Nervous System Revision Notes | A-Level Biology (All Boards)

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Chartered Biologist (CBiol MRSB)

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Overview Key Terms Neurones Resting Potential Action Potential Conduction Synapses Exam Boards Common Mistakes FAQ

Why the Nervous System Is a Mark-Scheme Minefield

Nervous coordination is one of the highest-yield topics in A-Level Biology – and one of the most unforgiving. The mechanisms are precise, the order of events matters, and the mark schemes are written around exact movements of specific ions. A student who writes “sodium goes in and the impulse travels” will lose most of the marks a student who explains depolarisation properly will gain. The biology is the same; the precision is not.

Having taught and examined this topic for many years, I can tell you exactly where students throw marks away. They confuse the resting potential with the action potential. They say potassium when they mean sodium. They describe the impulse as “electricity flowing down a wire” rather than a wave of depolarisation. And in synapse questions, they forget that transmission is one-way and cannot explain why. Every one of these is fixable once you see what the examiner is actually looking for.

On this page I will take you through the resting potential, the action potential, how impulses are conducted, and synaptic transmission – in the order examiners expect, using the precise language that earns full marks.

Board check: Nervous coordination appears in AQA 3.6.2.1–3.6.2.2, OCR A 5.1.3 (Neuronal Communication), OCR B (Module 5, Communication and Homeostasis), Edexcel A (Salters-Nuffield) Topic 8, Edexcel B Topic 8/9, and WJEC/Eduqas (A2, “The nervous system”). All boards require the resting potential, the action potential, conduction and the synapse. OCR A uniquely frames the action potential using positive feedback; Edexcel A names the Pacinian corpuscle and drug effects (L-Dopa, MDMA).

Key Terminology – The Words That Earn Marks

Get these definitions exactly right before attempting any question – examiners reject loose wording here more than almost anywhere else in the course.

Resting potential The potential difference across the axon membrane of a neurone that is not transmitting an impulse, typically about −70 mV (the inside is negative relative to the outside).

Action potential A brief reversal of the potential difference across the axon membrane (from about −70 mV to about +40 mV) caused by a rapid change in membrane permeability to sodium and potassium ions.

Depolarisation The change in potential difference that makes the inside of the axon less negative (and briefly positive), caused by sodium ions entering the axon.

Repolarisation The restoration of the negative resting potential, caused by potassium ions leaving the axon after the sodium channels close.

Threshold The potential difference (around −55 mV) that must be reached for voltage-gated sodium channels to open and an action potential to be triggered.

Refractory period The short period after an action potential during which the membrane cannot be depolarised again, because the sodium channels are recovering. It ensures impulses travel in one direction and are discrete.

Saltatory conduction The “jumping” of the action potential from one node of Ranvier to the next in a myelinated axon, which greatly increases the speed of conduction.

Examiner reject list: Do NOT say “the impulse is electricity” – it is a wave of depolarisation. Do NOT confuse sodium (depolarisation) with potassium (repolarisation). Do NOT write that channels “push” ions – ions move down their electrochemical gradient by facilitated diffusion. Do NOT confuse the sodium-potassium pump (active transport, restores the resting state) with voltage-gated channels (facilitated diffusion, generate the action potential).

Neurone Structure – Linking Structure to Function

All boards require you to know the structure of a motor neurone and to distinguish sensory, relay and motor neurones (AQA 3.6.2.1, OCR A 5.1.3b, Edexcel A 8.1, Eduqas/WJEC).

Cell body Contains the nucleus and most of the organelles. In a motor neurone it lies in the central nervous system; many short dendrites carry impulses towards the cell body.

Axon A long fibre carrying the impulse away from the cell body. Its length allows rapid transmission over a distance.

Myelin sheath (Schwann cells) An insulating layer of membrane wrapped around the axon by Schwann cells. It prevents ion movement except at gaps, speeding up conduction.

Nodes of Ranvier Gaps in the myelin sheath where the axon membrane is exposed. Action potentials are regenerated only at the nodes, allowing the impulse to “jump” (saltatory conduction).

Three types of neurone: Sensory neurones carry impulses from receptors to the CNS; relay (intermediate) neurones connect neurones within the CNS; motor neurones carry impulses from the CNS to effectors (muscles or glands). Examiners often ask you to identify the type from a diagram or describe its role in a reflex arc.

