What are the main components of the nervous system covered in Edexcel International A Levels Biology?

The nervous system, as outlined in the Edexcel International A Levels Biology curriculum, consists of two main components: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord, which are responsible for processing information and coordinating responses. The PNS comprises all the nerves that branch out from the CNS to the rest of the body, facilitating communication between the CNS and limbs and organs.

How does the nervous system maintain the internal environment of the body?

The nervous system maintains the body's internal environment through a process called homeostasis. It does this by detecting changes in the internal and external environments via sensory receptors and sending signals to the CNS. The CNS processes this information and sends appropriate responses to effectors, such as muscles or glands, to adjust bodily functions and maintain balance. This coordination is crucial for regulating processes like temperature, pH, and glucose levels.

What role does the nervous system play in coordination and response?

In the context of coordination and response, the nervous system plays a pivotal role by rapidly transmitting signals between different parts of the body. It coordinates voluntary and involuntary actions by processing sensory information and generating appropriate motor responses. For example, when you touch a hot surface, sensory neurons transmit the signal to the CNS, which quickly processes the information and sends a response via motor neurons to withdraw your hand, demonstrating a reflex action.

How do neurons transmit signals within the nervous system?

Neurons transmit signals through a combination of electrical and chemical processes. An electrical impulse, or action potential, travels along the axon of a neuron. When it reaches the synapse, the impulse triggers the release of neurotransmitters, which are chemical messengers. These neurotransmitters cross the synaptic gap and bind to receptors on the next neuron, continuing the transmission of the signal. This process is essential for communication within the nervous system.

What is the significance of the nervous system in gene technology studies?

In gene technology studies, the nervous system is significant because it provides insights into how genetic modifications can affect neural functions and behavior. Understanding the genetic basis of neurological disorders can lead to the development of targeted therapies and treatments. Additionally, gene technology can be used to study the expression of genes in the nervous system, helping scientists understand complex processes like learning, memory, and neural development.

Why is "Matching the wrong ion to the wrong phase of the action potential" a common mistake in The Nervous System?

Why it happens: Students memorise the ions but not which moves during depolarisation versus repolarisation. How to avoid it: Lock it in: Na⁺ IN = depolarisation (up to +40 mV); K⁺ OUT = repolarisation (back down). During repolarisation sodium does NOT leave — its channels are closed; it is potassium that leaves. Source: Edexcel IAL Biology Unit 5 examiner reports

Why is "Saying a stronger stimulus produces a bigger action potential" a common mistake in The Nervous System?

Why it happens: It feels intuitive that a bigger stimulus should give a bigger response. How to avoid it: Action potentials are all-or-nothing and always the same size. A stronger stimulus is coded by a HIGHER FREQUENCY of impulses (and more neurones recruited), not by a larger impulse. Source: Edexcel IAL Biology Unit 5 examiner reports

Why is "Getting the sodium–potassium pump ratio or directions wrong" a common mistake in The Nervous System?

Why it happens: The numbers (3 and 2) and the directions are easily reversed under pressure. How to avoid it: Memorise exactly: 3 Na⁺ OUT for every 2 K⁺ IN, using ATP. More positive ions leave than enter, helping make the inside negative. Source: Edexcel IAL Biology Unit 5 examiner reports

Why is "Confusing calcium ions and sodium ions at the synapse" a common mistake in The Nervous System?

Why it happens: Both are positive ions that flood in, so they are easily muddled. How to avoid it: Ca²⁺ enters the PREsynaptic knob to trigger vesicle fusion; Na⁺ enters the POSTsynaptic membrane (after ACh binds its receptor) to depolarise it. Keep the two sides of the synapse separate. Source: Edexcel IAL Biology Unit 5 examiner reports

Why is "Describing myelin as something that 'carries' or 'speeds up the electricity'" a common mistake in The Nervous System?

Why it happens: Students associate 'faster' with better conduction rather than insulation. How to avoid it: Myelin is an INSULATOR: it stops ion movement except at the nodes of Ranvier, so the impulse jumps node-to-node (saltatory conduction). Speed comes from not regenerating the action potential along the whole membrane. Source: Edexcel IAL Biology Unit 5 examiner reports

Why is "Saying adrenaline enters the target cell to have its effect" a common mistake in The Nervous System?

Why it happens: Students assume all signalling molecules act inside the cell. How to avoid it: Adrenaline is water-soluble and cannot cross the membrane. It binds a receptor on the OUTSIDE, activating adenylyl cyclase to make cAMP (the second messenger) inside the cell. The hormone stays outside; cAMP carries the message in. Source: Edexcel IAL Biology Unit 5 examiner reports

Edexcel International A Levels Biology (XBI11-YBI11)

The Nervous System

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Short Notes - The Nervous System

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Detailed Study Notes

Detailed notes on Respiration, Internal Environment, Coordination and Gene Technology for Edexcel International A Levels Biology, covering key concepts, explanations, examples, and exam-focused revision points.

The Nervous System — Pearson Edexcel International A Level Biology (XBI11/YBI11) Study Notes

How neurones transmit information: the structure of sensory, relay and motor neurones and the role of myelination; the resting potential maintained by the Na⁺/K⁺ pump; the action potential (depolarisation, repolarisation, hyperpolarisation); the all-or-nothing principle and the refractory period; how impulses are propagated and what affects conduction speed; synaptic transmission with acetylcholine; and the comparison of nervous with hormonal coordination, including the second-messenger action of adrenaline. Core Unit 5 (Topic 8) coordination material.

At a glance

Three neurone types: sensory (receptor → CNS), relay (within CNS), motor (CNS → effector). A myelin sheath (Schwann cells) speeds transmission by saltatory conduction — the impulse jumps between nodes of Ranvier.
Resting potential (−70 mV): the Na⁺/K⁺ pump moves 3 Na⁺ out for 2 K⁺ in (active transport, uses ATP) and the membrane is more permeable to K⁺ than Na⁺ → inside negative.
Action potential: a stimulus depolarises the membrane to threshold (≈ −55 mV); voltage-gated Na⁺ channels open, Na⁺ floods in → depolarisation to ≈ +40 mV.
Repolarisation: Na⁺ channels close and voltage-gated K⁺ channels open, K⁺ moves out → inside becomes negative again; a brief hyperpolarisation follows before the Na⁺/K⁺ pump restores −70 mV.
All-or-nothing: an impulse fires only if threshold is reached, always to the same size; a stronger stimulus is coded by a higher frequency of impulses, not bigger ones.
Refractory period: Na⁺ channels are inactivated, so no new action potential can fire — this makes impulses unidirectional and discrete and limits their frequency.
Conduction speed increases with myelination, larger axon diameter and higher temperature.
Synapse: impulse → Ca²⁺ influx → vesicles of acetylcholine fuse and release transmitter → diffuses across the cleft → binds receptors → Na⁺ channels open → EPSP/new action potential. Acetylcholinesterase breaks down the transmitter so it stops acting.
Nervous vs hormonal: nervous = fast, electrical + chemical, short-lived, localised; hormonal = slower, chemical (blood), longer-lasting, widespread. Adrenaline acts via the second-messenger model (receptor → adenylyl cyclase → cAMP → enzyme cascade).

