Types of neurons | AQA A-Level Psychology Revision
- Revision Notes
- Aug 3
- 12 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 Types of neurons A-Level Psychology revision notes explain the functions of sensory, relay and motor neurons and how they work together to transmit information through the nervous system. You will follow the pathway from detecting a stimulus to producing a response, while distinguishing the role played by each neuron type. This lesson builds on the organisation of the central and peripheral systems and prepares you to study communication across the gap between neurons. The AQA specification requires knowledge of the structure and function of all three neuron types.
Learning Objectives 🎯
By the end of this revision page, you should be able to:
Describe the function of a sensory neuron.
Describe the functions of relay and motor neurons.
Distinguish between sensory, relay and motor neurons.
Explain how the three neuron types work together.
Apply the neural pathway to an unfamiliar situation.
Identify the role of the central nervous system within neural communication.
Revision Notes 📚
Types of neurons A-Level Psychology revision overview
A neuron is a specialised cell involved in transmitting information through the nervous system.
The three neuron types required by the AQA specification are:
Sensory neurons
Relay neurons
Motor neurons
They perform different functions but work together as part of a communication pathway.
The basic sequence is:
Stimulus → sensory neuron → relay neuron → motor neuron → response
A response depends on information:
Being detected.
Travelling towards the central nervous system.
Being passed through the central nervous system.
Travelling from the central nervous system to the part of the body producing the response.
Structure of neurons
The specification requires students to know the structure and function of sensory, relay and motor neurons. However, the supplied specification does not identify or define the individual anatomical features of each neuron type.
The provided source material does not contain enough information to complete a detailed account of neuron structure accurately.
These notes therefore focus on the specification-supported functional differences between sensory, relay and motor neurons and the way they form a communication pathway. More detailed anatomical terminology should be added only from an approved course source.
The function of neurons
Neurons allow information to travel through the nervous system.
This communication connects:
Sensory information from the body.
The brain and spinal cord.
Muscles or other parts of the body that produce responses.
Different neurons carry information through different parts of this pathway.
Neuron type | Main function |
Sensory neuron | Carries information from sensory receptors towards the central nervous system |
Relay neuron | Passes information between neurons within the central nervous system |
Motor neuron | Carries information from the central nervous system to an effector |
Understanding the direction in which information travels is the easiest way to distinguish the three types.
Sensory neurons
A sensory neuron carries information from a sensory receptor towards the central nervous system.
A sensory receptor detects a change or stimulus.
The information then travels towards the brain or spinal cord.
The sequence is:
Stimulus → receptor → sensory neuron → central nervous system
Examples of sensory information could involve:
Light being detected.
A sound being detected.
Contact with an object.
A change in temperature.
Pressure on the body.
The sensory neuron does not make a decision about how to respond. Its function is to carry the detected information towards the central nervous system.
Sensory neurons and the peripheral nervous system
Sensory neurons form part of the communication between the body and the central nervous system.
They carry information from receptors located outside the brain and spinal cord towards the central nervous system.
This links sensory neurons with the peripheral nervous system, which connects the central nervous system with the rest of the body.
For example:
A person touches a hot object.
Receptors detect the heat.
Information travels along a sensory neuron.
The information reaches the central nervous system.
The divisions involved in this communication can be reviewed through the CNS and PNS.
Relay neurons
A relay neuron passes information between neurons within the central nervous system.
Relay neurons are found within the brain and spinal cord.
They form a connection between incoming sensory information and outgoing motor information.
The sequence is:
Sensory neuron → relay neuron → motor neuron
The relay neuron does not carry the information from the receptor itself, and it does not directly produce the muscular response. It passes information within the central nervous system.
The role of relay neurons
Relay neurons allow communication to occur between different parts of a neural pathway.
When sensory information enters the central nervous system, a relay neuron can pass that information to another neuron.
For example:
A sensory neuron carries information towards the spinal cord.
The sensory neuron communicates with a relay neuron.
The relay neuron passes the information within the central nervous system.
The information is communicated to a motor neuron.
