AQA A-Level Biology (7402)

## Learning Outcomes

- Explain how the resting potential is established and maintained in a neurone
- Describe the stages of an action potential, including depolarisation, repolarisation and the refractory period
- Distinguish clearly between a generator potential and an action potential
- Compare conduction in myelinated and non-myelinated axons
- Explain how myelination, axon diameter and temperature affect conduction speed
- Apply the all-or-nothing principle to exam questions about stimulus intensity and impulse frequency
- Use precise biological terminology in synoptic questions on receptors, neurones and responses

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand nerve impulses as part of nervous transmission, with a focus on the following syllabus points:

- Know that a nerve impulse is the passage of an action potential along an axon
- Explain how the resting potential is produced by sodium–potassium pumps, differential permeability and ion movement
- Describe how depolarisation and repolarisation occur through changes in membrane permeability to sodium ions and potassium ions
- Understand the refractory period and explain its role in making impulses discrete, limiting frequency and ensuring one-way transmission
- Explain the difference between a generator potential and an action potential
- Compare transmission in myelinated and non-myelinated axons, including the role of nodes of Ranvier
- Define saltatory conduction accurately and explain why it increases conduction speed
- Explain how stimulus intensity is coded by frequency of impulses rather than action potential size

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. How is the resting potential maintained across the membrane of a neurone?
2. Why does reaching threshold matter in the production of an action potential?
3. What is the difference between a generator potential and an action potential?
4. Why is conduction faster in a myelinated axon than in a non-myelinated axon?
5. How does the refractory period affect the direction and frequency of nerve impulses?

## Introduction

Organisms respond to changes in their internal and external environment to improve survival. In mammals, one of the fastest ways to coordinate a response is through the nervous system. This system depends on **nerve impulses**, which allow information to move rapidly from receptors to the central nervous system and then to effectors such as muscles or glands.

![Resting potential, action potential phases, and saltatory conduction in myelinated and non-myelinated axons.](https://static.pastpaperhero.com/images/aqa-a-level-biology-7402-responses-to-environmental-change-nerve-impulses_00.png)

_Nerve impulse transmission is outlined by membrane potential changes and the differing conduction patterns of myelinated and unmyelinated axons._

For AQA Biology, this topic is tested in detail and also appears in synoptic questions. You need more than a simple sequence of stages. Examiners want accurate ion movements, clear use of terms such as **depolarisation** and **saltatory conduction**, and a proper explanation of why stronger stimuli give a higher frequency of impulses rather than bigger impulses.

> **Key Term: nerve impulse**
> The passage of an action potential along an axon.

### Resting potential and how a neurone becomes ready to fire

A neurone at rest is described as **polarised**. This means there is a potential difference across its membrane: the inside is negative relative to the outside. In many exam questions this is given as about **–70 mV**.

This resting potential exists because the membrane controls the movement of ions. The phospholipid bilayer itself does not let sodium ions and potassium ions pass freely, so transport depends on membrane proteins.

Two features matter most.

First, the **sodium–potassium pump** actively transports ions using ATP. For each cycle, it moves **3 Na+ out** of the neurone and **2 K+ in**. This does two things at once: it creates ion concentration gradients, and it also makes the outside slightly more positive because more positive charge leaves than enters.

Second, the membrane is more permeable to potassium ions than to sodium ions at rest. Potassium channels are more open, so K+ diffuses out of the axon down its electrochemical gradient more easily than Na+ diffuses in. This further increases the negativity inside.

The result is a stable resting potential: more sodium ions outside, more potassium ions inside, and an inside that is negative relative to the outside.

> **Key Term: resting potential**
> The potential difference across the membrane of a neurone at rest, with the inside negative relative to the outside.
>
> **Key Term: sodium–potassium pump**
> A membrane carrier protein that uses ATP to transport 3 Na+ out of a neurone and 2 K+ into it.

Students often write that the pump “creates the action potential”. That is not correct. The pump establishes and maintains the ion gradients needed for an action potential, but the rapid spike itself happens because voltage-gated ion channels open.

