Synaptic transmission | AQA A-Level Psychology Revision
- Revision Notes
- Aug 3
- 13 min read
Updated: 7 days ago
For 7182 specification, first teach in September 2025
AQA A-Level Psychology | Free Revision Notes
Estimated study time: 40 minutes
These Synaptic transmission A-Level Psychology revision notes explain how neurons communicate across the small gap between them. You will follow the journey of an electrical signal to the end of one neuron, the release of neurotransmitters and the effect these chemicals have on the next neuron. This lesson builds on sensory, relay and motor communication and prepares you to compare neural communication with glands and hormonal communication.
Learning Objectives 🎯
By the end of this revision page, you should be able to:
Define a synapse.
Describe the process of synaptic transmission.
Explain the role of neurotransmitters.
Distinguish between presynaptic and postsynaptic neurons.
Distinguish between excitation and inhibition.
Apply synaptic transmission to unfamiliar situations.
Revision Notes 📚
Synaptic transmission A-Level Psychology revision overview
Neurons form communication pathways throughout the nervous system.
Information travels electrically along an individual neuron. However, neurons are not normally joined together as one continuous cell.
Between two neurons is a very small gap called a synapse.
Information must cross this gap before it can continue through the nervous system. The process through which information passes from one neuron to another is called synaptic transmission.
The basic sequence is:
Electrical signal in the first neuron → chemical transmission across the synapse → electrical effect in the next neuron
The AQA specification requires knowledge of:
The process of synaptic transmission.
The role of neurotransmitters.
Excitation.
Inhibition.
What is a synapse?
A synapse is the junction between two neurons.
It includes:
The end of the neuron sending the signal.
The small gap between the neurons.
The receiving part of the next neuron.
The two neurons can be described as:
Term | Meaning |
Presynaptic neuron | The neuron sending the signal |
Postsynaptic neuron | The neuron receiving the signal |
Synaptic gap | The small space between the two neurons |
The presynaptic neuron comes before the synapse in the direction of communication. The postsynaptic neuron comes after it.
Why is synaptic transmission necessary?
Sensory, relay and motor neurons must communicate to form a complete neural pathway.
For example:
A sensory neuron carries information towards the central nervous system.
It must communicate with a relay neuron.
The relay neuron must communicate with another neuron.
A motor neuron carries an instruction towards an effector.
The neurons do not simply pass an electrical signal directly from one cell into the next. Communication across the synapse uses neurotransmitters.
This allows information to move through the pathway studied in how the three types of neuron work together.
Electrical and chemical communication
Neural communication involves both electrical and chemical processes.
Location | Form of communication |
Along a neuron | Electrical |
Across a synapse | Chemical |
Along the next neuron | Electrical |
The signal therefore changes form during transmission:
Electrical → chemical → electrical
This distinction is important in examinations.
It would be inaccurate to say that neurotransmitters carry an electrical signal directly across the gap. Neurotransmitters are chemicals that affect the receiving neuron.
The process of synaptic transmission
Synaptic transmission occurs in a sequence.
Stage 1: An electrical signal reaches the end of the presynaptic neuron
An electrical signal travels along the presynaptic neuron.
When it reaches the end of the neuron, it triggers the release of chemical messengers.
Stage 2: Neurotransmitters are released
The chemical messengers are called neurotransmitters.
They are released from the end of the presynaptic neuron into the synaptic gap.
Stage 3: Neurotransmitters cross the synaptic gap
The neurotransmitters move across the small gap between the neurons.
The signal is chemical while it crosses the synapse.
Stage 4: Neurotransmitters bind to receptor sites
The neurotransmitters bind to matching receptor sites on the postsynaptic neuron.
A receptor site responds to a particular type of chemical messenger.
Stage 5: The postsynaptic neuron is affected
Binding changes the electrical state of the postsynaptic neuron.
The neurotransmitter may have:
An excitatory effect.
An inhibitory effect.
This affects how likely the postsynaptic neuron is to produce an electrical signal.
Presynaptic terminalSynaptic cleftPostsynaptic cell
Synapse is ready
The sending cell has neurotransmitter ready to release.