Edexcel A – the Pacinian corpuscle: Edexcel A names the Pacinian corpuscle as a pressure receptor that acts as a transducer, converting a mechanical stimulus into a generator potential. OCR A also expects sensory receptors as transducers. If you are not on these boards, this is extension content.

The Resting Potential – How −70 mV Is Established

Before a neurone can transmit an impulse, it maintains a resting potential of about −70 mV. Every board requires you to explain how this is set up in terms of ion movement and membrane permeability.

The sodium-potassium pump moves ions by active transport Using ATP, the pump moves 3 Na⁺ out of the axon for every 2 K⁺ in. This builds up more sodium outside and more potassium inside.

The membrane is more permeable to potassium than sodium Potassium ions diffuse back out through open potassium channels faster than sodium leaks back in. This loss of positive charge makes the inside more negative.

An electrochemical gradient is established The combined effect of the pump and the differential permeability leaves the inside of the axon at about −70 mV relative to the outside – the resting potential.

The mark-losing mistake: Students often say the membrane is “impermeable to sodium.” It is less permeable to sodium than to potassium – not completely impermeable. The exact comparison (more permeable to K⁺ than to Na⁺) is what earns the mark.

The Action Potential

When a neurone is stimulated above the threshold, an action potential is generated – a rapid reversal of the membrane potential. This is the heart of the topic and is worth learning as a precise sequence.

The Steps Examiners Want to See

Stimulus reaches threshold A stimulus causes some voltage-gated sodium channels to open. If the membrane depolarises to the threshold (about −55 mV), the action potential is triggered.

Depolarisation Voltage-gated sodium channels open fully and Na⁺ floods in down its electrochemical gradient. This makes the inside positive (about +40 mV). The entry of Na⁺ opens more channels – a positive feedback effect (OCR A names this explicitly).

Repolarisation At +40 mV the sodium channels close and voltage-gated potassium channels open. K⁺ diffuses out, restoring the negative potential inside.

Hyperpolarisation Potassium channels are slow to close, so slightly too much K⁺ leaves and the potential briefly overshoots below −70 mV.

Resting potential restored The sodium-potassium pump and channel closure restore the resting potential of −70 mV, ready for the next impulse.

The all-or-nothing principle: An action potential only occurs if the stimulus reaches the threshold. Below threshold, nothing happens; at or above threshold, a full action potential of the same size is produced – a bigger stimulus does not produce a bigger action potential. Instead, a stronger stimulus increases the frequency of action potentials. This is a frequently tested 2–3 mark point.

The refractory period – why it matters: During the refractory period the sodium channels cannot reopen, so no new action potential can form in that region. This (1) ensures impulses travel in one direction, (2) keeps action potentials discrete (separate), and (3) limits the maximum frequency of impulses. AQA may ask you to calculate the maximum frequency from the length of the refractory period.

Conduction Speed – What Makes Impulses Fast

All boards require the factors affecting the speed of conduction. Examiners like comparison and “explain why” questions here.

Myelination & saltatory conduction In a myelinated axon, the action potential cannot form under the myelin, so it “jumps” from node to node (saltatory conduction). This is much faster than the continuous conduction seen in non-myelinated axons.

Axon diameter A wider axon conducts faster because there is less resistance to the flow of ions along the axon (less leakage relative to volume).

Temperature Higher temperature increases the rate of diffusion of ions and the rate of respiration (more ATP for the sodium-potassium pump), so conduction is faster – up to the point where proteins denature. (Note: mammals are endothermic, so this is mainly relevant to investigations and to ectotherms.)

Common error: Students say myelin “makes the impulse stronger.” Myelin does not change the size of the action potential (all-or-nothing). It insulates the axon so the action potential only forms at the nodes, which makes conduction faster, not stronger.

Synaptic Transmission – Crossing the Gap

A synapse is the junction between two neurones (or between a neurone and an effector). Because the action potential cannot jump the synaptic cleft, the signal is carried across by a chemical neurotransmitter. All boards require the cholinergic synapse (using acetylcholine).

The Sequence Examiners Want to See

Action potential arrives at the presynaptic knob The depolarisation opens voltage-gated calcium channels, and Ca²⁺ diffuses into the presynaptic knob.

