The central nervous system and the structure and role of the neuron
Biological psychology on the Unit 2 paper (WPS02/01) starts here, with the cells that every later idea in this unit depends on.
Section A of WPS02 is Biological psychology, and it is the first 34 marks of a 2-hour, 96-mark paper.
The content to learn is "The role of the central nervous system (CNS) and neurotransmitters in human behaviour, including the structure and role of the neuron, the function of neurotransmitters and synaptic transmission."
This section teaches the CNS and the neuron; the next teaches neurotransmitters and synaptic transmission.
How the Unit 2 paper works
Part of WPS02
Marks
What it holds
Section A, Biological psychology
34
Four short-answer questions (1 to 4 marks a part), then one 8-mark item as the last question
Section B, Learning theories and development
34
The same shape on Topic D
Section C
12 + 16
A 12-mark item on either topic, then a 16-mark item that joins a Topic C idea to a Topic D idea
Front pages
Formulae and statistical tables (standard deviation, Spearman's rho, chi-squared, Wilcoxon)
Allow about 1.25 minutes a mark.
The commands set on this paper are State, Name, Give, Identify, Define, Describe, Explain, Calculate, Determine, Draw, then Discuss, Assess and Evaluate at 8 marks, and Evaluate or To what extent at 12 and 16 marks.
There is no multiple choice.
Aggression (this unit) is the single most examined area of the paper: in the four series from January 2024 to June 2025 it supplied about a quarter of all marks, including a share of every 16-mark question.
The question: how does a thought become an action?
Farid is reversing out of a parking space when another driver hoots at him.
In under a second his face is hot, his hands grip the wheel and he shouts.
Something had to carry the sound of the horn from his ears into his brain, compare it with what he knows about rude drivers, and send instructions to the muscles of his face, chest and hands.
That "something" is the nervous system, built from billions of cells called neurons.
Every explanation of aggression in this unit (brain areas, neurotransmitters, hormones, genes) is in the end a claim about how these cells are built, how they signal and what changes their signals.
The two main parts of the nervous system
The central nervous system (CNS) is the brain and the spinal cord.
It is the control centre: it receives information, combines it with what is stored, makes decisions and sends out commands.
The brain does the complex processing (perceiving a threat, judging whether to strike back, holding back an impulse); the spinal cord carries messages between the brain and the body and handles simple reflexes on its own.
Everything outside the CNS is the peripheral nervous system (PNS).
Its somatic part carries information from the senses to the CNS and commands from the CNS to the skeletal muscles (Farid's grip on the wheel).
Its autonomic part runs the organs and glands without conscious control.
The autonomic system has two branches: the sympathetic branch prepares the body for action (heart rate up, breathing faster, adrenaline released from the adrenal glands) and the parasympathetic branch calms it again.
You will meet the sympathetic branch again in the hormones section, because the arousal it creates is part of how anger becomes aggression.
The diagram below shows the divisions.
The neuron: structure
A neuron is a nerve cell specialised to carry electrical signals.
Its parts, numbered as in the diagram below:
1. Dendrites: short branches that receive signals from other neurons.
2. Cell body (soma): contains the nucleus and keeps the cell alive; it adds up the signals arriving from the dendrites.
3. Nucleus: holds the cell's genetic material (DNA), including the genes that code for the enzymes and receptors the neuron uses.
4. Axon: a long fibre that carries the electrical impulse away from the cell body.
5. Myelin sheath: a fatty insulating layer around many axons that speeds up the impulse.
6. Gaps in the myelin (nodes of Ranvier): the impulse jumps from gap to gap, which is why myelinated axons conduct so fast.
7. Axon terminals: the endings of the axon, where the neuron passes its message to the next cell across a synapse.
Three kinds of neuron
- Sensory neurons carry information from sense organs (the ear hearing the horn) to the CNS.
- Relay neurons (interneurons) sit inside the CNS and connect neurons to each other; most of the brain's neurons are relay neurons.
- Motor neurons carry commands from the CNS to muscles and glands.
The neuron: role
Why is a neuron built like this?
Its job is to receive, decide and pass on.