The complete sequence
The process can be summarised as:
An electrical signal travels along the presynaptic neuron.
The signal reaches the end of the neuron.
Neurotransmitters are released.
The neurotransmitters cross the synaptic gap.
They bind to receptor sites on the postsynaptic neuron.
The postsynaptic neuron becomes more or less likely to produce an electrical signal.
Stage | Main event |
1 | Electrical signal reaches the presynaptic ending |
2 | Neurotransmitters are released |
3 | Neurotransmitters cross the synaptic gap |
4 | Neurotransmitters bind to postsynaptic receptors |
5 | The electrical state of the postsynaptic neuron changes |
6 | Excitation or inhibition affects further transmission |
A strong examination answer presents these events in the correct order.
The role of neurotransmitters
A neurotransmitter is a chemical messenger that carries information across a synapse.
Its role is to:
Be released from the presynaptic neuron.
Cross the synaptic gap.
Bind to receptor sites.
Affect the postsynaptic neuron.
Influence whether the neural signal continues.
Without neurotransmitters, information could not cross the gap between neurons in this way.
Neurotransmitters and receptor sites
Neurotransmitters affect the postsynaptic neuron by binding to receptor sites.
The receptor must be able to respond to the neurotransmitter.
A useful simplified comparison is that the neurotransmitter and receptor need to fit together appropriately.
However, avoid writing only that neurotransmitters work “like a key in a lock”. This comparison may help you remember the idea, but an examination answer should use the correct terminology:
Neurotransmitters cross the synaptic gap and bind to specific receptor sites on the postsynaptic neuron.
What happens after binding?
When neurotransmitters bind to receptor sites, they affect the electrical state of the postsynaptic neuron.
The effect may be excitatory or inhibitory.
An excitatory effect increases the likelihood that the postsynaptic neuron will produce an electrical signal.
An inhibitory effect decreases the likelihood that the postsynaptic neuron will produce an electrical signal.
The neurotransmitter does not physically turn into an electrical signal. Instead, its binding changes the receiving neuron in a way that affects electrical activity.
Excitation
Excitation occurs when a neurotransmitter increases the likelihood that the postsynaptic neuron will produce an electrical signal.
An excitatory neurotransmitter makes the receiving neuron more likely to become active.
The sequence is:
Excitatory neurotransmitter binds → postsynaptic neuron becomes more likely to produce an electrical signal
Excitation therefore supports the continuation of neural communication.
For example, if the postsynaptic neuron is a motor neuron, excitation may contribute to an instruction continuing towards a muscle.
Inhibition
Inhibition occurs when a neurotransmitter decreases the likelihood that the postsynaptic neuron will produce an electrical signal.
An inhibitory neurotransmitter makes the receiving neuron less likely to become active.
The sequence is:
Inhibitory neurotransmitter binds → postsynaptic neuron becomes less likely to produce an electrical signal
Inhibition can reduce or prevent further transmission through that pathway.
Comparing excitation and inhibition
Excitation | Inhibition |
Makes the postsynaptic neuron more likely to produce an electrical signal | Makes the postsynaptic neuron less likely to produce an electrical signal |
Supports further neural activity | Reduces further neural activity |
Increases the probability of transmission continuing | Decreases the probability of transmission continuing |
Has an activating effect | Has a limiting effect |
The terms do not describe whether a behaviour is desirable.
An excitatory effect is not automatically “good”, and an inhibitory effect is not automatically “bad”. They describe the effect on neural activity.
Excitation and inhibition work together
A postsynaptic neuron may receive chemical messages from several neurons.
Some messages may be excitatory, while others may be inhibitory.
The overall effect depends on the balance of the input received.
Pattern of input | Likely effect |
Excitatory influence is stronger | The postsynaptic neuron is more likely to produce an electrical signal |
Inhibitory influence is stronger | The postsynaptic neuron is less likely to produce an electrical signal |
Excitatory and inhibitory influences are balanced | Further activity may be limited, depending on the combined effect |
This balance allows neural activity to be regulated rather than every incoming message automatically producing a response.
Why inhibition is important
The nervous system needs to control as well as activate responses.
If every chemical message automatically excited the next neuron:
Neural activity could continue without sufficient regulation.
