AQA A-Level Biology (7402)

## Learning Outcomes

- Describe the sequence of events in transmission across a cholinergic synapse
- Explain why synaptic transmission is unidirectional
- Distinguish clearly between excitatory and inhibitory synapses
- Explain how temporal summation and spatial summation affect whether a postsynaptic neurone fires
- Compare a cholinergic synapse with a neuromuscular junction
- Apply synaptic transmission knowledge to unfamiliar contexts such as drug effects or disease

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand synaptic transmission, with a focus on the following syllabus points:

- Know the structure of a cholinergic synapse, including the presynaptic membrane, synaptic cleft and postsynaptic membrane
- Explain the sequence of transmission across a cholinergic synapse, including the role of voltage-gated calcium ion channels and acetylcholine
- Understand why transmission across a synapse is one-way only
- Explain the role of acetylcholinesterase in stopping continuous stimulation
- Distinguish between excitatory and inhibitory synapses in terms of depolarisation and hyperpolarisation
- Define and apply temporal summation and spatial summation
- Understand transmission across a neuromuscular junction and how it compares with neurone-to-neurone synapses
- Use precise biological terminology when explaining ion movement, receptors and threshold

## 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. What happens when an action potential reaches the presynaptic membrane of a cholinergic synapse?
2. Why is transmission across a synapse unidirectional?
3. How does an inhibitory synapse reduce the chance of an action potential in the postsynaptic neurone?
4. What is the difference between temporal summation and spatial summation?
5. Name two ways a neuromuscular junction differs from a neurone-to-neurone cholinergic synapse.

## Introduction

Synaptic transmission is the process by which one neurone passes information to another neurone, or to an effector such as a muscle or gland cell. This is a high-value AQA topic because it brings together membrane structure, diffusion, facilitated diffusion, exocytosis, enzyme action, ATP use and cell signalling in one mechanism.

![Cholinergic synaptic transmission from presynaptic depolarisation and calcium influx to acetylcholine release, postsynaptic sodium entry, and acetylcholinesterase action.](https://static.pastpaperhero.com/images/aqa-a-level-biology-7402-responses-to-environmental-change-synaptic-transmission_00.png)

_Cholinergic synaptic signalling includes vesicle exocytosis, acetylcholine receptor binding, postsynaptic depolarisation, and acetylcholine hydrolysis in the synaptic cleft._

In exams, students often know the general story but lose marks through imprecise wording. AQA rewards exact terms such as **presynaptic membrane**, **receptors on the postsynaptic membrane**, and **sodium ions enter the postsynaptic neurone**. This means you need more than a vague outline: you need the sequence, the locations, and the reason each step matters.

> **Key Term: synapse**
> The junction between two neurones, or between a neurone and an effector cell.

### Structure of a cholinergic synapse

A cholinergic synapse uses **acetylcholine (ACh)** as its neurotransmitter. The basic layout has three parts. First, the **presynaptic neurone** ends in a swollen region called the synaptic knob. This contains many mitochondria and synaptic vesicles filled with ACh. Second, there is a very narrow gap called the **synaptic cleft**. Third, there is the **postsynaptic membrane**, which contains specific receptor proteins.

The arrangement is not random. Each structural feature supports a stage in transmission. Vesicles allow rapid release of neurotransmitter. Mitochondria provide ATP for resynthesising ACh. The cleft is narrow, so diffusion is fast. Receptors are only on the postsynaptic membrane, which is one reason transmission goes in one direction.

> **Key Term: cholinergic synapse**
> A synapse that uses acetylcholine as its neurotransmitter.
>
> **Key Term: synaptic cleft**
> The small gap between the presynaptic membrane and the postsynaptic membrane.
>
> **Key Term: postsynaptic membrane**
> The membrane of the cell receiving the signal, containing specific receptors for the neurotransmitter.

A common exam mistake is to write that ACh binds to a “binding site” without saying where. That is too vague. The better answer is that ACh binds to **receptors on the postsynaptic membrane**. Another common mistake is to say sodium channels open, but not that **Na+ enters** the postsynaptic neurone. Channel opening alone does not explain depolarisation.

### Transmission across a cholinergic synapse step by step

The sequence matters. If you miss one step, later steps make less sense.

First, an action potential arrives at the end of the presynaptic neurone. This changes the potential difference across the presynaptic membrane and opens **voltage-gated calcium ion channels**. Calcium ions then enter the synaptic knob by facilitated diffusion, down their electrochemical gradient.

Next, the rise in Ca2+ concentration inside the synaptic knob causes synaptic vesicles to move towards and fuse with the presynaptic membrane. This releases ACh into the synaptic cleft by **exocytosis**.