1. At rest, the inside of the neuron is negatively charged compared with the outside (the resting potential).
2. When enough excitatory signals arrive at the dendrites and cell body, the charge across the membrane flips briefly. This is an action potential, or nerve impulse.
3. The action potential travels down the axon. It is all or none: it either fires at full strength or not at all. A stronger stimulus does not make a bigger impulse; it makes the neuron fire more often.
4. When the impulse reaches the axon terminals, the electrical signal has to cross a tiny gap to reach the next neuron. That crossing is chemical, and it is where neurotransmitters work (the next section).
So the neuron is an electrical cable joined to other cables by chemical links.
This matters for aggression because the chemical links can be strengthened, weakened or disturbed: by drugs, by hormones, and by genes that change the enzymes and receptors at the synapse.
A worked chain: a reflex and a decision
When Farid touches a hot car door handle, a sensory neuron sends an impulse to the spinal cord, a relay neuron there passes it straight to a motor neuron, and his hand pulls away before the brain has noticed: a reflex arc.
Shouting at the other driver is different.
The sound travels by sensory neurons to the brain, where millions of relay neurons in the areas that detect threat and control impulses process it; only then do motor neurons carry commands to his voice and hands.
Aggression is almost never a reflex.
It passes through the brain, which is why brain areas (the next sections) can make it more or less likely.
Connecting the ideas
- To brain structure: the pre-frontal cortex and limbic system are networks of relay neurons; how well they communicate decides how well impulses are controlled.
- To hormones: hormones such as testosterone travel in the blood and change how neurons in the hypothalamus and amygdala respond.
- To genes: the nucleus of every neuron holds genes such as MAOA, which codes for an enzyme that breaks down neurotransmitters at the synapse.
- To learning theories (Topic D): learning by conditioning or observation changes the strength of connections between neurons. Biology and learning are two levels of description of the same brain, a point worth making in a 16-mark answer.
- Central nervous system (CNS): the brain and the spinal cord, which receive information from the body, process it and send out commands.
- Peripheral nervous system (PNS): all the nerves outside the brain and spinal cord, which carry information between the CNS and the senses, muscles, organs and glands.
- Neuron: a nerve cell that carries information as electrical impulses and passes it to other cells by releasing chemicals at synapses.
- Action potential: a brief, all-or-none electrical impulse that travels along the axon of a neuron when it fires.
- Myelin sheath: a fatty layer around an axon that insulates it and speeds up the nerve impulse.
Describe the structure of a neuron.
(4)
Top answer (4 of 4).
1. A neuron has a cell body containing the nucleus, which keeps the cell alive. (AO1 1)
2. Dendrites branch from the cell body and receive signals from other neurons. (AO1 1)
3. A long axon carries the electrical impulse away from the cell body, often covered by a fatty myelin sheath that speeds the impulse up. (AO1 1)
4. At the end of the axon are axon terminals, which pass the message on to the next neuron across a synapse. (AO1 1)
Weaker answer (2 of 4). "A neuron has dendrites, a cell body, an axon and terminals.
It sends messages."
Four parts are named but none is described, so this is close to a list.
Examiners credit developed points: each part with what it is or does.
Outcome: one part per point, each with its function in the same sentence.
Grace is cooking when the oven timer beeps loudly.
She jumps, turns round and shouts at her brother, who has just walked in and startled her.
Describe the role of neurons in Grace's reaction.
(2)
Top answer (2 of 2).
1. Sensory neurons carried the sound of the beeping timer and the sight of her brother from Grace's ears and eyes to her brain. (AO2 1)
2. Relay neurons in her brain processed this as a threat and motor neurons then sent impulses to the muscles that turned her round and made her shout at her brother. (AO2 1)
Generic answer (0 of 2). "Sensory neurons carry information to the CNS and motor neurons carry it to muscles."
True, but nothing about Grace, the timer or the shout.
On a scenario item, generic answers score 0 marks.
Explain one reason why damage to the myelin sheath could affect behaviour.
(2)
Top answer (2 of 2). "The myelin sheath insulates the axon so that impulses jump between the gaps and travel quickly (AO1 1), so if it is damaged, impulses travel slowly or fail, and messages between brain areas, for example from the pre-frontal cortex that holds back impulses, would arrive too late to control a reaction.
