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AQA A-Level Biology: Structure and Function of Neurons in Nerve Impulse Transmission — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Biology specificationlast verified 3 July 2026

The short answer

The structure and function of neurons are fundamental to understanding how organisms respond to changes in their internal and external environments. This section delves into the detailed structure of a myelinated motor neurone, the establishment of resting potential, the generation and conduction of action potentials, and factors affecting the speed of nerve impulse transmission.

The question

Explain how a myelinated motor neurone differs from a non-myelinated motor neurone in terms of structure and function.

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For explanation questions, marks are typically awarded for clear and detailed descriptions that demonstrate a deep understanding of the topic. For comparison questions, marks are given for identifying both similarities and differences accurately. For description questions, marks are awarded for providing a clear and concise account of the structure or process. For discussion questions, marks are given for examining and evaluating different aspects of the topic with evidence and reasoning.

What the command words demand

Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Compare: Identify similarities and differences between two or more concepts or processes.
Describe: Give a clear and concise account of the structure, function, or process.
Discuss: Examine and evaluate different aspects of a topic, providing evidence and reasoning.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.

Identify the key structural differences between myelinated and non-myelinated motor neurones.2 marks
Myelinated motor neurones have a myelin sheath, which is an insulating layer of fatty tissue wrapped around the axon in segments by Schwann cells. Non-myelinated motor neurones lack this myelin sheath.
Describe how these structural differences affect function.2 marks
The myelin sheath in myelinated motor neurones allows for saltatory conduction, where action potentials jump from one node of Ranvier to the next. This significantly increases the speed of nerve impulse transmission. In non-myelinated motor neurones, action potentials propagate continuously along the entire length of the axon, which is slower and requires more energy.
Summarize the key points.0 marks
Myelinated motor neurones have a myelin sheath that enables faster conduction through saltatory conduction. Non-myelinated motor neurones lack this myelin sheath and rely on continuous conduction, which is slower.

Final answer: Myelinated motor neurones have a myelin sheath that allows for faster nerve impulse transmission through saltatory conduction. Non-myelinated motor neurones lack this myelin sheath and rely on continuous conduction, which is slower.

Work through every step correctly and you earn all 4 marks.

Another worked example

Describe the process of generating an action potential in a neuron, including the roles of Na + and K + ions.

6 marks

Explain the resting potential and how it is maintained.1 mark
The resting potential is the electrical potential difference across the cell membrane when the neuron is not transmitting a signal. It is maintained by differential membrane permeability to K + and Na + , the sodium-potassium pump, and electrochemical gradients.
Describe the depolarization phase.1 mark
A stimulus causes Na + channels to open, allowing Na + ions to rush into the cell, depolarizing the membrane and reaching a threshold potential of about -55 mV.
Explain the action potential phase.2 marks
Once the threshold is reached, more Na + channels open, leading to a rapid influx of Na + ions and further depolarization. The membrane potential rises to around +30 mV.
Describe the repolarization phase.1 mark
Na + channels close, and K + channels open, allowing K + ions to leave the cell, repolarizing the membrane back to its resting potential.
Mention the hyperpolarization phase (if applicable).1 mark
Excess K + efflux can cause a brief period of hyperpolarization, where the membrane potential drops below the resting level before returning to normal.

Final answer: The resting potential is maintained by differential permeability to K + and Na + , the sodium-potassium pump, and electrochemical gradients. A stimulus causes depolarization as Na + ions enter the cell. Once the threshold is reached, more Na + channels open, leading to an action potential. Repolarization occurs as K + ions leave the cell, returning the membrane to its resting potential.

Work through every step correctly and you earn all 6 marks.

Common mistakes

Confusing the roles of Na + and K + ions in resting potential and action potentials.
Why it happens: Students may mix up which ion is more permeable to the cell membrane and which channels open during depolarization and repolarization.
Fix: Always remember that the cell membrane is more permeable to K + than Na + , leading to a negative resting potential. During depolarization, Na + channels open, and during repolarization, K + channels open.
Forgetting the all-or-nothing principle in action potentials.
Why it happens: Students may overlook the fact that once an action potential is initiated, its amplitude and duration are always the same.
Fix: Emphasize that the all-or-nothing principle means that if the threshold potential is reached, the action potential will occur with a consistent amplitude and duration, regardless of the strength of the initial stimulus.
Misunderstanding the role of myelination in nerve impulse speed.
Why it happens: Students may not fully grasp how saltatory conduction works or why it is faster than continuous conduction.
Fix: Practice explaining that myelin sheaths allow action potentials to jump from one node of Ranvier to the next, significantly increasing the speed of nerve impulse transmission compared to continuous conduction in non-myelinated axons.
Confusing absolute and relative refractory periods.
Why it happens: Students may mix up the definitions and roles of these two types of refractory periods.
Fix: Remember that the absolute refractory period is when Na + channels are inactivated, preventing any new action potentials. The relative refractory period is when the membrane is less excitable and requires a stronger stimulus to reach the threshold potential.
Failing to explain how axon diameter affects conduction speed.
Why it happens: Students may not understand the relationship between axon diameter and resistance to ion flow.
Fix: Practice explaining that larger axons have less resistance to ion flow, leading to faster conduction speeds. Smaller axons have more resistance, making conduction slower.
Not considering the impact of temperature on nerve impulse transmission.
Why it happens: Students may overlook how changes in temperature affect molecular movement and conduction speed.
Fix: Remember that higher temperatures generally increase the rate of molecular movement, leading to faster nerve impulse transmission. Lower temperatures have the opposite effect.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Compare Neurone Structure	Explain how myelinated and non-myelinated motor neurones differ in structure and function.	4
Describe Action Potential	Describe how an action potential is generated, detailing the roles of Na+ and K+ ions.	6
Explain With Example	Explain the all-or-nothing principle and illustrate it with a real-world example.	3
Compare Conduction Types	Compare saltatory and continuous conduction, highlighting differences in speed and energy efficiency.	5
Total across these question types		18

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.