Unnecessary responses could be produced.
Different signals could interfere with one another.
The nervous system would have difficulty controlling activity.
Inhibition contributes to the regulation of communication by reducing the likelihood of activity in particular pathways.
Why excitation is important
Excitation allows information to continue through a neural pathway.
For example:
A receptor detects a stimulus.
A sensory neuron carries the information towards the CNS.
Neurons communicate across synapses.
Excitatory effects help the relevant pathway continue.
A motor instruction may eventually reach an effector.
A response is produced.
Excitation is therefore important in transmitting information and coordinating behaviour.
Direction of synaptic transmission
Synaptic transmission normally occurs in one direction:
Presynaptic neuron → synaptic gap → postsynaptic neuron
This direction is maintained because:
Neurotransmitters are released from the presynaptic side.
Receptor sites receive them on the postsynaptic side.
The terms presynaptic and postsynaptic are therefore based on the direction of communication.
A neuron may be postsynaptic at one synapse and presynaptic at another. Its label depends on its position within that particular junction.
One-way communication through a pathway
Consider three neurons:
Neuron A → Neuron B → Neuron C
At the first synapse:
Neuron A is presynaptic.
Neuron B is postsynaptic.
At the next synapse:
Neuron B is presynaptic.
Neuron C is postsynaptic.
This shows why presynaptic and postsynaptic describe a neuron’s role at a specific synapse rather than a permanent neuron type.
Synaptic transmission between neuron types
The same general process can occur between different neuron types.
Sensory neuron to relay neuron
A sensory neuron carries information towards the CNS and communicates with a relay neuron across a synapse.
Relay neuron to another neuron
A relay neuron passes information within the CNS and may communicate with another relay neuron or a motor neuron.
Relay neuron to motor neuron
A relay neuron communicates with a motor neuron, allowing an instruction to travel away from the CNS.
The sequence can be represented as:
Sensory neuron → synapse → relay neuron → synapse → motor neuron
The detailed functions of these cells can be reviewed through incoming, connecting and outgoing neural communication.
Applying synaptic transmission to a response
Consider the following situation:
Finley touches a hot object and quickly moves his hand away.
The pathway involves several stages:
Receptors detect the heat.
A sensory neuron carries an electrical signal towards the CNS.
At the end of the sensory neuron, neurotransmitters are released.
The neurotransmitters cross the synapse.
They bind to receptors on a relay neuron.
The relay neuron communicates with a motor neuron across another synapse.
The motor neuron carries an instruction towards a muscle.
The muscle produces the response.
The transmission within and between neurons therefore contributes to the coordinated response.
A method for answering application questions
Use the following structure.
1. Identify the presynaptic neuron
State which neuron is sending the information.
2. Describe the electrical signal arriving
Explain that the signal reaches the end of the presynaptic neuron.
3. Explain neurotransmitter release
State that neurotransmitters are released into the synaptic gap.
4. Explain movement across the gap
State that the neurotransmitters cross the synapse.
5. Identify the postsynaptic neuron
Explain that the neurotransmitters bind to receptor sites on the receiving neuron.
6. Explain the effect
State whether the neurotransmitter makes further activity more or less likely.
For example:
When the signal reaches the end of the sensory neuron, neurotransmitters are released into the synaptic gap. They cross the gap and bind to receptor sites on the relay neuron. If their effect is excitatory, the relay neuron becomes more likely to produce an electrical signal.
Worked application example
A motor neuron receives excitatory and inhibitory chemical messages. The overall excitatory influence is stronger.
A developed explanation would state:
Neurotransmitters are released by presynaptic neurons.
They cross synaptic gaps and bind to receptors on the motor neuron.
Excitatory messages make the motor neuron more likely to produce an electrical signal.
Inhibitory messages make it less likely.
Because the overall excitatory influence is stronger, the motor neuron is more likely to become active.
An instruction may then continue towards an effector.
Synaptic transmission and the central nervous system
Synaptic transmission allows neurons in the central nervous system to communicate.
For example, relay neurons within the brain and spinal cord transmit information through networks containing many synapses.
The CNS can therefore:
Receive information.
Pass information between neurons.