Relay neurons are therefore sometimes described as connecting neurons because they connect incoming and outgoing communication.
Relay neurons and the central nervous system
Relay neurons are associated with the central nervous system.
Remember:
The brain and spinal cord form the central nervous system.
Relay neurons pass information within these central structures.
Sensory and motor neurons connect central processing with the rest of the body.
This distinction is useful in examination questions asking students to identify where a particular neuron functions.
Neuron type | Relationship with the CNS |
Sensory neuron | Carries information towards the CNS |
Relay neuron | Carries information within the CNS |
Motor neuron | Carries information away from the CNS |
Motor neurons
A motor neuron carries information from the central nervous system to an effector.
An effector is a part of the body that produces a response.
The sequence is:
Central nervous system → motor neuron → effector → response
For example, a motor neuron may carry an instruction that results in a muscle producing movement.
The motor neuron carries the outgoing instruction. It does not detect the original stimulus.
Motor neurons and responses
Motor neurons allow the central nervous system to produce a physical response.
For example:
The central nervous system processes information.
An instruction travels along a motor neuron.
The instruction reaches a muscle.
The muscle produces a movement.
Possible responses include:
Moving a hand.
Turning the head.
Stepping away from an object.
Contracting a muscle.
The motor neuron is named after its role in producing motor activity or movement.
Motor neurons and the peripheral nervous system
Motor neurons carry information from the central nervous system towards other parts of the body.
They therefore contribute to the peripheral nervous system’s role in connecting the brain and spinal cord with the rest of the body.
Information can travel through the peripheral nervous system in both directions:
Towards the CNS: through sensory neurons.
Away from the CNS: through motor neurons.
This creates a continuous communication pathway between the body and the central nervous system.
Comparing sensory, relay and motor neurons
Feature | Sensory neuron | Relay neuron | Motor neuron |
Main function | Carries sensory information | Connects neurons within the CNS | Carries an instruction to an effector |
Direction of information | Towards the CNS | Within the CNS | Away from the CNS |
Starting point | Sensory receptor | Another neuron in the CNS | The CNS |
End point | CNS | Another neuron in the CNS | Effector |
Role in the pathway | Input | Connection and processing pathway | Output |
A useful memory rule is:
Sensory = information in
Relay = information across
Motor = instruction out
How the three neuron types work together
The three types of neuron form a coordinated pathway.
Stage 1: A stimulus is detected
A receptor detects a change inside or outside the body.
This produces sensory information that must be transmitted towards the central nervous system.
Stage 2: The sensory neuron carries information
The sensory neuron carries the information from the receptor towards the brain or spinal cord.
Stage 3: A relay neuron passes information within the CNS
The sensory information is communicated to a relay neuron.
The relay neuron passes the information through the central nervous system.
Stage 4: A motor neuron carries an instruction
Information is communicated to a motor neuron.
The motor neuron carries an instruction away from the central nervous system.
Stage 5: An effector produces a response
The motor neuron communicates with an effector.
The effector then produces a response, such as muscular movement.
The complete neural pathway
The full pathway can be represented as:
Stimulus → receptor → sensory neuron → relay neuron → motor neuron → effector → response
Each stage has a separate role.
Stage | Function |
Stimulus | A detectable change occurs |
Receptor | Detects the stimulus |
Sensory neuron | Carries information towards the CNS |
Relay neuron | Passes information within the CNS |
Motor neuron | Carries information away from the CNS |
Effector | Produces the response |
Response | The resulting action or bodily change |
⚠️ The information does not travel from a motor neuron to a sensory neuron. Sensory neurons carry incoming information, while motor neurons carry outgoing instructions.
Communication between neurons
The neurons in a pathway do not function as one uninterrupted cell.
Information must pass from one neuron to another.
For example:
A sensory neuron communicates with a relay neuron.
A relay neuron communicates with a motor neuron.
The process by which information passes between neurons is called synaptic transmission.
This involves chemical communication across the gap between neurons and is covered fully in the process of synaptic communication.
Applying the pathway to a simple response
Consider the following situation:
While cooking, Owen touches a very hot baking tray and quickly moves his hand away.