What you’ll learn

Mapped to the Pearson Edexcel International A Levels XBI11-YBI11 syllabus (2018-onwards).

8.1 — Describe the structure of sensory, relay and motor neurones and explain the role of myelination in increasing the speed of conduction (saltatory conduction).
8.2 — Explain how a resting potential is established and maintained by the sodium–potassium pump and the differential permeability of the membrane to ions.
8.3 — Describe how an action potential is generated and transmitted along a neurone in terms of depolarisation, repolarisation and hyperpolarisation, and the movement of Na⁺ and K⁺ through voltage-gated channels.
8.4 — Explain the all-or-nothing principle and the importance of the refractory period in ensuring discrete, unidirectional impulses and limiting their frequency.
8.5 — Explain the effect of myelination, axon diameter and temperature on the speed of conduction of an impulse.
8.6 — Describe the structure of a cholinergic synapse and explain how an impulse is transmitted across it, including the roles of calcium ions, acetylcholine, receptors and acetylcholinesterase.
8.7 — Explain how synapses ensure one-way transmission and allow summation.
8.8 — Compare nervous and hormonal (endocrine) coordination and describe the action of adrenaline using the second-messenger model (cell signalling — synoptic with Units 1 and 2).

Neurones and myelination

Sensory, relay and motor neurones; how the myelin sheath speeds transmission.

A neurone is a specialised nerve cell that carries information as electrical impulses. Three types work together in a reflex arc:

Sensory neurone — carries impulses from a receptor to the central nervous system (CNS). It has a cell body on a side branch part-way along the axon (in a dorsal root ganglion), a long dendron leading to the cell body and a long axon leading away.
Relay (intermediate) neurone — lies entirely within the CNS and connects sensory to motor neurones. It is short with many connections.
Motor neurone — carries impulses from the CNS to an effector (a muscle or gland). Its cell body is in the CNS, with many dendrites and one long axon ending in axon terminals.

Key features of a motor neurone:

Cell body — contains the nucleus and many ribosomes/rough ER (to make neurotransmitter).
Dendrites — receive impulses from other neurones and conduct them towards the cell body.
Axon — a long fibre that carries the impulse away from the cell body to the axon terminals.
Myelin sheath — an insulating layer of many membranes wrapped around the axon by Schwann cells, broken at intervals by gaps called nodes of Ranvier.
Axon terminals — the ends that form synapses with the next cell.

Myelination and saltatory conduction. The myelin sheath is rich in the lipid myelin, which acts as an electrical insulator: ions cannot cross the membrane where it is myelinated. Depolarisation (the action potential) can therefore only occur at the nodes of Ranvier. The local currents flow from one node to the next, so the impulse "jumps" from node to node — this is saltatory conduction (Latin saltare, to jump). Because the action potential is not regenerated along every part of the membrane (only at the nodes), the impulse travels much faster along a myelinated axon than along an unmyelinated one of the same diameter.

A myelinated motor neurone. The myelin sheath (oval Schwann cells) insulates the axon, so depolarisation occurs only at the nodes of Ranvier and the impulse jumps node-to-node by saltatory conduction — making transmission faster.

Sensory = receptor → CNS; relay = within CNS; motor = CNS → effector.
Myelin (Schwann cells) insulates the axon; gaps = nodes of Ranvier.
Saltatory conduction: impulse jumps node-to-node → much faster than unmyelinated.

See the full worked example for the nervous system →

The resting potential

How the Na⁺/K⁺ pump and selective permeability give −70 mV.

When a neurone is not conducting an impulse, the inside of the axon is negative relative to the outside — about −70 mV. This potential difference across the membrane is the resting potential, and the membrane is said to be polarised.

It is established and maintained in two linked ways:

The sodium–potassium pump (Na⁺/K⁺ pump). This is a carrier protein that uses ATP (active transport — synoptic link to Unit 2 membrane transport) to pump 3 Na⁺ ions OUT of the axon for every 2 K⁺ ions pumped IN. This moves three positive charges out for every two brought in, so it makes the inside more negative and sets up concentration gradients: a high Na⁺ concentration outside and a high K⁺ concentration inside.
Differential (selective) membrane permeability. At rest the membrane is much more permeable to K⁺ than to Na⁺ because more potassium channels are open. K⁺ therefore diffuses back out down its concentration gradient faster than Na⁺ can leak back in. The loss of positive K⁺ ions leaves the inside negative. An equilibrium is reached at about −70 mV, where the electrical attraction pulling K⁺ back in balances the concentration gradient pushing it out.

Large negatively charged proteins and other anions trapped inside the axon also contribute to the negative interior.

So the resting potential is not a "do-nothing" state — the Na⁺/K⁺ pump is constantly working (using ATP) to keep the gradients ready, so that the cell can fire an action potential instantly when stimulated.

Resting potential ≈ −70 mV; membrane polarised, inside negative.
Na⁺/K⁺ pump: 3 Na⁺ out / 2 K⁺ in, using ATP (active transport).
Membrane more permeable to K⁺ → K⁺ diffuses out → inside left negative.
Sets up gradients: high Na⁺ outside, high K⁺ inside.

See the full worked example for the nervous system →

The action potential

Depolarisation, repolarisation and hyperpolarisation in terms of ion channels.

An action potential is a rapid, temporary reversal of the membrane potential. It is triggered when a stimulus depolarises the membrane to a threshold value (about −55 mV). The stages are:

Resting state (−70 mV). Voltage-gated Na⁺ and K⁺ channels are closed; the membrane is polarised.
Depolarisation (rising to ≈ +40 mV). A stimulus opens a few voltage-gated sodium ion channels. Na⁺ diffuses in, making the inside less negative. If the depolarisation reaches the threshold (−55 mV), more voltage-gated Na⁺ channels open in a positive-feedback rush, so Na⁺ floods in down its electrochemical gradient. The inside becomes positive, peaking at about +40 mV — the membrane is now depolarised.
Repolarisation. At about +40 mV the voltage-gated Na⁺ channels close (inactivate) and the voltage-gated potassium ion channels open. K⁺ diffuses out down its concentration gradient, removing positive charge from inside, so the potential difference falls back towards negative — the membrane repolarises.
Hyperpolarisation (overshoot). The K⁺ channels are slow to close, so slightly too much K⁺ leaves, taking the inside below −70 mV for a moment. The Na⁺/K⁺ pump and closing of the K⁺ channels then restore the resting potential of −70 mV.
Recovery. The Na⁺/K⁺ pump re-establishes the ion gradients (3 Na⁺ out / 2 K⁺ in) so the membrane is ready to fire again.

