2.3 The 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. Edexcel additionally requires a detailed understanding of the synapse: vesicles in the presynaptic terminal release neurotransmitter molecules into the synaptic cleft, where they diffuse across and bind to receptors on the postsynaptic membrane to generate a new impulse. Edexcel also expects you to know the adaptations of a motor neurone — long axon for conducting impulses over long distances, myelin sheath insulating the axon and speeding nerve impulses by saltatory conduction, and many dendrites providing a large surface area to receive impulses.

How to learn this topic

Build on what you already know

• Cells have specialised structures — neurones have a long axon and dendrites.
• The body is organised into systems — the nervous system is one of two coordination systems.
• Basic idea of homeostasis (introduced earlier in Edexcel 1BI0 topic 2).
• Diffusion — needed to understand neurotransmitter movement across the synaptic cleft.
• Comparison with the endocrine system — 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. Walk through the adaptations of a motor neurone: long axon, myelin sheath, dendrites (Edexcel-specific emphasis).
5. Build the reflex arc step by step: stimulus → receptor → sensory → relay → motor → effector → response.
6. Contrast a reflex (no brain) with a conscious response (brain involved, slower).
7. Explain synapses in detail — vesicles, presynaptic terminal, neurotransmitter diffusion across the cleft, receptors on the postsynaptic membrane, new impulse generation.
8. Practise four- and five-mark answers using the exact Edexcel 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. (Edexcel phrase: 'sensory carries impulse from receptor to CNS'.)

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. (Edexcel phrase: 'motor carries impulse from CNS to effector'. Adaptations: long axon, myelin sheath, dendrites.)

axon

The long extension of a neurone that conducts the electrical impulse away from the cell body. In a motor neurone, the axon stretches from the CNS to the effector. (Edexcel phrase: 'long axon conducts impulses over long distance'.)

dendrite

A branched projection from a neurone's cell body that receives impulses from other neurones. Many dendrites give a large surface area to receive multiple inputs. (Edexcel phrase: 'dendrites provide large surface area to receive impulses'.)

myelin sheath

A fatty insulating layer wrapped around the axon of many sensory and motor neurones. It insulates the axon and speeds up nerve impulses via saltatory conduction. (Edexcel phrases: 'myelin sheath insulates axon' AND 'myelin sheath speeds up nerve impulses' — both are required.)

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. (Edexcel phrase: 'effector produces the response'. Two and only two kinds: muscles and glands.)

synapse

The tiny junction between two neurones (or between a neurone and an effector). The electrical impulse is briefly converted to a chemical signal — a neurotransmitter — that diffuses across the synaptic cleft and binds to receptors on the next cell. (Edexcel expect detail: vesicles, presynaptic, cleft, receptors on postsynaptic membrane, new impulse generated.)

synaptic cleft

The tiny gap (~20 nm) between the presynaptic and postsynaptic membranes that the neurotransmitter must diffuse across. (Edexcel phrase: 'neurotransmitters diffuse across the gap'.)

presynaptic terminal

The swollen end of the first neurone at a synapse. Contains vesicles of neurotransmitter that are released when the impulse arrives. (Edexcel phrase: 'neurotransmitters released into synaptic cleft'.)

postsynaptic membrane

The membrane of the next neurone (or effector) on the far side of the synaptic cleft. Carries receptor proteins that bind neurotransmitter. (Edexcel phrase: 'neurotransmitters attach to receptors on next neurone'.)

neurotransmitter

The chemical released from vesicles in the presynaptic terminal of a synapse. It diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane, triggering a new impulse. (Always pair with 'released into synaptic cleft', 'diffuses across', and 'binds to receptors'.)

vesicle

A small, membrane-bound sac inside the presynaptic terminal that stores and releases neurotransmitter molecules when an impulse arrives.

reflex action

An automatic and rapid response that does not involve the conscious part of the brain. Protects the body from damage. (Edexcel phrase: 'rapid response minimises damage'; also 'impulse does not travel to brain'.)

reflex arc

The pathway of neurones that produces a reflex: stimulus → receptor → sensory neurone → relay neurone in spinal cord → motor neurone → effector → response. (Edexcel chain: sensory to CNS/spinal cord → motor from spinal cord to effector → bypasses brain → rapid, minimises damage.)

saltatory conduction

The 'jumping' of a nerve impulse between gaps in the myelin sheath (nodes of Ranvier). It makes conduction along a myelinated axon much faster than along an unmyelinated one.

Notes

The job of the nervous system

The nervous system lets organisms respond to stimuli and coordinate their 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.

The Edexcel marking-phrase chain for a typical (non-reflex) response is:

> sensory carries impulse from receptor to CNS | CNS coordinates the response | motor carries impulse from CNS to effector | effector produces the response.

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.

Motor neurone — Edexcel adaptations

Edexcel often asks you to explain the adaptations of a motor neurone. The marking-phrase chain to memorise is:

> long axon conducts impulses over long distance | myelin sheath insulates axon | myelin sheath speeds up nerve impulses | dendrites provide large surface area to receive impulses.