The neural pathway can be applied as follows:
Heat from the tray acts as the stimulus.
Receptors detect the heat.
A sensory neuron carries information towards the central nervous system.
A relay neuron passes the information within the central nervous system.
A motor neuron carries an instruction away from the central nervous system.
A muscle acts as the effector.
The muscle contracts, moving Owen’s hand away.
A strong application answer uses both specialist terminology and details from the scenario.
Applying the pathway to sensory information and movement
Consider another example:
Erin hears her name being called and turns her head towards the sound.
This can be explained through the following sequence:
Receptors detect the sound.
A sensory neuron carries information towards the central nervous system.
Information is passed through the central nervous system.
A motor neuron carries instructions to muscles.
The muscles produce the movement of Erin’s head.
The key distinction is between:
Detecting and carrying information towards the CNS.
Carrying instructions away from the CNS to produce movement.
A method for answering application questions
Use the following structure.
1. Identify the stimulus
What change was detected?
2. Identify the receptor
What detected the stimulus?
3. Apply the sensory neuron
State that it carried information towards the central nervous system.
4. Apply the relay neuron
State that it passed information within the brain or spinal cord.
5. Apply the motor neuron
State that it carried an instruction away from the central nervous system.
6. Identify the effector and response
Name the muscle or other body part producing the response described in the scenario.
For example:
The heat was detected by receptors. A sensory neuron carried this information towards the central nervous system. A relay neuron passed the information within the central nervous system, and a motor neuron carried an instruction to the muscles in the arm. These muscles contracted, moving the hand away.
Neurons and voluntary movement
The three neuron types can contribute to deliberate movement.
For example, when a person chooses to pick up a book:
Sensory information helps the person detect the book’s position.
The central nervous system processes the information.
Motor instructions travel towards the muscles.
The muscles produce the movement.
The somatic nervous system is associated with sensory information and voluntary control of skeletal muscles.
The role of the somatic division can be reviewed through somatic and autonomic communication.
Neurons and automatic bodily responses
Neural communication is also involved in automatic responses.
The autonomic nervous system regulates involuntary internal processes.
In a demanding or threatening situation, neural communication contributes to changes in bodily activity.
The broader nervous and hormonal response is explored through automatic responses to threatening situations.
Neurons and the nervous system
The three neuron types should not be confused with the main divisions of the nervous system.
Divisions of the nervous system | Types of neuron |
Central nervous system | Sensory neuron |
Peripheral nervous system | Relay neuron |
Somatic nervous system | Motor neuron |
Autonomic nervous system |
A division describes a larger organisational part of the nervous system.
A neuron is a cell involved in communication within that system.
For example:
A relay neuron functions within the CNS.
Sensory and motor neurons contribute to communication between the CNS and the body.
Neurons and hormones
Neurons should also be distinguished from hormones.
Neural communication | Hormonal communication |
Involves neurons | Involves hormones |
Information travels through the nervous system | Hormones are released by glands |
Includes sensory, relay and motor neurons | Forms part of the endocrine system |
Neurons communicate across synapses | Hormones travel through the bloodstream |
Hormonal communication is covered separately in glands and hormones.
Writing an effective explanation
A strong explanation connects all three neuron types.
For example:
Sensory neurons carry information from receptors towards the central nervous system. Within the brain or spinal cord, relay neurons pass information between neurons. Motor neurons then carry instructions away from the central nervous system to effectors, which produce a response.
This answer:
States the function of each neuron.
Gives the direction of information.
Identifies the role of the central nervous system.
Explains how the neurons work together.
A weaker answer might simply state that all three neurons “send messages”. This does not distinguish their functions.