The action potential. Threshold at −55 mV triggers a flood of Na⁺ (depolarisation to +40 mV); Na⁺ channels then close and K⁺ channels open (repolarisation); excess K⁺ loss causes a brief hyperpolarisation before the Na⁺/K⁺ pump restores −70 mV. The refractory period spans the recovery.

Threshold ≈ −55 mV must be reached to trigger an action potential.
Depolarisation: voltage-gated Na⁺ channels open, Na⁺ in → +40 mV.
Repolarisation: Na⁺ channels close, voltage-gated K⁺ channels open, K⁺ out.
Hyperpolarisation: too much K⁺ leaves; Na⁺/K⁺ pump restores −70 mV.

See the full worked example for the nervous system →

All-or-nothing, the refractory period and conduction speed

Why impulses are discrete and one-way, and what makes them faster.

The all-or-nothing principle. A stimulus must reach the threshold to trigger an action potential. If it does, an action potential of the same size is always produced; if it does not reach threshold, no action potential fires. A bigger action potential cannot be produced — so a stronger stimulus is not coded by a larger impulse but by a higher frequency of impulses (and by stimulating more neurones).

The refractory period. Immediately after an action potential, the voltage-gated Na⁺ channels are inactivated and cannot reopen for a short time, so no new action potential can be generated in that region. This is the refractory period. It has three important consequences:

It ensures the impulse is propagated in one direction only — the region just behind the action potential is refractory, so the impulse cannot travel backwards.
It ensures action potentials are discrete (separate) events rather than merging into one continuous depolarisation.
It limits the frequency of impulses, setting a maximum firing rate.

How the impulse is propagated. When one part of the membrane depolarises, the influx of Na⁺ raises the charge inside; this sets up local currents that depolarise the adjacent region to threshold, which then fires its own action potential. In this way the action potential is regenerated along the axon (in a myelinated axon, only at the nodes of Ranvier). Because the region behind is refractory, the wave only moves forwards.

Factors affecting conduction speed:

Myelination — a myelinated axon conducts much faster because of saltatory conduction (the impulse jumps between nodes of Ranvier rather than being regenerated continuously).
Axon diameter — a wider axon conducts faster, because there is less resistance (less "leakage") to the flow of local currents along the inside of the axon.
Temperature — a higher temperature speeds conduction (up to a point) because ions diffuse faster and the Na⁺/K⁺ pump and respiration (ATP supply) work faster. Above about 40 °C the channel proteins denature and conduction fails.

All-or-nothing: threshold reached → full-size impulse; below threshold → nothing.
Stimulus strength is coded by frequency of impulses, not their size.
Refractory period: Na⁺ channels inactivated → one-way, discrete impulses; limits frequency.
Faster conduction with myelination, larger axon diameter and higher temperature.

See the full worked example for the nervous system →

The cholinergic synapse

How the impulse crosses a gap using acetylcholine.

A synapse is the junction between two neurones (or between a neurone and an effector). The neurones do not touch — there is a tiny gap, the synaptic cleft (~20 nm), so the electrical impulse cannot jump it directly. Instead it is carried across chemically by a neurotransmitter. A synapse that uses acetylcholine is called a cholinergic synapse.

Sequence of events at a cholinergic synapse:

The action potential arrives at the end of the presynaptic neurone (the synaptic knob), depolarising the presynaptic membrane.
Voltage-gated calcium ion channels open, so Ca²⁺ diffuses into the synaptic knob.
The influx of Ca²⁺ causes synaptic vesicles containing acetylcholine (ACh) to move to and fuse with the presynaptic membrane, releasing ACh into the synaptic cleft by exocytosis.
ACh diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane.
This opens sodium ion channels in the postsynaptic membrane; Na⁺ enters, depolarising it. If enough ACh binds, the depolarisation reaches threshold (an excitatory postsynaptic potential, EPSP) and a new action potential is generated in the postsynaptic neurone.
Acetylcholinesterase (an enzyme in the cleft) hydrolyses the ACh into choline and ethanoic (acetic) acid. This stops the stimulation, so the receptors are freed and the postsynaptic membrane can repolarise. The products are reabsorbed into the presynaptic knob and recombined (using ATP) to re-form ACh — synaptic vesicles are recycled.

Why synapses ensure one-way transmission: the neurotransmitter is only made and released by the presynaptic neurone, and the receptors are only on the postsynaptic membrane. So an impulse can only cross from pre- to postsynaptic — never backwards.

Summation. A single impulse may release too little ACh to reach threshold. Summation is the addition of several small EPSPs to reach threshold:

Spatial summation — several different presynaptic neurones release transmitter at the same time onto one postsynaptic neurone.
Temporal summation — one presynaptic neurone fires rapidly in quick succession, so its EPSPs add up. Synapses therefore act as decision points that filter weak signals and integrate information.

A cholinergic synapse. Impulse arrival → Ca²⁺ influx → vesicles fuse and release acetylcholine → ACh diffuses across the cleft and binds receptors → Na⁺ enters the postsynaptic membrane → depolarisation/new action potential. Acetylcholinesterase then breaks down ACh so stimulation stops.

Impulse → Ca²⁺ influx → vesicles fuse → ACh released by exocytosis.
ACh diffuses across cleft, binds receptors → Na⁺ channels open → postsynaptic depolarisation.
Acetylcholinesterase hydrolyses ACh so stimulation stops and receptors are freed.
One-way: transmitter only released presynaptically, receptors only postsynaptic. Summation (spatial + temporal) reaches threshold.

See the full worked example for the nervous system →

Nervous vs hormonal coordination and adrenaline

Comparing the two systems; the second-messenger model.

Animals coordinate their responses using two communication systems that complement each other.

Feature	Nervous coordination	Hormonal (endocrine) coordination
Transmission	Electrical impulses along neurones, then chemical at synapses	Chemical (hormones) carried in the blood
Speed	Very fast (milliseconds)	Slower (seconds to minutes)
Target	Localised — a specific effector	Widespread — any cell with the right receptor
Duration of effect	Short-lived	Longer-lasting
Pathway	Neurone → effector	Gland → blood → target cell

Nervous control suits rapid, precise, short-term responses (e.g. a reflex withdrawing the hand); hormonal control suits slower, longer-term, widespread responses (e.g. growth, metabolism, the response to long-term stress).

The action of adrenaline — the second-messenger model. Adrenaline is a hormone released from the adrenal glands in the "fight-or-flight" response. It is water-soluble, so it cannot cross the cell-surface membrane. Instead it acts as a first messenger that binds to a specific receptor on the outside of the target-cell membrane (synoptic link — cell signalling, protein receptors). This triggers a cascade:

Adrenaline (first messenger) binds to its receptor on the cell-surface membrane.
This activates the membrane-bound enzyme adenylyl cyclase (adenyl cyclase).
Adenylyl cyclase converts ATP into cyclic AMP (cAMP) — the second messenger — inside the cell.
cAMP activates enzymes (e.g. a kinase), which activate further enzymes in a cascade, amplifying the signal.
In a liver cell this cascade ultimately activates the enzymes that break down glycogen to glucose (glycogenolysis), raising blood glucose ready for activity.

