower — but it gives the brain the option to choose how to respond.

For any 'response to a stimulus' question that is not a reflex, the AQA marking phrases are: receptors detect the stimulus → impulses travel to the CNS → the brain coordinates the response → impulses sent to effectors.

The brain (Higher tier)

The brain is made of billions of interconnected neurones. Three regions you need:

• Cerebral cortex — the wrinkled outer layer; controls consciousness, intelligence, memory, language.
• Cerebellum — at the back, below the cortex; controls balance and coordination of muscle movements.
• Medulla — at the base, where the brain meets the spinal cord; controls unconscious activities such as heart rate and breathing.

Neuroscientists study the brain by: (1) examining patients with brain damage to see which functions are lost; (2) electrically stimulating specific regions in patients undergoing brain surgery; (3) MRI scans that produce detailed images of brain structure, and functional MRI that shows which regions are active during particular tasks. Investigating the brain is difficult because it is incredibly complex, surrounded by the skull, and damaging it during research can cause permanent harm.

The eye (briefly, Bio-only)

For completeness — the eye is a sense organ packed with light receptors:

• cornea — transparent front; refracts light
• iris — coloured ring; controls how much light enters by changing the size of the pupil
• lens — focuses light onto the retina (changes shape — accommodation)
• retina — back of the eye; contains the light receptors (rods and cones)
• optic nerve — carries impulses from retina to the brain

Accommodation: the ciliary muscles change the shape of the lens to focus light from near or distant objects. Myopia (short-sighted) — the lens is too curved or eyeball too long, so distant objects focus in front of the retina; corrected with a concave lens. Hyperopia (long-sighted) — opposite problem; corrected with a convex lens.

Exam tips

• For any reflex-arc question, write the full pathway in order: receptor → sensory neurone → relay neurone in spinal cord → motor neurone → effector. Each is potentially a mark.
• Always state the LOCATION of the relay neurone (in the spinal cord) — examiners reward this specifically.
• When asked WHY reflexes matter, use the trio 'rapid', 'automatic', 'protects from damage' plus the killer phrase 'does not involve the conscious part of the brain'.
• For a non-reflex (conscious) response, the marking phrase chain is: receptors detect stimulus → impulses travel to CNS → brain coordinates response → impulses sent to effectors.
• Effectors are muscles OR glands — name both for a definition mark, and pick the right one for the question (muscle to move; gland to secrete).
• On synapse questions, say neurotransmitter is RELEASED FROM VESICLES, DIFFUSES across the cleft, and BINDS TO RECEPTORS on the postsynaptic membrane.
• Compare with endocrine system in any 'why two control systems' question: nervous is electrical / fast / short-lived; hormonal is chemical / slow / long-lasting.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in AQA GCSE Biology (8461)

• 4.5.1 Homeostasis
• 4.5.3 Hormonal coordination in humans
• 4.5.4 Plant hormones (Bio only)