What you'll learn

• How sensory, relay and motor neurons are structured and what each one does.
• How a nerve impulse travels within a neuron and between neurons.
• How neurotransmitters can cause excitation or inhibition.
• How to apply and evaluate this content in AQA A-Level Psychology answers.

The big picture: communication in the nervous system

Biopsychology looks at how biological systems help explain behaviour. A key idea is that the body and brain communicate using specialised cells called neurons.

A message usually travels electrically within a neuron, then chemically across the gap between neurons. This allows the nervous system to respond quickly to changes, such as touching something hot or hearing your name.

The central nervous system, or CNS, is the brain and spinal cord. The peripheral nervous system, or PNS, is the network of nerves outside the CNS.

Basic neuron structure

Although neurons come in different shapes, they share some important parts.

• Dendrites are branch-like structures that receive signals from other neurons or from sensory receptors.
• The cell body, sometimes called the soma, contains the nucleus, which controls the cell’s activities.
• The axon is a long fibre that carries the nerve impulse away from the cell body.
• The myelin sheath is a fatty insulating layer around some axons. It speeds up transmission.
• Nodes of Ranvier are small gaps in the myelin sheath. The impulse effectively jumps between these gaps, making transmission faster.
• Terminal buttons are the endings of the neuron. They help pass the message on to the next neuron, muscle or gland.
• An action potential is a brief electrical impulse that travels along the axon when the neuron has been sufficiently stimulated.

The three main types of neuron

AQA expects you to know the structure and function of sensory neurons, relay neurons and motor neurons.

A stimulus is a change in the environment, such as heat, pressure or light. A receptor is a specialised cell that detects a stimulus. An effector is a muscle or gland that produces a response.

Sensory neurons

A sensory neuron carries information from sensory receptors towards the CNS. For example, receptors in your skin detect heat, and sensory neurons carry that message to the spinal cord or brain.

Relay neurons

A relay neuron, also called an interneuron, is found in the CNS. It connects sensory neurons to motor neurons. Relay neurons are important because they allow information to be processed before a response is produced.

Motor neurons

A motor neuron carries messages away from the CNS to an effector. If the effector is a muscle, the message may cause it to contract. If the effector is a gland, the message may cause it to release a chemical.

Synapses: where neurons communicate

A synapse is the junction between two neurons. More precisely, it includes the end of the first neuron, the tiny gap between neurons, and the membrane of the next neuron.

The neuron sending the message is the presynaptic neuron. The neuron receiving the message is the postsynaptic neuron. The small gap between them is the synaptic cleft.

Synaptic transmission step by step

Synaptic transmission is the process by which neurons communicate across the synapse.

1. An action potential reaches the terminal button of the presynaptic neuron.
2. Tiny sacs called synaptic vesicles release neurotransmitters into the synaptic cleft.
3. The neurotransmitters diffuse across the gap.
4. They bind to specific receptor sites on the postsynaptic membrane. A receptor site is a molecule that only certain neurotransmitters can attach to.
5. This changes the activity of the postsynaptic neuron, making it either more likely or less likely to fire.
6. The neurotransmitter is then removed from the synaptic cleft, for example through reuptake, where it is taken back into the presynaptic neuron, or through enzymatic breakdown, where enzymes break it down.

Synaptic transmission is usually one-way because neurotransmitters are released from the presynaptic side and receptor sites are found on the postsynaptic side.

Neurotransmitters, excitation and inhibition

Examples include dopamine, serotonin, acetylcholine and GABA. You do not need detailed chemistry for this topic, but you should understand that neurotransmitters affect whether the next neuron is likely to fire.

Excitation means the postsynaptic neuron becomes more likely to fire an action potential. Inhibition means it becomes less likely to fire an action potential.

A single neuron can receive many excitatory and inhibitory signals at the same time. Summation is the adding together of these signals. If the overall effect reaches the threshold, meaning the minimum level of stimulation needed, the postsynaptic neuron fires.

Applying synaptic transmission

Understanding synapses helps explain how drugs can affect behaviour. For example, some drugs influence the amount of neurotransmitter in the synaptic cleft, or block receptor sites so neurotransmitters cannot bind.

This is a useful AO2 point because it shows you can apply the biological mechanism to an unfamiliar scenario, such as a drug, toxin or neurological condition.

AO3: evaluating this biological approach

Although this topic is mainly AO1 description, you can still evaluate it when writing broader biopsychology essays.

Strengths

One strength is that neurons and synapses give a scientific and objective explanation of behaviour. Neural communication can be investigated using controlled laboratory methods, brain-recording techniques and drug studies. This gives the explanation credibility because it is based on observable biological processes.

A second strength is real-world application. Many treatments work by changing synaptic activity. For example, some drugs affect reuptake, neurotransmitter release or receptor binding. This shows that understanding synaptic transmission has practical value in medicine and mental health.

Limitations

One limitation is biological reductionism. This means reducing complex behaviour to biological mechanisms alone. Synapses are important, but behaviour is also influenced by thoughts, learning, social context and culture.

Another limitation is that neurotransmitter explanations can be oversimplified. It is tempting to say “low serotonin causes depression” or “dopamine causes pleasure”, but real behaviour involves multiple neurotransmitters, brain areas and environmental factors.

Ethics and methodology

Research into neural processes may involve drug manipulation, brain scanning, animal studies or invasive procedures. In human research, psychologists must consider informed consent, right to withdraw, protection from harm, confidentiality and debriefing. In animal research, researchers must justify the scientific value and minimise suffering.