The great advantage of the second-messenger / cascade model is amplification: one adrenaline molecule can lead to the production of many cAMP molecules, each activating many enzyme molecules, so a tiny hormone concentration produces a large, rapid response.

Nervous = fast, electrical+chemical, localised, short-lived; hormonal = slower, chemical via blood, widespread, longer-lasting.
Adrenaline is water-soluble → cannot enter cell → acts as a first messenger at a membrane receptor.
Receptor → adenylyl cyclase → ATP converted to cyclic AMP (second messenger).
cAMP activates an enzyme cascade → amplifies the signal (e.g. glycogen → glucose in liver).

See the full worked example for the nervous system →

Quick recap

Sensory (receptor→CNS), relay (in CNS) and motor (CNS→effector) neurones; myelin sheath (Schwann cells) gives saltatory conduction between nodes of Ranvier → faster.
Resting potential −70 mV: Na⁺/K⁺ pump moves 3 Na⁺ out / 2 K⁺ in (ATP); membrane more permeable to K⁺, which diffuses out leaving the inside negative.
Action potential: threshold −55 mV → Na⁺ channels open, Na⁺ in (depolarisation +40 mV) → Na⁺ channels close, K⁺ channels open, K⁺ out (repolarisation) → hyperpolarisation → pump restores −70 mV.
All-or-nothing: full-size impulse only above threshold; stimulus strength coded by frequency. Refractory period (Na⁺ channels inactivated) → one-way, discrete impulses, limits frequency.
Conduction speed rises with myelination, larger axon diameter and higher temperature.
Synapse: impulse → Ca²⁺ in → vesicles release ACh → diffuses across cleft → binds receptors → Na⁺ in → postsynaptic depolarisation/EPSP; acetylcholinesterase breaks ACh down. One-way; summation reaches threshold.
Nervous (fast, localised, short) vs hormonal (slow, widespread, long). Adrenaline: receptor → adenylyl cyclase → cAMP (second messenger) → enzyme cascade.

Memorise this

Verbatim phrases and definitions Edexcel mark schemes credit.

Resting potential ≈ −70 mV; Na⁺/K⁺ pump moves 3 Na⁺ OUT and 2 K⁺ IN using ATP; membrane more permeable to K⁺.
Depolarisation: voltage-gated Na⁺ channels open, Na⁺ in → +40 mV. Repolarisation: Na⁺ channels close, K⁺ channels open, K⁺ out.
All-or-nothing: same-size impulse above threshold; stimulus strength coded by FREQUENCY of impulses.
Refractory period (Na⁺ channels inactivated) ensures unidirectional, discrete impulses and limits frequency.
Saltatory conduction: impulse jumps node-to-node along a myelinated axon → faster.
Synapse: Ca²⁺ influx → vesicles release acetylcholine → binds postsynaptic receptors → Na⁺ in → depolarisation; acetylcholinesterase hydrolyses ACh.
Adrenaline (first messenger) → receptor → adenylyl cyclase → cyclic AMP (second messenger) → enzyme cascade.

How it’s examined

The nervous system is examined in Unit 5 (paper code WBI15), often synoptically with membrane transport and ATP from Units 1–2. Expect: short structure questions on neurones and myelination (2–3 marks); a frequently set 'describe the changes during an action potential' question that requires the ion (Na⁺/K⁺) and channel detail tied to each phase, very often as the interpretation of an action-potential graph (state values such as −70, −55, +40 mV and link them to channel events); a 6-mark levels-based question on synaptic transmission (sequence with Ca²⁺, acetylcholine, receptors and acetylcholinesterase) or on the refractory period; comparison-table questions on nervous vs hormonal coordination; and a second-messenger/adrenaline question linking to cell signalling. The most-penalised errors are matching the wrong ion to a phase, saying a stronger stimulus gives a bigger impulse, and saying adrenaline enters the cell.

Sources: Pearson Edexcel International Advanced Level Biology (YBI11/XBI11) specification (Issue 2, 2018 onwards); Edexcel IAL Biology Unit 5 examiner reports; Edexcel IAL Biology Unit 5 sample assessment materials and mark schemes. Last reviewed 2026-06-25.

Take this whole topic with you

Step-by-step worked examples — The Nervous System

Step-by-step solutions to past-paper-style questions on the nervous system, written exactly the way a tutor would explain them at the board.

Question type:

