The CNS, neurons and synaptic transmission

The central nervous system

The central nervous system (CNS) is the brain and the spinal cord. The brain makes decisions and controls behaviour; the spinal cord carries messages between the brain and the rest of the body (the peripheral nervous system).

The CNS is built from neurons: cells that carry electrical signals. Messages pass between neurons chemically, using neurotransmitters. Behaviour such as aggression, mood and sleep depends on how this signalling works.

Structure of a neuron

• Dendrites receive signals from other neurons.
• The cell body contains the nucleus and keeps the cell alive.
• The axon carries the electrical signal (the action potential) away from the cell body. Many axons are covered by a fatty myelin sheath, which speeds up the signal.
• Terminal buttons at the end of the axon hold vesicles filled with neurotransmitter.

The action potential is all or none: once the neuron reaches its threshold it fires fully.

Synaptic transmission

The gap between two neurons is the synaptic cleft. The process:

1. The action potential reaches the terminal button of the presynaptic neuron.

2. Vesicles fuse with the membrane and release neurotransmitter into the cleft.

3. Neurotransmitter crosses the cleft and binds to matching receptors on the postsynaptic neuron (like a key in a lock).

4. The message is excitatory (makes the next neuron more likely to fire, e.g. glutamate) or inhibitory (less likely to fire, e.g. GABA; serotonin is usually described as inhibitory).

5. The cleft is cleared by reuptake into the presynaptic neuron or by enzymes breaking the neurotransmitter down.

Transmission is one way only: from presynaptic to postsynaptic.

Neurotransmitters and behaviour; evaluation

• Serotonin is linked to mood, sleep and impulse control. Low serotonin is linked to poorer control of impulsive, aggressive behaviour.
• Dopamine is linked to reward and motivation.
• The enzyme monoamine oxidase A, made by the MAOA gene, breaks down serotonin, dopamine and noradrenaline (see the genes lesson).

Evaluation: neurotransmitter explanations are objective and can be measured, and drugs that change neurotransmitter levels give practical applications. But they are reductionist: much of the evidence is correlational or from animals, and it is hard to show whether a chemical imbalance causes behaviour or results from it.

Worked example

Explain how a message passes from one neuron to the next.

1. The action potential travels down the axon to the terminal button.
2. Vesicles release neurotransmitter into the synaptic cleft.
3. The neurotransmitter binds to receptors on the postsynaptic neuron, making it more or less likely to fire.
4. Leftover neurotransmitter is removed by reuptake or broken down by enzymes.

Answer: Electrical signal to terminal, chemical release into the cleft, binding to receptors, then reuptake or breakdown.

Check yourself

1. Which structures make up the central nervous system?
   • The brain and the spinal cord
   • The brain and the sense organs
   • The spinal cord and the nerves to the muscles
   • The heart and the brain
   Show answer

   The brain and the spinal cord. The CNS is the brain plus the spinal cord; everything else is peripheral.

2. Which part of a neuron receives incoming signals?
   • Axon
   • Terminal buttons
   • Dendrites
   • Myelin sheath
   Show answer

   Dendrites. Dendrites receive; the axon sends.

3. What does the myelin sheath do?
   • Speeds up the electrical signal along the axon
   • Stores neurotransmitter
   • Breaks down serotonin
   • Receives messages from other neurons
   Show answer

   Speeds up the electrical signal along the axon. Myelin insulates the axon so the signal travels faster.

4. Neurotransmitter is released into the
   • cell body
   • myelin sheath
   • spinal cord
   • synaptic cleft
   Show answer

   synaptic cleft. The cleft is the gap between the presynaptic and postsynaptic neurons.

5. A neurotransmitter makes the postsynaptic neuron less likely to fire. Its effect is
   • excitatory
   • inhibitory
   • all or none
   • reuptake
   Show answer

   inhibitory. Inhibitory messages reduce the chance of firing; excitatory increase it.

6. What is reuptake?
   • The postsynaptic neuron releasing neurotransmitter
   • An enzyme building neurotransmitter
   • The action potential travelling back to the dendrites
   • The presynaptic neuron reabsorbing neurotransmitter from the cleft
   Show answer

   The presynaptic neuron reabsorbing neurotransmitter from the cleft. Reuptake recycles neurotransmitter back into the presynaptic neuron.

7. Describe the process of synaptic transmission.
   Show answer

   When the action potential reaches the terminal button, vesicles release neurotransmitter into the synaptic cleft. It binds to receptors on the postsynaptic neuron, which then becomes more or less likely to fire. The neurotransmitter is removed by reuptake or broken down by enzymes.. Release, cross, bind, then excite or inhibit and clear the cleft.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification, Issue 5 (February 2021)
• Pearson IAL Psychology component guide: WPS02 (Issue 2, November 2024)
• Simply Psychology: What happens at the synapse between two neurons?

Last updated 8 November 2026. Spotted a mistake? Tell us.