Vesicles release neurotransmitter Calcium ions cause synaptic vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis.

Neurotransmitter diffuses across the cleft ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane.

Postsynaptic membrane depolarises Binding opens sodium channels in the postsynaptic membrane; Na⁺ enters and, if the threshold is reached, a new action potential is generated in the postsynaptic neurone.

Neurotransmitter is broken down The enzyme acetylcholinesterase hydrolyses ACh into choline and ethanoic acid. These are reabsorbed into the presynaptic knob and recombined into ACh using ATP, so the receptor is freed and the synapse is reset.

Why transmission is unidirectional: Neurotransmitter is only released from the presynaptic knob, and receptors are only on the postsynaptic membrane. So the signal can only travel one way across the synapse – a guaranteed exam point.

Summation (AQA, OCR A): A single impulse may release too little neurotransmitter to reach threshold. Temporal summation – several impulses from one neurone in quick succession – or spatial summation – impulses from several neurones at once – can add together to reach threshold. Inhibitory synapses make the postsynaptic membrane more negative (e.g. by letting Cl⁻ in), making an action potential less likely.

Neuromuscular junction (AQA): AQA requires a comparison of a cholinergic synapse with a neuromuscular junction. Both use ACh, but the neuromuscular junction is between a motor neurone and a muscle, is always excitatory, and the postsynaptic membrane (the muscle’s sarcolemma) has many folds with many receptors.

Drug effects (Edexcel A & OCR A): You may be asked to predict how a drug affects a synapse – e.g. a drug that blocks acetylcholinesterase causes ACh to remain and the postsynaptic neurone to keep firing. AQA states you will not need to recall named drugs, but you must be able to apply the mechanism to unfamiliar examples. Edexcel A names L-Dopa (Parkinson’s) and MDMA (Ecstasy).

Exam Board Comparison – What Your Board Requires

This is the table no other revision site provides. Use it to check exactly what your board requires – do not waste time learning content your specification does not examine.

Subtopic	AQA	OCR A	OCR B	Edexcel A	Edexcel B	WJEC / Eduqas
Sensory / relay / motor neurones	✔	✔	✔	✔	✔	✔
Pacinian corpuscle / receptors as transducers	❌	✔	❌	✔	❌	❌
Resting potential	✔	✔	✔	✔	✔	✔
Action potential	✔	✔	✔	✔	✔	✔
Positive feedback in depolarisation	Implicit	✔	❌	❌	❌	❌
Refractory period & frequency	✔	✔	✔	✔	✔	✔
Saltatory conduction	✔	✔	✔	✔	✔	✔
Cholinergic synapse	✔	✔	✔	✔	✔	✔
Summation (temporal/spatial)	✔	✔	❌	❌	❌	❌
Neuromuscular junction comparison	✔	❌	❌	❌	❌	❌
Drug effects on synapses	✔*	✔	❌	✔	✔	❌

*AQA note: AQA expects you to apply drug effects to unfamiliar synapses but states that recall of named drugs and their modes of action is not required.

8 Common Mistakes from Examiner Reports

These are the errors I see again and again, both as an examiner and as a tutor. Every one of them costs marks.

#	The mistake	The correction
1	Confusing sodium and potassium	Na⁺ in = depolarisation; K⁺ out = repolarisation. Swapping them loses the mark.
2	“The impulse is electricity flowing along the axon”	It is a wave of depolarisation caused by ions moving across the membrane, not a current flowing along a wire.
3	Saying a bigger stimulus gives a bigger action potential	Action potentials are all-or-nothing. A bigger stimulus increases the frequency, not the size.
4	“Myelin makes the impulse stronger”	Myelin insulates the axon so the impulse jumps node to node, making it faster, not stronger.
5	Confusing the pump with the channels	The Na⁺/K⁺ pump uses ATP and sets up the resting state; voltage-gated channels let ions diffuse to make the action potential.
6	Forgetting why transmission is one-way	Neurotransmitter is released only from the presynaptic knob, and receptors are only on the postsynaptic membrane.
7	Forgetting acetylcholinesterase	ACh must be broken down by acetylcholinesterase, or the postsynaptic neurone would keep firing. This is a common missed mark.
8	Saying the membrane is “impermeable” to sodium at rest	It is less permeable to Na⁺ than to K⁺ – not completely impermeable.