Key Words 🔑
Key word | Student-friendly definition | How it may be used in an exam |
Neuron | A specialised cell involved in transmitting information through the nervous system. | Introduce the three types involved in neural communication. |
Sensory neuron | A neuron that carries information from sensory receptors towards the central nervous system. | Identify the neuron carrying incoming information. |
Relay neuron | A neuron that passes information between neurons within the central nervous system. | Explain communication within the brain or spinal cord. |
Motor neuron | A neuron that carries information from the central nervous system to an effector. | Identify the neuron carrying an outgoing instruction. |
Stimulus | A change that can be detected. | Identify what begins the communication pathway. |
Receptor | A structure that detects a stimulus. | Explain where sensory information originates. |
Central nervous system | The brain and spinal cord. | Identify where relay neurons function. |
CNS | The abbreviation for central nervous system. | Use accurate terminology in short-answer questions. |
Effector | A part of the body that produces a response. | Identify where motor information is directed. |
Response | The action or bodily change produced after information has been processed. | Complete the neural pathway. |
Synaptic transmission | The process through which information passes between neurons. | Link neuron types to the next biopsychology lesson. |
Peripheral nervous system | The nerves connecting the CNS with the rest of the body. | Explain the wider system involving sensory and motor neurons. |
Common Mistakes ⚠️
Mistake: Saying that sensory neurons carry instructions to muscles.
Why this is incorrect:Sensory neurons carry information from receptors towards the central nervous system. Motor neurons carry instructions to effectors.
How to improve:Remember: sensory information travels in, motor instructions travel out.
Mistake: Saying that relay neurons carry information from receptors.
Why this is incorrect:Sensory neurons carry information from receptors. Relay neurons pass information within the central nervous system.
How to improve:Associate relay neurons with the brain and spinal cord.
Mistake: Describing motor neurons as detecting stimuli.
Why this is incorrect:Stimuli are detected by receptors, and the information is carried by sensory neurons.
How to improve:Describe motor neurons as carrying outgoing instructions to effectors.
Mistake: Placing the neuron types in the wrong order.
Why this is incorrect:Incoming sensory information must reach the CNS before a motor instruction can travel towards an effector.
How to improve:Learn the sequence:
Sensory → relay → motor
Mistake: Saying that relay neurons are found throughout the peripheral nervous system.
Why this is incorrect:Relay neurons pass information within the central nervous system.
How to improve:Link relay neurons with the brain and spinal cord.
Mistake: Describing an effector as the original detector of a stimulus.
Why this is incorrect:A receptor detects the stimulus. An effector produces the response.
How to improve:Remember: receptor receives, effector acts.
Mistake: Giving the same function for all three neuron types.
Why this is incorrect:Although all are involved in communication, they carry information through different parts of the pathway.
How to improve:Include the starting point, direction and destination for each neuron.
Mistake: Naming the neurons without applying them to the scenario.
Why this is incorrect:Application requires the answer to identify the stimulus, information pathway and response in the named context.
How to improve:Use details such as the heat, sound, movement or muscle described in the question.
Mistake: Confusing neurons with divisions of the nervous system.
Why this is incorrect:Sensory, relay and motor describe types of cell. Central, peripheral, somatic and autonomic describe divisions of the nervous system.
How to improve:State whether the question is asking about a cell type or an organisational division.
Exam-Style Questions ✍️
Question 1
Which one of the following carries information from a receptor towards the central nervous system?
A. A relay neuron
B. A motor neuron
C. A sensory neuron
D. An effector
[1 mark]
Question 2
Define a relay neuron.
[2 marks]
Question 3
State one difference between the functions of sensory and motor neurons.
[2 marks]
Question 4
Explain how sensory, relay and motor neurons work together to produce a response.
[6 marks]
Question 5
A student accidentally touches a hot metal surface and immediately moves their hand away.
Using your knowledge of neuron types, explain how information travels through the nervous system to produce this response.
[6 marks]
Question 6
Complete the table by identifying the correct type of neuron.
Function | Type of neuron |
Carries information towards the CNS | |
Passes information within the CNS | |
Carries an instruction to an effector |
[3 marks]
Question 7
A psychologist presents the following pathway:
Sound detected → Neuron A → CNS → Neuron B → neck muscle contracts
Identify Neuron A and Neuron B. Explain the function of each.
[4 marks]
Question 8
Explain the difference between a receptor and an effector.
Refer to the roles of sensory and motor neurons in your answer.
[4 marks]
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