In detail:

• A long axon lets a single cell carry the impulse all the way from the CNS to a distant effector (e.g. from your lower spinal cord to a muscle in your foot) without ever needing to cross a synapse — fewer synapses mean fewer delays.
• The myelin sheath is a fatty insulating layer wrapped around the axon by specialised supporting cells. It prevents the electrical signal from leaking sideways and forces the impulse to jump between gaps in the sheath (nodes of Ranvier). This is called saltatory conduction — the impulse travels much faster along a myelinated axon than an unmyelinated one.
• The dendrites at the cell-body end branch into many fine projections. The large total surface area lets the neurone receive impulses from many other neurones at the same time, so the CNS can integrate multiple inputs into a single output.

The synapse — a closer look (Edexcel)

Edexcel goes deeper on synapses than some other specifications. You need to be able to describe — in order — exactly what happens at the gap between two neurones.

Neurones don't physically touch. Between the end of one neurone (the presynaptic terminal) and the start of the next neurone (the postsynaptic membrane) there is a tiny gap — the synaptic cleft — about 20 nanometres wide. The electrical impulse can't jump across the gap directly, so it's temporarily turned into a chemical signal:

1. The electrical impulse arrives at the presynaptic terminal at the end of the first neurone.
2. This causes vesicles (small membrane-bound sacs) inside the presynaptic terminal to move to and fuse with the presynaptic membrane.
3. The vesicles release neurotransmitter molecules into the synaptic cleft.
4. The neurotransmitters diffuse across the gap (high concentration on the presynaptic side → low concentration on the postsynaptic side).
5. The neurotransmitters attach (bind) to receptors on the postsynaptic membrane of the next neurone — only neurotransmitter molecules with the right shape will fit each receptor.
6. This binding triggers a new electrical impulse in the postsynaptic neurone, which then travels on.

The Edexcel marking-phrase chain for a synapse question is therefore:

> neurotransmitters released into synaptic cleft | neurotransmitters diffuse across the gap | neurotransmitters attach to receptors on next neurone | new impulse generated in next neurone.

A few extra points Edexcel reward:

• The whole process is one-way — vesicles of neurotransmitter are only on the presynaptic side, and receptors are only on the postsynaptic side. The signal cannot travel backwards.
• Each synapse adds a small delay of a couple of milliseconds — which is why responses with many synapses (conscious responses involving the brain) are slower than reflexes (only two synapses).
• Many drugs and toxins work by interfering with synapses — either by mimicking neurotransmitters at receptors, by blocking receptors, or by preventing the breakdown / reuptake of neurotransmitter.

Myelin sheath — saltatory conduction

The myelin sheath is worth singling out because Edexcel will ask about it directly. Key facts:

• It is a fatty insulating layer wrapped around the axons of most sensory and motor neurones (but not relay neurones).
• It insulates the axon, stopping the electrical impulse leaking out sideways.
• It is broken up by small gaps called nodes of Ranvier, between which the impulse jumps.
• Jumping between gaps — saltatory conduction — means the impulse travels much faster than along an unmyelinated axon of the same diameter.
• Damage to the myelin sheath (e.g. in multiple sclerosis) slows nerve impulses and disrupts coordination.

In an exam, the two phrases to give are: "the myelin sheath insulates the axon" and "the myelin sheath speeds up nerve impulses".

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

The Edexcel marking-phrase chain for a reflex-arc question is:

> sensory carries impulse from receptor to CNS / spinal cord | motor carries impulse from CNS/spinal cord to effector/muscle | impulse does not travel to brain / bypasses brain | rapid response minimises damage.

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.

Motor neurone → effector (Edexcel detail)

When the motor neurone reaches its effector (e.g. a muscle), the impulse must still cross a small gap — the neuromuscular junction, which works like any other synapse. The Edexcel marking-phrase chain to describe what happens here is:

> impulse travels along the axon | myelin sheath insulates the axon | neurotransmitters released | neurotransmitters bind to receptors on the effector.

Neurotransmitter binding to receptors on a muscle fibre triggers the muscle to contract — i.e. the effector produces the response.

Exam tips

• On any reflex-arc question, use the Edexcel chain: sensory carries impulse from receptor to CNS/spinal cord → motor carries impulse from CNS to effector → impulse bypasses the brain → rapid response minimises damage.
• Always state the LOCATION of the relay neurone (in the spinal cord) — examiners reward this specifically.
• For a non-reflex (conscious) response, the Edexcel chain is: sensory carries impulse from receptor to CNS → CNS coordinates the response → motor carries impulse from CNS to effector → effector produces the response.
• 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).
• For SYNAPSE questions, give all four Edexcel marking steps in order: neurotransmitters RELEASED into synaptic cleft → DIFFUSE across the gap → ATTACH to RECEPTORS on the next neurone → NEW IMPULSE generated.
• For MOTOR NEURONE adaptations, hit all four phrases: long axon (long distance), myelin sheath insulates the axon, myelin sheath speeds up nerve impulses, dendrites provide large surface area to receive impulses.
• For the motor-neurone-to-effector question, use: impulse travels along the axon → myelin sheath insulates the axon → neurotransmitters released → neurotransmitters bind to receptors on the effector.
• If asked WHY myelin matters, the two-mark phrasing is 'insulates the axon' AND 'speeds up nerve impulses' — both are needed.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in Edexcel GCSE Biology (1BI0)

• 2.1 Mitosis and the cell cycle
• 2.2 Cell differentiation, growth and stem cells