1Classifying neurones and the role of myelin (2 marks)
Getting startedIdentify & classify• neurones, myelination, Unit 5
Question
A neurone carries impulses from the central nervous system to a muscle. Name this type of neurone and state one way in which its myelin sheath affects impulse transmission. (2 marks)
Step-by-step solution
1. Step 1
  A neurone carrying impulses from the CNS to an effector (such as a muscle) is a motor neurone.
2. Step 2
  Its myelin sheath insulates the axon so depolarisation only occurs at the nodes of Ranvier; the impulse jumps from node to node (saltatory conduction), which speeds up transmission.
Answer
Motor neurone; the myelin sheath insulates the axon so the impulse jumps node-to-node (saltatory conduction), increasing the speed of transmission.
Examiner tip
1 mark for 'motor neurone'; 1 mark for any correct effect of myelin (insulation / saltatory conduction / faster transmission). 'Carries the electricity better' scores 0 — myelin is an insulator.
2The sodium–potassium pump and the resting potential (2 marks)
Getting startedWord problem• resting potential, Na/K pump, Unit 5
Question
Describe the role of the sodium–potassium pump in maintaining the resting potential of a neurone. (2 marks)
Step-by-step solution
1. Step 1
  The pump uses ATP (active transport) to move 3 Na⁺ ions out of the axon for every 2 K⁺ ions moved in.
2. Step 2
  This sets up a concentration gradient (high Na⁺ outside, high K⁺ inside) and, by moving more positive ions out than in, helps make the inside negative (about −70 mV).
Answer
The Na⁺/K⁺ pump uses ATP to pump 3 Na⁺ out and 2 K⁺ in, establishing the ion gradients and contributing to the negative interior (−70 mV).
Examiner tip
1 mark for '3 Na⁺ out / 2 K⁺ in' (correct numbers and directions); 1 mark for using ATP/active transport OR for establishing the gradient/negative inside. Reversing the ratio (3 K⁺ out, 2 Na⁺ in) is a common error that loses the mark.
3Interpreting an action-potential graph (4 marks)
Building confidenceGraph or diagram• Adapted from Edexcel IAL Biology Unit 5 SAM• action potential, graph, ion channels, Unit 5
Question
A graph shows membrane potential rising from −70 mV to +40 mV (region X) and then falling back below −70 mV (region Y). Identify the ion movements responsible for region X and region Y, and explain what causes the brief dip below −70 mV. (4 marks)
Step-by-step solution
1. Step 1
  Region X (rise to +40 mV) = depolarisation. Voltage-gated sodium ion channels open and Na⁺ diffuses in down its electrochemical gradient, making the inside positive.
2. Step 2
  Region Y (fall from +40 mV) = repolarisation. The Na⁺ channels close and voltage-gated potassium ion channels open, so K⁺ diffuses out, removing positive charge from inside.
3. Step 3
  The K⁺ channels are slow to close, so slightly too much K⁺ leaves, taking the membrane potential below −70 mV — this is hyperpolarisation.
4. Step 4
  The Na⁺/K⁺ pump (and the closing of K⁺ channels) then restores the resting potential of −70 mV.
Answer
X = depolarisation (Na⁺ channels open, Na⁺ in); Y = repolarisation (K⁺ channels open, K⁺ out); the dip is hyperpolarisation caused by too much K⁺ leaving before the pump restores −70 mV.
Examiner tip
Each correct ion/channel link scores: Na⁺ in for the rise, K⁺ out for the fall. The dip mark needs 'too much K⁺ leaves / K⁺ channels slow to close'. Saying Na⁺ leaves during the fall is the classic error.
4Sequence of events at a cholinergic synapse (4 marks)
Building confidenceMulti-step problem• synapse, acetylcholine, Unit 5
Question
Describe how an impulse arriving at a presynaptic neurone leads to a new action potential in the postsynaptic neurone at a cholinergic synapse. (4 marks)
Step-by-step solution
1. Step 1
  The arriving action potential depolarises the presynaptic membrane, opening voltage-gated calcium ion channels so Ca²⁺ diffuses into the synaptic knob.
2. Step 2
  Ca²⁺ causes synaptic vesicles of acetylcholine to fuse with the presynaptic membrane and release acetylcholine into the cleft by exocytosis.
3. Step 3
  Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening sodium ion channels so Na⁺ enters and depolarises it.
4. Step 4
  If enough ACh binds, the depolarisation reaches threshold and a new action potential is generated in the postsynaptic neurone.
Answer
Ca²⁺ in → vesicles release ACh → ACh diffuses across cleft, binds receptors → Na⁺ in → depolarisation to threshold → new action potential.
Examiner tip
Marks for: Ca²⁺ influx; ACh release by vesicle fusion/exocytosis; ACh binds receptors → Na⁺ in; threshold → new action potential. Confusing Ca²⁺ (presynaptic) with Na⁺ (postsynaptic) is the commonest error.
5Coding stimulus intensity (6 marks)
StretchMulti-step problem• Adapted from Edexcel IAL Biology Unit 5 examiner reports• all-or-nothing, refractory period, extended, Unit 5
Question
Action potentials are described as 'all-or-nothing' and are always the same size. Explain how the nervous system can nevertheless distinguish a weak stimulus from a strong one, and explain how the refractory period influences this. (6 marks)
Step-by-step solution
1. Step 1
  By the all-or-nothing principle, an action potential is only produced if the stimulus reaches threshold, and it is always the same size — its size does not depend on stimulus strength.
2. Step 2
  So stimulus intensity cannot be coded by the size of the impulse.
3. Step 3
  A stronger stimulus produces action potentials at a higher frequency (more impulses per second) along a given neurone.
4. Step 4
  A stronger stimulus may also stimulate a greater number of neurones, and may activate neurones with higher thresholds that a weak stimulus could not reach.
5. Step 5
  The brain interprets this higher frequency / greater number of active neurones as a stronger stimulus.
6. Step 6
  The refractory period (Na⁺ channels inactivated) sets a maximum frequency of impulses, so a neurone cannot fire faster than this — placing an upper limit on the intensity it can signal.
Answer
Impulse size is fixed (all-or-nothing); intensity is coded by frequency of impulses and number of neurones recruited; the refractory period caps the maximum frequency.
Examiner tip
A 6-mark levels-based question. Top band: all-or-nothing → size fixed → frequency codes intensity → more neurones recruited → refractory period limits maximum frequency. Many candidates only say 'more impulses' without explaining why size cannot vary.
6The second-messenger action of adrenaline (6 marks)
StretchMulti-step problem• adrenaline, second messenger, cell signalling, Unit 5
Question
Adrenaline is a water-soluble hormone. Explain how it brings about a response inside a liver cell using the second-messenger model. (6 marks)
Step-by-step solution
1. Step 1
  Because adrenaline is water-soluble, it cannot cross the phospholipid cell-surface membrane, so it acts on the cell from outside.
2. Step 2
  Adrenaline (the first messenger) binds to a specific receptor protein on the cell-surface membrane (complementary shape).
3. Step 3
  This activates the membrane enzyme adenylyl cyclase on the inside of the membrane.
4. Step 4
  Adenylyl cyclase converts ATP into cyclic AMP (cAMP) — the second messenger — inside the cell.
5. Step 5
  cAMP activates enzymes (a kinase cascade); each activated enzyme activates many more, amplifying the signal.
6. Step 6
  In the liver cell this cascade activates the enzymes that break down glycogen to glucose (glycogenolysis), raising the blood glucose concentration ready for activity.
Answer
Water-soluble → binds membrane receptor (first messenger) → activates adenylyl cyclase → ATP to cAMP (second messenger) → enzyme cascade (amplification) → glycogen broken down to glucose.
Examiner tip
Top band requires the full named chain: receptor → adenylyl cyclase → cAMP → enzyme cascade, plus the amplification point and the glycogen→glucose outcome. Saying adrenaline 'enters the cell' caps the answer at Level 1.

Model Answers — The Nervous System

High-scoring sample answers for the nervous system on the Cambridge IGCSE paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
3 marks
Q (3 marks). State three features of a motor neurone and give the function of each.
Model answer
A motor neurone has a cell body containing the nucleus and many ribosomes/rough endoplasmic reticulum; this synthesises the proteins and the neurotransmitter needed by the cell.

It has a long axon that carries the impulse away from the cell body towards the effector (muscle or gland) over a long distance.

It has a myelin sheath made of Schwann cells wrapped around the axon, which insulates it so the impulse jumps from node to node (saltatory conduction), speeding up transmission.
Why this scores
Why this scores 3/3. One mark per feature correctly linked to a function: (1) cell body → makes proteins/neurotransmitter; (2) axon → carries impulse to effector; (3) myelin sheath → insulation/saltatory conduction/faster. Naming a feature without its function does not score.
Question 2
3 marks
Q (3 marks). Explain how the resting potential of a neurone is established.
Model answer
The sodium–potassium pump uses ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions moved in. This sets up a concentration gradient with a high sodium concentration outside and a high potassium concentration inside the axon.

The cell-surface membrane is more permeable to potassium ions than to sodium ions at rest, because more potassium channels are open.