Coordinate responses.
Send instructions through motor pathways.
The organisation of the CNS and PNS is reviewed in central and peripheral nervous-system divisions.
Synaptic transmission and the biological approach
The biological approach explains behaviour partly through biological structures and neurochemistry.
Synaptic transmission illustrates both:
Neurons are biological structures.
Neurotransmitters are involved in neurochemistry.
Chemical activity affects communication within the nervous system.
Neural communication can influence behaviour.
This connects the topic with biological structures and neurochemistry.
Neural and endocrine communication
Neural communication should be distinguished from endocrine communication.
Neural communication | Endocrine communication |
Uses neurons and neurotransmitters | Uses glands and hormones |
Neurotransmitters cross synapses | Hormones travel through the bloodstream |
Connects cells through neural pathways | Communicates with target parts of the body |
Includes excitation and inhibition | Involves hormonal effects |
Forms part of the nervous system | Forms part of the endocrine system |
Both are biological communication systems, but they use different messengers and pathways.
This comparison is developed in nervous and endocrine communication.
Synaptic transmission and fight or flight
Synaptic communication contributes to the nervous-system activity involved in responding to demanding or threatening situations.
Information must be transmitted:
From sensory systems.
Through the central nervous system.
Along pathways controlling bodily responses.
The nervous system also works with the endocrine system during the automatic response to threat.
This shows how synaptic transmission forms one part of a wider coordinated biological response.
Writing an effective description of synaptic transmission
A strong description might state:
Synaptic transmission begins when an electrical signal reaches the end of a presynaptic neuron. This triggers the release of neurotransmitters into the synaptic gap. The neurotransmitters cross the gap and bind to receptor sites on the postsynaptic neuron. Their effect may be excitatory, making the postsynaptic neuron more likely to produce an electrical signal, or inhibitory, making it less likely.
This answer:
Uses the correct sequence.
Names the presynaptic and postsynaptic neurons.
Explains the role of neurotransmitters.
Distinguishes excitation from inhibition.
Connects chemical transmission with electrical activity.
What a complete answer needs
For a question asking you to describe synaptic transmission, include:
An electrical signal arriving at the presynaptic ending.
The release of neurotransmitters.
Movement across the synaptic gap.
Binding to postsynaptic receptor sites.
An effect on the postsynaptic neuron.
For a question asking you to explain excitation and inhibition, include:
The direction of the effect.
The likelihood of the postsynaptic neuron producing an electrical signal.
Avoid giving only vague statements such as “excitation turns the neuron on”.
Key Words 🔑
Key word | Student-friendly definition | How it may be used in an exam |
Synapse | The junction between two neurons. | Identify where chemical transmission occurs. |
Synaptic transmission | The process through which information passes from one neuron to another across a synapse. | Describe neural communication in sequence. |
Presynaptic neuron | The neuron sending the signal across a particular synapse. | Identify where neurotransmitters are released. |
Postsynaptic neuron | The neuron receiving the chemical message at a particular synapse. | Identify where receptor sites are located. |
Synaptic gap | The small space between the presynaptic and postsynaptic neurons. | Explain where neurotransmitters travel. |
Neurotransmitter | A chemical messenger that carries information across a synapse. | Explain the chemical stage of neural communication. |
Receptor site | A site on the postsynaptic neuron to which a neurotransmitter binds. | Explain how a neurotransmitter affects the receiving neuron. |
Excitation | An effect that makes the postsynaptic neuron more likely to produce an electrical signal. | Distinguish activating and limiting influences. |
Inhibition | An effect that makes the postsynaptic neuron less likely to produce an electrical signal. | Explain how neural activity may be reduced. |
Electrical signal | Information transmitted electrically along a neuron. | Distinguish communication within and between neurons. |
Chemical transmission | Communication using neurotransmitters across a synapse. | Explain why the signal changes form at the synapse. |
Sensory neuron | A neuron carrying information from receptors towards the CNS. | Apply synaptic transmission to incoming information. |
Relay neuron | A neuron passing information within the CNS. | Apply synaptic transmission within the brain or spinal cord. |
Motor neuron | A neuron carrying instructions from the CNS towards an effector. | Apply synaptic transmission to an outgoing response. |
Effector | A part of the body that produces a response. | Complete an applied neural pathway. |
Common Mistakes ⚠️
Mistake: Saying that neurons directly touch one another.