Can’t Keep the Action Potential Steps Straight?

Nervous coordination is where precise sequencing earns the grade. If sodium and potassium keep swapping in your answers, or you can’t explain why a synapse only works one way, tutoring will fix it. I teach this topic in the exact order the mark scheme rewards, with the language examiners accept.

Tyrone John • CBiol MRSB • Former WJEC/Eduqas & Edexcel Examiner • 25+ Years Teaching A-Level Biology

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Frequently Asked Questions – The Nervous System

How is the resting potential of a neurone established?

The resting potential (about −70 mV) is established by the sodium-potassium pump, which actively transports 3 sodium ions out of the axon for every 2 potassium ions in, using ATP. The membrane is then more permeable to potassium than to sodium, so potassium diffuses back out faster than sodium leaks in. This loss of positive charge leaves the inside of the axon negative relative to the outside, creating the resting potential.

What happens during an action potential?

When a stimulus depolarises the membrane to threshold (about −55 mV), voltage-gated sodium channels open and sodium floods in, making the inside positive (about +40 mV) – this is depolarisation. The sodium channels then close and potassium channels open, so potassium leaves and the membrane repolarises. Potassium channels are slow to close, causing a brief hyperpolarisation, before the resting potential is restored. The whole event is all-or-nothing.

What is the all-or-nothing principle?

The all-or-nothing principle states that an action potential is only generated if the stimulus reaches the threshold. Below threshold, no action potential occurs; at or above threshold, a full action potential of fixed size is produced. A stronger stimulus does not produce a bigger action potential – instead it produces action potentials at a higher frequency. This is how the nervous system encodes the intensity of a stimulus.

Why does saltatory conduction make impulses faster?

In a myelinated axon, the myelin sheath insulates the membrane so that ions cannot cross except at the gaps called nodes of Ranvier. The action potential is therefore only regenerated at the nodes, and it effectively “jumps” from one node to the next – this is saltatory conduction. Because the impulse does not have to be regenerated along the whole length of the axon, it travels much faster than the continuous conduction in a non-myelinated axon.

Why is the refractory period important?

During the refractory period the sodium channels are recovering and cannot reopen, so no new action potential can form in that region of membrane. This has three important effects: it ensures the impulse travels in one direction only (the region behind cannot be re-excited), it keeps action potentials discrete and separate from one another, and it limits the maximum frequency at which a neurone can fire.

How does a nerve impulse cross a synapse?

When the action potential reaches the presynaptic knob, voltage-gated calcium channels open and calcium ions enter. This causes synaptic vesicles of acetylcholine to fuse with the presynaptic membrane and release the neurotransmitter by exocytosis. Acetylcholine diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane, opening sodium channels. If enough sodium enters to reach threshold, a new action potential is generated. Acetylcholinesterase then breaks down the acetylcholine to reset the synapse.

Why is synaptic transmission only one-way?

Synaptic transmission is unidirectional because neurotransmitter is only stored in and released from the presynaptic knob, and the receptors that respond to it are only present on the postsynaptic membrane. There are no neurotransmitter vesicles on the postsynaptic side and no receptors on the presynaptic side, so the signal can only travel from the presynaptic neurone to the postsynaptic neurone.

What is the difference between temporal and spatial summation?

Both are ways of reaching the threshold when a single impulse releases too little neurotransmitter. In temporal summation, several impulses arrive from the same presynaptic neurone in quick succession, so the neurotransmitter builds up over time until threshold is reached. In spatial summation, impulses arrive at the same time from several different presynaptic neurones, and their combined neurotransmitter release reaches threshold. Summation is required by AQA and OCR A.

Written by Tyrone John

CBiol MRSB • Former WJEC/Eduqas & Edexcel Examiner • PGCE • 25+ Years Teaching A-Level Biology • Published Scientific Research

Tyrone has over 25 years of experience teaching A-Level Biology and is a Chartered Biologist and member of the Royal Society of Biology. As a former examiner for WJEC/Eduqas and Edexcel, he has first-hand knowledge of how mark schemes are applied and what examiners look for in student answers. Learn more →