Potassium ions therefore diffuse out of the axon down their concentration gradient faster than sodium ions can move back in. The loss of these positive ions leaves the inside of the axon negative relative to the outside (about −70 mV) — the resting potential.
Why this scores
Why this scores 3/3. Marks: (1) Na⁺/K⁺ pump moves 3 Na⁺ out / 2 K⁺ in using ATP; (2) membrane more permeable to K⁺; (3) K⁺ diffuses out → inside negative ≈ −70 mV. Candidates who omit 'more permeable to K⁺' rarely reach 3 marks.
Question 3
4 marks
Q (4 marks). State four differences between coordination by the nervous system and coordination by the endocrine (hormonal) system.
Model answer
In the nervous system the signal travels as electrical impulses along neurones (with chemical transmission at synapses), whereas in the endocrine system the signal is a chemical hormone carried in the blood.

Nervous coordination is very fast (acting in milliseconds), whereas hormonal coordination is slower (acting over seconds to minutes or longer).

The effect of a nervous impulse is localised to a specific effector, whereas a hormone has a widespread effect on all cells that possess the appropriate receptor.

The effects of nervous coordination are short-lived, whereas the effects of hormones are usually longer-lasting.
Why this scores
Why this scores 4/4. One mark per valid PAIRED difference: (1) electrical impulse vs chemical in blood; (2) fast vs slow; (3) localised vs widespread; (4) short-lived vs long-lasting. Each point must compare BOTH systems — a statement about only one scores 0.
Question 4
Practice question — Unit 55 marks
Q (5 marks). Describe the changes that occur in the permeability of a neurone's membrane to sodium and potassium ions during an action potential, and the effect of each change on the membrane potential.
Model answer
When the membrane is depolarised to the threshold (about −55 mV), the voltage-gated sodium ion channels open, so the membrane becomes very permeable to sodium ions. Sodium ions diffuse rapidly into the axon down their electrochemical gradient.

This influx of positive sodium ions causes depolarisation: the inside of the axon becomes positive, and the membrane potential rises to about +40 mV.

At about +40 mV the sodium ion channels close (inactivate) and the voltage-gated potassium ion channels open, so the membrane becomes permeable to potassium ions. Potassium ions diffuse out of the axon down their concentration gradient.

This loss of positive ions causes repolarisation: the inside becomes negative again and the membrane potential falls back towards the resting value.

The potassium channels are slow to close, so slightly too much potassium leaves, causing a brief hyperpolarisation (below −70 mV); the sodium–potassium pump then restores the resting potential of −70 mV.
Why this scores
Why this scores 5/5. Marks: (1) Na⁺ channels open at threshold → membrane permeable to Na⁺; (2) Na⁺ in → depolarisation to +40 mV; (3) Na⁺ channels close and K⁺ channels open; (4) K⁺ out → repolarisation; (5) too much K⁺ out → hyperpolarisation → pump restores −70 mV. Wrong ion/phase pairing is the most penalised error.
Question 5
5 marks
Q (5 marks). Explain what is meant by the refractory period and why it is important in the transmission of nerve impulses.
Model answer
The refractory period is the short time immediately after an action potential during which the voltage-gated sodium ion channels are inactivated and cannot reopen, so a new action potential cannot be generated in that region of the membrane no matter how strong the stimulus.

It is important for three reasons.

First, it ensures that the impulse travels in one direction only. The region of membrane just behind the action potential is in its refractory period, so it cannot be re-stimulated; the impulse can therefore only depolarise the membrane ahead of it, moving the action potential forward.

Second, it ensures that action potentials remain discrete, separate events rather than merging into one prolonged depolarisation, so information is carried as distinct impulses.

Third, it limits the frequency of impulses, because a neurone cannot fire again until the refractory period is over. This sets a maximum firing rate and so an upper limit on the intensity of stimulus that can be signalled.
Why this scores
Why this scores 5/5. Marks: (1) refractory period = Na⁺ channels inactivated → no new AP for a short time; (2) ensures one-way conduction (region behind is refractory); (3) keeps impulses discrete/separate; (4) limits frequency / sets maximum firing rate; (5) clear cause (inactivated Na⁺ channels) linked to the consequences. A common slip is to attribute one-way conduction to myelin rather than to the refractory period.
Question 6
Practice question — Unit 5 (levels-based)6 marks
Q (6 marks). Describe how an impulse is transmitted across a cholinergic synapse, including the role of acetylcholinesterase.
Model answer
When the action potential arrives at the end of the presynaptic neurone it depolarises the presynaptic membrane, causing voltage-gated calcium ion channels to open so that calcium ions diffuse into the synaptic knob.

The increase in calcium ion concentration causes the synaptic vesicles containing acetylcholine to move to and fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft by exocytosis.

The acetylcholine diffuses across the cleft and binds to specific receptor proteins on the postsynaptic membrane. This opens sodium ion channels, so sodium ions diffuse into the postsynaptic neurone and depolarise it (an excitatory postsynaptic potential). If enough acetylcholine binds, the depolarisation reaches threshold and a new action potential is generated in the postsynaptic neurone.

The enzyme acetylcholinesterase, present in the synaptic cleft, then hydrolyses the acetylcholine into choline and ethanoic acid. This stops the acetylcholine continuing to stimulate the postsynaptic membrane, so the sodium channels close and the postsynaptic membrane can repolarise, preventing continuous, uncontrolled firing. The breakdown products are reabsorbed into the presynaptic knob and recombined, using ATP, to re-form acetylcholine and refill the vesicles.
Why this scores
Why this reaches Level 3 (5–6). A complete, ordered chain: Ca²⁺ influx → vesicle fusion/ACh release → ACh binds receptors → Na⁺ in → depolarisation/new AP → acetylcholinesterase hydrolyses ACh → stimulation stops / vesicles recycled. Level 1 answers omit Ca²⁺ or the role of acetylcholinesterase. A* discriminator: explaining WHY acetylcholinesterase matters (prevents continuous firing) and that ATP is needed to re-form ACh.
Question 7
Practice question — Unit 5 (levels-based)6 marks
Q (6 marks). Explain how myelination, axon diameter and temperature each affect the speed at which an impulse is conducted along a neurone.
Model answer
Myelination. In a myelinated axon the myelin sheath acts as an electrical insulator, so depolarisation (the action potential) can only occur at the nodes of Ranvier. The local currents flow from one node to the next, so the impulse jumps from node to node — saltatory conduction. Because the action potential does not have to be regenerated along every part of the membrane, a myelinated axon conducts an impulse much faster than an unmyelinated one of the same diameter.

Axon diameter. A wider (greater diameter) axon conducts faster. A wider axon offers less resistance to the flow of the local currents inside it (there is proportionally less ion leakage across the membrane relative to the volume of cytoplasm), so the adjacent region is brought to threshold more quickly and the impulse moves on faster.