ACh then diffuses across the synaptic cleft. Because the cleft is very narrow, this happens quickly. The ACh molecules bind to specific receptors on the postsynaptic membrane. In a cholinergic excitatory synapse, this causes sodium ion channels to open. Sodium ions enter the postsynaptic neurone, making the inside less negative. This is **depolarisation**.

If enough sodium ions enter to reach threshold, a new action potential is generated in the postsynaptic neurone. If threshold is not reached, no action potential is produced. This detail is important because a synapse does not automatically create a new impulse every time neurotransmitter is released.

Finally, ACh must be removed quickly. The enzyme **acetylcholinesterase** hydrolyses ACh into choline and ethanoate. These products are taken back up into the presynaptic neurone, where ACh can be resynthesised using ATP from mitochondria. This prevents the postsynaptic membrane from being stimulated continuously.

> **Key Term: acetylcholine (ACh)**
> A neurotransmitter released at cholinergic synapses.
>
> **Key Term: acetylcholinesterase**
> The enzyme that hydrolyses acetylcholine in the synaptic cleft, stopping continuous stimulation.
>
> **Key Term: depolarisation**
> A reduction in the potential difference across a membrane, making the inside less negative than at resting potential.

A useful cause-and-effect chain is:

action potential arrives → Ca2+ channels open → Ca2+ enters presynaptic knob → vesicles fuse with membrane → ACh released by exocytosis → ACh diffuses across cleft → ACh binds to receptors on postsynaptic membrane → Na+ channels open → Na+ enters postsynaptic neurone → depolarisation → threshold may be reached → new action potential

Why does ACh need to be broken down so quickly? Because if it stayed in the cleft, receptors would remain activated, sodium channels would stay open longer, and the postsynaptic membrane could keep depolarising. The nervous system depends on brief, separate impulses, not one long uncontrolled signal.

> **Test Tip:** In a 4- to 6-mark mechanism question, write the sequence in order and include the ion movements. “Calcium ions enter the presynaptic neurone” and “sodium ions enter the postsynaptic neurone” are often essential marking points.

Synaptic transmission is also slower than conduction along an axon. Along an axon, depolarisation spreads electrically from one region of membrane to the next. At a synapse, the signal must trigger vesicle fusion, diffusion of neurotransmitter, receptor binding and then fresh ion movement. This creates a small synaptic delay.

The structure of the synapse also explains why transmission is **unidirectional**. Neurotransmitter vesicles and voltage-gated Ca2+ channels are located in the presynaptic neurone. Receptors for that neurotransmitter are on the postsynaptic membrane. So even if an action potential reaches a synapse, the next signal can only pass from pre- to postsynaptic side.

> **Key Term: unidirectional transmission**
> One-way transmission across a synapse from the presynaptic neurone to the postsynaptic cell.

### Excitatory and inhibitory synapses, and the role of summation

Not all synapses increase the chance of firing. Some make the postsynaptic neurone less likely to produce an action potential.

At an **excitatory synapse**, neurotransmitter binding leads to depolarisation. That means the membrane potential moves closer to threshold. At an **inhibitory synapse**, neurotransmitter binding causes the postsynaptic membrane to become more negative than resting potential, usually by opening chloride ion channels so Cl- enters, and sometimes by opening potassium ion channels so K+ leaves. This is called **hyperpolarisation**.

Hyperpolarisation matters because it moves the membrane potential further away from threshold. So a bigger excitatory input would now be needed to trigger an action potential.

> **Key Term: inhibitory synapse**
> A synapse that reduces the likelihood of the postsynaptic neurone generating an action potential.
>
> **Key Term: hyperpolarisation**
> An increase in membrane potential so that the inside of the membrane becomes more negative than resting potential.

Students often memorise “inhibitory means stop” without understanding how. The mechanism is electrical: more negative inside means the membrane is further from the threshold needed to open enough voltage-gated sodium channels for a new action potential.

This links directly to **summation**. A single weak stimulus may release only a small amount of neurotransmitter. That may not be enough to depolarise the postsynaptic membrane to threshold. However, inputs can add together.

In **temporal summation**, one presynaptic neurone releases neurotransmitter repeatedly in quick succession. Because each release happens before the effect of the previous one has fully disappeared, the depolarisations add together. If the total reaches threshold, the postsynaptic neurone fires.

In **spatial summation**, several presynaptic neurones release neurotransmitter onto the same postsynaptic neurone at the same time. Their combined effect may bring the membrane to threshold.

> **Key Term: temporal summation**
> The additive effect of neurotransmitter released from one presynaptic neurone in quick succession.
>
> **Key Term: spatial summation**
> The additive effect of neurotransmitter released from several presynaptic neurones at the same time.

Summation explains why the nervous system can filter out weak background stimulation. A low-level stimulus may produce too little excitatory input to reach threshold. A stronger or repeated stimulus can generate enough input for summation and trigger a response. This is why you may ignore quiet traffic noise but react to a sudden horn.