A useful chain of reasoning is:

- pump sets up gradients
- membrane is more permeable to K+ than Na+ at rest
- K+ diffuses out more than Na+ diffuses in
- inside becomes negative
- neurone is polarised and ready to respond to a stimulus

This explains both **how** the resting potential is formed and **why** the neurone can respond quickly when stimulated.

> **Test Tip:** In “explain resting potential” questions, include both active transport and differential permeability. Many students lose marks by mentioning only the pump or only diffusion.

### From generator potential to action potential

A stimulus does not automatically create a nerve impulse. First, it causes a local change in membrane potential. In receptors, this is called a **generator potential**.

For example, in a pressure receptor such as a Pacinian corpuscle, pressure deforms the receptor and opens stretch-mediated sodium ion channels. Sodium ions diffuse into the sensory neurone ending, making the inside less negative. This local depolarisation is the generator potential.

Generator potentials are **graded**. That means their size depends on the strength of the stimulus. A weak pressure causes a small generator potential; a stronger pressure causes a larger one.

If this depolarisation reaches the **threshold**, an action potential is triggered. If threshold is not reached, no action potential forms.

> **Key Term: generator potential**
> A local graded depolarisation in a receptor caused by a stimulus; if threshold is reached, it triggers an action potential.
>
> **Key Term: threshold**
> The minimum level of depolarisation needed to trigger an action potential.

Once threshold is reached, voltage-gated sodium ion channels open. Sodium ions diffuse rapidly into the axon down their electrochemical gradient. This causes further depolarisation, which opens more sodium channels. This is a positive feedback effect, so the membrane potential rises very quickly.

The membrane potential may rise from around **–70 mV** to about **+40 mV**. This rapid reversal of charge is the **action potential**.

> **Key Term: action potential**
> A rapid change in membrane potential in which the inside of the neurone becomes positive relative to the outside.

After this peak, sodium channels close or become inactivated, and voltage-gated potassium channels open. Potassium ions diffuse out of the neurone, so the membrane begins to return to its resting state. This is **repolarisation**.

Because potassium channels are slow to close, too much K+ may leave. The membrane becomes even more negative than the resting potential for a short time. This is sometimes described as hyperpolarisation. After that, the normal resting potential is restored as channels reset and the sodium–potassium pump continues to maintain ion gradients.

> **Key Term: depolarisation**
> A reduction in the potential difference across the membrane, caused here by sodium ions entering the neurone.
>
> **Key Term: repolarisation**
> The return of the membrane towards resting potential, caused by potassium ions leaving the neurone.

The most tested principle here is the **all-or-nothing** rule. If threshold is reached, a full action potential is produced. If threshold is not reached, none is produced. A stronger stimulus does not create a taller action potential. Instead, it creates action potentials more frequently.

Why? Because a stronger stimulus usually causes a larger generator potential, which reaches threshold more often over time. The neurone therefore fires more often, not more strongly.

This is a very common source of error. Students often assume “stronger stimulus = bigger impulse”. That is wrong.

> **Test Tip:** If asked how the nervous system detects stimulus intensity, write “higher frequency of nerve impulses” rather than “larger action potentials”.

### Passage of the impulse along the axon

An action potential at one region of membrane causes local currents. Sodium ions entering that region spread sideways inside the axon, making the adjacent region less negative. If that neighbouring region reaches threshold, its voltage-gated sodium channels open and a new action potential forms there.

In a **non-myelinated axon**, this happens continuously along the whole length of the membrane. Every adjacent section must depolarise, then repolarise, one after another. This is effective but relatively slow.

> **Key Term: non-myelinated axon**
> An axon without a myelin sheath, where depolarisation occurs along the whole length of the membrane.

In a **myelinated axon**, the myelin sheath acts as an electrical insulator. Ions cannot move across the membrane where myelin is present, so depolarisation can only occur at the gaps in the sheath, called the **nodes of Ranvier**.

> **Key Term: myelinated axon**
> An axon covered by a myelin sheath, so depolarisation occurs only at gaps in the sheath.
>
> **Key Term: nodes of Ranvier**
> Gaps in the myelin sheath where ions move across the membrane and action potentials are generated.

Because only the nodes depolarise, fewer parts of the membrane need to go through the full action potential cycle. The local currents pass quickly beneath the myelin to the next node, where a new action potential is produced. This is called **saltatory conduction**.