Temperature. A higher temperature increases conduction speed (up to about 40 °C). Ions diffuse faster at higher temperatures, and the rate of respiration is greater so more ATP is available for the sodium–potassium pump to restore the gradients quickly. Above about 40 °C, however, the channel and pump proteins begin to denature and conduction slows or fails.
Why this scores
Why this reaches Level 3 (5–6). Each factor explained with a mechanism, not just stated: myelin → saltatory conduction (faster); wider axon → less resistance/leakage (faster); higher temperature → faster ion diffusion + more ATP for the pump (faster, until denaturation). Level 1 just lists 'myelin, diameter, temperature' without explanation.
Question 8
6 marks
Q (6 marks). In multiple sclerosis the myelin sheaths around neurones are progressively destroyed. Using your knowledge of how impulses are conducted, explain why this would slow down or block the conduction of nerve impulses.
Model answer
In a healthy myelinated neurone the myelin sheath insulates the axon, so ions can only cross the membrane at the nodes of Ranvier. The action potential is therefore only regenerated at the nodes, and the impulse jumps from node to node by saltatory conduction, which makes conduction very fast.

If the myelin is destroyed, the axon membrane is exposed along its whole length instead of only at the nodes. The action potential can no longer "jump"; instead it must be regenerated continuously along every part of the membrane (as in an unmyelinated axon). Because the action potential has to be re-created at every point, conduction becomes much slower.

In addition, with the insulation gone, ions leak across the now-exposed membrane and local currents dissipate. The depolarisation reaching the next region may then fail to reach the threshold, so an action potential is not regenerated and the impulse is blocked entirely.

Because the impulse to the effector is slowed or blocked, responses become slow, weak or fail altogether — explaining symptoms such as muscle weakness and loss of coordination. This is a synoptic application: the same all-or-nothing thresholding that normally makes signalling reliable means that, once local currents fall below threshold, no impulse passes at all.
Why this scores
Why this reaches Level 3 (5–6). Applies the mechanism to an unfamiliar context (AO2): (1) normally myelin → saltatory conduction; (2) loss of myelin → membrane exposed; (3) action potential must be regenerated continuously → slower; (4) ions leak / local currents dissipate; (5) depolarisation may not reach threshold → impulse blocked; (6) links to the symptom (slow/failed responses). Top answers explicitly use the threshold/all-or-nothing idea to explain complete blocking, not just slowing.
Question 9
Practice question — Unit 5 (data interpretation / calculation)8 marks
Q (8 marks). A recording electrode measured the membrane potential of an axon. With a weak stimulus, 3 action potentials were recorded in 0.30 s; with a strong stimulus, 9 action potentials were recorded in the same 0.30 s, but every action potential reached the same peak of +40 mV. Calculate the frequency of impulses for each stimulus, and use the all-or-nothing principle and the refractory period to explain these results.
Model answer
Calculation of frequency. Frequency = number of impulses ÷ time.

For the weak stimulus:

$\text{frequency} = \frac{3}{0.30\ \text{s}} = 10\ \text{impulses s}^{-1}\ (10\ \text{Hz}).$

For the strong stimulus:

$\text{frequency} = \frac{9}{0.30\ \text{s}} = 30\ \text{impulses s}^{-1}\ (30\ \text{Hz}).$

So the strong stimulus produced a three-fold higher frequency of impulses than the weak stimulus.

Explanation using the all-or-nothing principle. Every action potential reached the same peak of +40 mV regardless of stimulus strength. This is the all-or-nothing principle: provided the stimulus reaches the threshold, an action potential of a fixed size is produced; if it does not reach threshold, no action potential occurs. The size of the action potential therefore cannot encode how strong the stimulus is.

Instead, a stronger stimulus is coded by a higher frequency of action potentials — here 30 Hz compared with 10 Hz — and by the recruitment of more neurones. The brain interprets the higher frequency as a more intense stimulus.

The role of the refractory period. After each action potential, the voltage-gated sodium ion channels are inactivated during the refractory period, so a new action potential cannot be generated until this has passed. This sets the maximum possible frequency of impulses. The fact that even the strong stimulus produced a regular, finite number of impulses (rather than a continuous depolarisation) shows the refractory period spacing them out and capping the firing rate. The refractory period also ensures the impulses travel in one direction only and remain discrete, so the frequency code is reliable.
Why this scores
Why this scores 7–8/8. Calculation marks: (1) weak = 3 ÷ 0.30 = 10 Hz with working; (2) strong = 9 ÷ 0.30 = 30 Hz with working (units required). Explanation marks: (3) all-or-nothing → same peak/+40 mV → size fixed; (4) threshold must be reached; (5) intensity coded by frequency (10 vs 30 Hz) / more neurones; (6) refractory period = Na⁺ channels inactivated; (7) sets maximum frequency / impulses discrete; (8) reasoned link of data to the principle. A* answers quote BOTH frequencies and explicitly tie the constant +40 mV peak to all-or-nothing.
Question 10
Practice question — Unit 5 (synoptic / cell signalling)7 marks
Q (7 marks). Adrenaline raises the blood glucose concentration without entering its target cells. Explain how the second-messenger model achieves this, and explain the advantage of the resulting enzyme cascade.
Model answer
Adrenaline is a water-soluble hormone, so it cannot pass through the hydrophobic phospholipid bilayer of the cell-surface membrane and therefore cannot enter the target (liver) cell. It acts instead as a first messenger at the outside of the cell.

Adrenaline binds to a specific complementary receptor protein on the cell-surface membrane. The binding changes the shape of the receptor and activates the membrane-bound enzyme adenylyl cyclase on the inner face of the membrane.

Activated adenylyl cyclase catalyses the conversion of ATP into cyclic AMP (cAMP) inside the cell. cAMP is the second messenger: it carries the signal onward inside the cell even though adrenaline itself remained outside.

cAMP activates a protein kinase, which in turn activates other enzymes in a series of steps — an enzyme cascade. The end of the cascade activates the enzymes that catalyse the breakdown of glycogen into glucose (glycogenolysis) in the liver cell, so glucose is released into the blood and the blood glucose concentration rises.

The advantage of the cascade is amplification: a single adrenaline molecule activates one receptor, which can activate many adenylyl cyclase molecules, each producing many cAMP molecules, each activating many enzyme molecules at each step. As a result a very low concentration of hormone produces a large, rapid response, which is ideal for the fast mobilisation of glucose needed in the fight-or-flight response.
Why this scores
Why this scores 6–7/7. Marks: (1) water-soluble → cannot cross membrane → first messenger; (2) binds specific membrane receptor; (3) activates adenylyl cyclase; (4) ATP → cyclic AMP (second messenger); (5) cAMP activates a kinase / enzyme cascade; (6) cascade → glycogen broken down to glucose; (7) amplification — small hormone concentration → large rapid response. Saying adrenaline 'enters the cell' or 'binds DNA' loses the opening marks.