Summation also allows integration. A coordinator such as the CNS receives many inputs at once. Some are excitatory, some inhibitory. The postsynaptic neurone effectively “adds up” these inputs. Whether it fires depends on the balance between them.

For example, imagine a postsynaptic neurone receiving:

- one weak excitatory input: not enough to reach threshold
- two excitatory inputs at once: threshold reached by spatial summation
- one excitatory input plus one inhibitory input: no action potential because the inhibitory effect offsets the excitatory one
- rapid repeated input from one neurone: threshold reached by temporal summation

This is far more useful than a simple yes/no system. It allows fine control of responses.

> **Test Tip:** If an exam asks why a person ignores weak stimuli, use summation and threshold. Say that low neurotransmitter release may not depolarise the postsynaptic membrane to threshold, and inhibitory input may prevent threshold being reached.

### Neuromuscular junctions and applying synapse knowledge

A **neuromuscular junction** is a specialised cholinergic synapse between a motor neurone and a muscle fibre. The same core steps occur: action potential arrives, Ca2+ enters the presynaptic terminal, vesicles release ACh, ACh diffuses across the cleft, receptors on the muscle membrane are activated, the membrane depolarises, and if threshold is reached an action potential is produced.

> **Key Term: neuromuscular junction**
> A specialised cholinergic synapse between a motor neurone and a muscle fibre.

However, there are some important differences from a neurone-to-neurone cholinergic synapse. The postsynaptic cell is a muscle fibre, not another neurone. The postsynaptic membrane has many folds, increasing surface area. There are many ACh receptors. The response is always excitatory because the role of the junction is to trigger muscle contraction.

This means a neuromuscular junction is designed for reliability. A motor neurone impulse normally leads to a response in the muscle. By contrast, at many neurone-to-neurone synapses, whether the postsynaptic neurone fires depends on the balance of excitatory and inhibitory inputs and on summation.

A simple comparison is:

- cholinergic synapse: may be excitatory or inhibitory depending on context; connects neurones or neurone to gland
- neuromuscular junction: always excitatory; connects motor neurone to muscle fibre

You can also apply synapse knowledge to unfamiliar material. If a drug mimics ACh and binds to cholinergic receptors, it acts as an agonist and will increase stimulation of the postsynaptic membrane. If a drug blocks ACh receptors, less sodium enters the postsynaptic cell, so depolarisation is reduced and an action potential is less likely.

If calcium ion channels in the presynaptic membrane are blocked, vesicles will not fuse effectively, so little or no ACh will be released. At a neuromuscular junction, this would reduce muscle contraction and could cause muscle weakness. The chain of reasoning is more important than the name of the disease or drug.

Another good application is pain relief. If a neurotransmitter acts as an inhibitory transmitter on pain pathways, it may cause hyperpolarisation of the postsynaptic neurone. That makes action potentials less likely, so fewer pain signals reach the CNS.

These questions test whether you understand mechanism, not just recall words. If the exam changes the neurotransmitter or gives a drug you have never met, ask:

1. Does it increase or decrease neurotransmitter release?
2. Does it mimic the neurotransmitter or block its receptor?
3. Does it increase depolarisation or cause hyperpolarisation?
4. Will threshold be more or less likely to be reached?

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A synapse is the junction between two neurones or between a neurone and an effector
- A cholinergic synapse uses acetylcholine as its neurotransmitter
- An action potential at the presynaptic membrane opens voltage-gated calcium ion channels
- Calcium ions enter the presynaptic neurone and trigger synaptic vesicles to fuse with the presynaptic membrane
- Acetylcholine is released by exocytosis and diffuses across the synaptic cleft
- Acetylcholine binds to receptors on the postsynaptic membrane, causing sodium ion channels to open
- Sodium ions enter the postsynaptic neurone, causing depolarisation; if threshold is reached, a new action potential is generated
- Acetylcholinesterase hydrolyses acetylcholine, preventing continuous stimulation
- Transmission across a synapse is unidirectional because vesicles are presynaptic and receptors are postsynaptic
- Excitatory synapses increase the chance of firing, while inhibitory synapses cause hyperpolarisation and reduce the chance of firing
- Temporal summation involves repeated input from one neurone; spatial summation involves simultaneous input from several neurones
- A neuromuscular junction is a specialised cholinergic synapse between a motor neurone and a muscle fibre and is always excitatory

## Key Terms and Concepts

- synapse
- cholinergic synapse
- synaptic cleft
- postsynaptic membrane
- acetylcholine (ACh)
- acetylcholinesterase
- depolarisation
- unidirectional transmission
- inhibitory synapse
- hyperpolarisation
- temporal summation
- spatial summation
- neuromuscular junction


Responses to environmental change - Synaptic transmission