> **Key Term: saltatory conduction**
> Conduction in a myelinated axon where depolarisation occurs only at nodes of Ranvier.

This is the exact point examiners want. Do not rely only on the phrase “the impulse jumps from node to node”. That phrase is too vague on its own. The better explanation is:

- myelin insulates the membrane
- depolarisation occurs only at nodes of Ranvier
- local currents travel between nodes
- fewer sections of membrane are depolarised
- conduction is faster

The action potential itself does not get weaker as it moves. At each new region, it is regenerated to the same size. So a nerve impulse travels without decreasing in amplitude.

A short comparison helps:

- **Non-myelinated axon:** depolarisation along entire membrane, slower
- **Myelinated axon:** depolarisation only at nodes, faster
- **Same key rule in both:** action potentials are regenerated and remain the same size

### Refractory period, direction of travel and conduction speed

After an action potential, the membrane enters the **refractory period**. During this time, another action potential cannot be generated immediately because sodium channels are inactivated or the membrane is still returning to resting potential.

> **Key Term: refractory period**
> The short period after an action potential when another action potential cannot be initiated immediately.

This matters for three reasons.

First, it keeps impulses **discrete**. Without a refractory period, action potentials could merge into one continuous event, which would make nervous communication less precise.

Second, it limits the **maximum frequency** of impulses. There must be a short delay between one action potential and the next.

Third, it helps ensure **one-way transmission** along an axon. The region behind the action potential is refractory, so it cannot depolarise again straight away. That means the impulse moves forwards into the next resting region, not backwards.

A simple calculation may be tested.

If the refractory period is **2 ms**, the maximum possible frequency is:

- convert 2 ms to seconds: 2 ms = 0.002 s
- frequency = 1 / time
- frequency = 1 / 0.002
- frequency = **500 Hz**

That means the neurone can transmit up to 500 impulses per second.

Students often forget unit conversion here. If you divide by 2 instead of 0.002, the answer is wrong by a factor of 1000.

> **Test Tip:** In calculation questions on impulse frequency, always convert milliseconds to seconds before using frequency = 1 / time.

Two other factors affect conduction speed.

A **larger axon diameter** increases speed because there is less resistance to the flow of ions in the cytoplasm. Local currents spread more easily, so neighbouring regions reach threshold faster.

A **higher temperature** usually increases speed because ions diffuse faster and membrane transport processes happen more quickly. However, very high temperatures would damage proteins and membranes, so there is a limit.

A useful exam explanation is:

- larger diameter → lower internal resistance → faster spread of local currents
- higher temperature → faster ion diffusion → faster depolarisation of neighbouring regions
- myelination → fewer regions depolarised → greatest increase in speed

These ideas also explain biological patterns. Reflex pathways need rapid responses, so fewer synapses and fast-conducting neurones are useful. Myelinated motor neurones are especially suitable where speed matters.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A nerve impulse is the passage of an action potential along an axon
- At resting potential, the inside of the neurone is negative relative to the outside
- The sodium–potassium pump moves 3 Na+ out and 2 K+ in using ATP
- The membrane is more permeable to K+ than Na+ at rest, so K+ diffuses out more easily
- A generator potential is graded, but an action potential follows the all-or-nothing principle
- Depolarisation occurs when voltage-gated sodium channels open and Na+ enters the neurone
- Repolarisation occurs when sodium channels close and potassium channels open so K+ leaves
- The refractory period makes impulses discrete, limits maximum frequency and helps ensure one-way transmission
- In non-myelinated axons, depolarisation occurs along the whole membrane
- In myelinated axons, depolarisation occurs only at nodes of Ranvier
- Saltatory conduction is faster because fewer sections of membrane need to depolarise
- Stronger stimuli are coded by a higher frequency of nerve impulses, not by larger action potentials

## Key Terms and Concepts

- nerve impulse
- resting potential
- sodium–potassium pump
- generator potential
- threshold
- action potential
- depolarisation
- repolarisation
- non-myelinated axon
- myelinated axon
- nodes of Ranvier
- saltatory conduction
- refractory period


Responses to environmental change - Nerve impulses