Key Definitions and Keywords — The Nervous System

Definitions to memorise and the exact keywords mark schemes credit for the nervous system answers — sharpened from recent examiner reports for the 2026 Cambridge IGCSE sitting.

Neurone
Examiner keyword
A specialised cell that transmits information rapidly as electrical impulses, consisting of a cell body, dendrites and an axon.
Myelin sheath
Examiner keyword
An insulating layer of lipid-rich membranes wrapped around an axon by Schwann cells, interrupted by nodes of Ranvier, that increases the speed of impulse conduction.
Saltatory conduction
Examiner keyword
The transmission of an impulse along a myelinated axon by the action potential jumping from one node of Ranvier to the next, which greatly increases conduction speed.
Resting potential
Examiner keyword
The potential difference of about −70 mV across the membrane of a neurone that is not transmitting an impulse, with the inside negative relative to the outside.
Sodium–potassium pump
Examiner keyword
A carrier protein that uses ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions moved in, establishing the ion gradients needed for the resting potential.
Action potential
Examiner keyword
A rapid, temporary reversal of the membrane potential (to about +40 mV) that occurs when a stimulus depolarises the membrane to the threshold, and which is propagated along the axon.
Depolarisation
Examiner keyword
The change in membrane potential from negative towards positive caused by voltage-gated sodium ion channels opening and sodium ions diffusing into the axon.
Repolarisation
Examiner keyword
The return of the membrane potential towards the resting value, caused by sodium channels closing and voltage-gated potassium ion channels opening so that potassium ions diffuse out.
Hyperpolarisation
Examiner keyword
A brief overshoot in which the membrane potential becomes more negative than the resting potential because slightly too much potassium leaves before the channels close.
Threshold
Examiner keyword
The membrane potential (about −55 mV) that must be reached for voltage-gated sodium channels to open fully and trigger an action potential.
All-or-nothing principle
Examiner keyword
The principle that an action potential is only produced if the stimulus reaches the threshold, and that every action potential is the same size regardless of how strong the stimulus is.
Refractory period
Examiner keyword
The short period after an action potential during which the voltage-gated sodium channels are inactivated so that no new action potential can be generated, ensuring impulses are unidirectional and discrete and limiting their frequency.
Synapse
Examiner keyword
The junction between two neurones (or a neurone and an effector), across which an impulse is transmitted chemically by a neurotransmitter diffusing across the synaptic cleft.
Acetylcholine (ACh)
Examiner keyword
A neurotransmitter released from vesicles at a cholinergic synapse that diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening sodium channels.
Acetylcholinesterase
Examiner keyword
An enzyme in the synaptic cleft that hydrolyses acetylcholine into choline and ethanoic acid, stopping its stimulating effect so the postsynaptic membrane can repolarise.
Second messenger (cAMP)
Examiner keyword
A molecule, such as cyclic AMP, produced inside a cell when a hormone (the first messenger) binds to a membrane receptor, which relays and amplifies the signal via an enzyme cascade.
Summation
The adding together of several small postsynaptic potentials (from several synapses, spatial, or from rapid repeated firing, temporal) so that the threshold is reached and an action potential is triggered.

Common Mistakes and Misconceptions — The Nervous System

The traps other students keep falling into on the nervous system questions — taken from recent Cambridge IGCSE examiner reports and mark schemes — and how to avoid them.

✕Matching the wrong ion to the wrong phase of the action potential
Edexcel IAL Biology Unit 5 examiner reports
Why it happens
Students memorise the ions but not which moves during depolarisation versus repolarisation.
How to avoid it
Lock it in: Na⁺ IN = depolarisation (up to +40 mV); K⁺ OUT = repolarisation (back down). During repolarisation sodium does NOT leave — its channels are closed; it is potassium that leaves.
✕Saying a stronger stimulus produces a bigger action potential
Edexcel IAL Biology Unit 5 examiner reports
Why it happens
It feels intuitive that a bigger stimulus should give a bigger response.
How to avoid it
Action potentials are all-or-nothing and always the same size. A stronger stimulus is coded by a HIGHER FREQUENCY of impulses (and more neurones recruited), not by a larger impulse.
✕Getting the sodium–potassium pump ratio or directions wrong
Edexcel IAL Biology Unit 5 examiner reports
Why it happens
The numbers (3 and 2) and the directions are easily reversed under pressure.
How to avoid it
Memorise exactly: 3 Na⁺ OUT for every 2 K⁺ IN, using ATP. More positive ions leave than enter, helping make the inside negative.
✕Confusing calcium ions and sodium ions at the synapse
Edexcel IAL Biology Unit 5 examiner reports
Why it happens
Both are positive ions that flood in, so they are easily muddled.
How to avoid it
Ca²⁺ enters the PREsynaptic knob to trigger vesicle fusion; Na⁺ enters the POSTsynaptic membrane (after ACh binds its receptor) to depolarise it. Keep the two sides of the synapse separate.
✕Describing myelin as something that 'carries' or 'speeds up the electricity'
Edexcel IAL Biology Unit 5 examiner reports
Why it happens
Students associate 'faster' with better conduction rather than insulation.
How to avoid it
Myelin is an INSULATOR: it stops ion movement except at the nodes of Ranvier, so the impulse jumps node-to-node (saltatory conduction). Speed comes from not regenerating the action potential along the whole membrane.
✕Saying adrenaline enters the target cell to have its effect
Edexcel IAL Biology Unit 5 examiner reports
Why it happens
Students assume all signalling molecules act inside the cell.
How to avoid it
Adrenaline is water-soluble and cannot cross the membrane. It binds a receptor on the OUTSIDE, activating adenylyl cyclase to make cAMP (the second messenger) inside the cell. The hormone stays outside; cAMP carries the message in.

The Nervous System — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

The Nervous System

Short Notes - The Nervous System

Detailed Study Notes

At a glance

What you’ll learn

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Classifying neurones and the role of myelin (2 marks)

2The sodium–potassium pump and the resting potential (2 marks)

3Interpreting an action-potential graph (4 marks)

4Sequence of events at a cholinergic synapse (4 marks)

5Coding stimulus intensity (6 marks)

6The second-messenger action of adrenaline (6 marks)

Neurone

Myelin sheath

Saltatory conduction

Resting potential

Sodium–potassium pump

Action potential

Depolarisation

Repolarisation

Hyperpolarisation

Threshold

All-or-nothing principle

Refractory period

Synapse

Acetylcholine (ACh)

Acetylcholinesterase

Second messenger (cAMP)

Summation

✕Matching the wrong ion to the wrong phase of the action potential

✕Saying a stronger stimulus produces a bigger action potential

✕Getting the sodium–potassium pump ratio or directions wrong

✕Confusing calcium ions and sodium ions at the synapse

✕Describing myelin as something that 'carries' or 'speeds up the electricity'

✕Saying adrenaline enters the target cell to have its effect

The Nervous System — frequently asked questions