(AO3 1)"
Weaker answer (1 of 2). "The myelin sheath speeds up impulses, so damage would slow them down."
The function is right, but "slow them down" only restates it; the answer never says why slower impulses would change behaviour.
The neuron and the CNS were not asked as items in the four series from January 2024 to June 2025, but they can be set at any time, and a Describe [4] on neuron structure is exactly the kind of untested item that appears.
Learn the parts with their functions, and the three kinds of neuron with where they carry information.
In a scenario item, attach each part to a detail: the sound, the brain's processing, the muscles that act.
- Confusing neurons with neurotransmitters. A neuron is a cell; a neurotransmitter is a chemical the neuron releases. "Serotonin neurons" means neurons that release serotonin.
- Thinking a stronger stimulus makes a stronger impulse. The action potential is all or none. Intensity is coded by how often neurons fire and how many fire.
- Putting the spinal cord in the PNS. The CNS is the brain and the spinal cord; nerves running out to the body are the PNS.
- Treating aggression as a reflex. A reflex arc bypasses the brain; aggression involves processing in brain areas, which is why brain explanations make sense.
Check your understanding
1. State the two parts of the central nervous system. (2) Total [2]
2. Explain one reason why the axon of a neuron is covered by a myelin sheath. (2) Total [2]
3. A student says: "If a neuron is stimulated more strongly, it sends a bigger impulse." Explain why this statement is wrong. (2) Total [2]
4. Mateo is playing football. An opponent kicks his ankle hard; Mateo feels a sharp pain, decides the kick was deliberate and pushes the opponent to the ground. Describe the role of sensory, relay and motor neurons in Mateo's behaviour. (3) Total [3]
5. Describe the role of the neuron in human behaviour. (4) Total [4]
6. Leila works in a busy call centre in Amman. When a caller insults her she feels her heart pound and her face go hot. Usually she pauses, takes a breath and answers politely, but after a night of very little sleep she snapped back at a caller and was warned by her manager. Discuss the role of the central nervous system and neurons in Leila's behaviour. You must make reference to the context in your answer. (8) Total [8]
Answers
1. The brain (AO1 1) and the spinal cord (AO1 1).
2. Identification (AO1 1) plus justification (AO3 1). The myelin sheath insulates the axon so the impulse jumps between the gaps in the sheath (1), which makes the impulse travel much faster, so messages reach the brain or muscles quickly enough for rapid responses (1).
3. Identification (AO1 1) plus justification (AO3 1). An action potential is all or none: it fires at full strength or not at all (1), so a stronger stimulus makes the neuron fire more often (or makes more neurons fire), not produce a bigger impulse (1).
4. One mark per point tied to the scenario (AO2 3). Sensory neurons carried the pain from Mateo's ankle to his CNS (1); relay neurons in his brain processed the kick and his judgement that it was deliberate (1); motor neurons carried commands to his arm muscles to push the opponent over (1). Generic answers score 0.
5. One mark per developed point (AO1 4). Neurons carry information around the body as electrical impulses (1); sensory neurons bring information from the senses to the CNS (1); relay neurons in the CNS connect neurons and process information, for example deciding how to respond (1); motor neurons carry commands to muscles and glands so the person acts (1); neurons pass messages to each other by releasing neurotransmitters at synapses (1).
6. AO1 4 + AO2 4, no credit for evaluation. Indicative AO1: CNS as brain and spinal cord, the control centre; sensory, relay and motor neurons; action potentials; synapses and neurotransmitters; the autonomic (sympathetic) branch producing arousal; brain areas such as the pre-frontal cortex holding back impulses. Indicative AO2: sensory neurons carry the caller's insult from Leila's ears to her brain; her sympathetic nervous system raises her heart rate and makes her face hot; relay neurons in her brain, including the frontal areas, usually process the insult and hold back the urge to reply, so she pauses and breathes; motor neurons then carry the commands for a calm, polite reply; after a night without sleep the control processes in her brain work less well, so the impulse to snap back reaches her motor neurons and she answers rudely. Level 4 (7-8): accurate knowledge applied to different parts of Leila's day throughout (insult, pounding heart, usual pause, the snapped reply after no sleep).