Why this is incorrect:There is a small synaptic gap between the presynaptic and postsynaptic neurons.
How to improve:Explain that neurotransmitters must cross the gap between the neurons.
Mistake: Saying that neurotransmitters travel electrically across the synapse.
Why this is incorrect:Communication along a neuron is electrical, while transmission across the synaptic gap is chemical.
How to improve:Use the sequence: electrical, chemical, electrical.
Mistake: Saying that neurotransmitters are electrical impulses.
Why this is incorrect:Neurotransmitters are chemical messengers.
How to improve:Explain that they are released when an electrical signal reaches the presynaptic ending.
Mistake: Reversing the presynaptic and postsynaptic neurons.
Why this is incorrect:The presynaptic neuron sends the signal, while the postsynaptic neuron receives its effect.
How to improve:Remember: pre means before the synapse, post means after it.
Mistake: Saying that receptors release neurotransmitters.
Why this is incorrect:Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron.
How to improve:Separate the two roles clearly: release before, receptors after.
Mistake: Describing excitation as a positive emotion.
Why this is incorrect:Excitation refers to an increased likelihood of activity in the postsynaptic neuron.
How to improve:Define it in terms of neural firing rather than feelings or behaviour.
Mistake: Describing inhibition as stopping all activity permanently.
Why this is incorrect:Inhibition reduces the likelihood that the postsynaptic neuron will produce an electrical signal.
How to improve:Use the phrase “less likely” rather than claiming that activity must stop completely.
Mistake: Saying that excitatory neurotransmitters are always beneficial.
Why this is incorrect:Excitatory describes the effect on neural activity, not whether the outcome is helpful.
How to improve:Keep biological activity separate from value judgements.
Mistake: Listing the stages of transmission in the wrong order.
Why this is incorrect:Neurotransmitters cannot bind to postsynaptic receptors before they have been released and crossed the gap.
How to improve:Learn the sequence from electrical arrival to postsynaptic effect.
Mistake: Naming neurotransmitters without explaining their role.
Why this is incorrect:The learning objective requires an explanation of how chemical messengers transmit information.
How to improve:State where they are released, where they travel and what they bind to.
Mistake: Describing transmission across one synapse without explaining its effect on the next neuron.
Why this is incorrect:The process is incomplete until the effect on the postsynaptic neuron has been explained.
How to improve:Finish by referring to excitation or inhibition.
Exam-Style Questions ✍️
Question 1
Which one of the following best describes a neurotransmitter?
A. An electrical signal travelling along a neuron
B. A chemical messenger carrying information across a synapse
C. A receptor that detects an external stimulus
D. A division of the peripheral nervous system
[1 mark]
Question 2
Define the term synapse.
[2 marks]
Question 3
Distinguish between a presynaptic neuron and a postsynaptic neuron.
[2 marks]
Question 4
Describe the process of synaptic transmission.
[4 marks]
Question 5
Explain one difference between excitation and inhibition.
[4 marks]
Question 6
Place the following stages of synaptic transmission in the correct order:
Neurotransmitters bind to receptor sites.
An electrical signal reaches the end of the presynaptic neuron.
The postsynaptic neuron becomes more or less likely to produce an electrical signal.
Neurotransmitters cross the synaptic gap.
Neurotransmitters are released.
[3 marks]
Question 7
A sensory neuron communicates with a relay neuron.
Explain how information passes from the sensory neuron to the relay neuron.
[4 marks]
Question 8
A motor neuron receives both excitatory and inhibitory chemical messages. The overall inhibitory influence is stronger.
Explain how this may affect the activity of the motor neuron.
[4 marks]
Question 9
Maya touches a hot surface and moves her hand away.
Using your knowledge of neuron types and synaptic transmission, explain how information is transmitted through the nervous system to produce Maya’s response.
[6 marks]
Question 10
Explain synaptic transmission. Refer to neurotransmitters, excitation and inhibition in your answer.
[8 marks]
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