What you'll learn

• What the central nervous system (CNS) is and how it helps control behaviour.
• The main parts of a neuron and how nerve impulses travel.
• How neurotransmitters carry messages across a synapse.
• How to apply this biological knowledge to human behaviour in AO1, AO2 and AO3 answers.

The nervous system: the starting point

Your nervous system is the body’s communication network. It allows you to sense the world, process information, and respond through actions, thoughts, emotions and bodily changes.

The brain is heavily involved in complex behaviour, including memory, emotion, decision-making, movement and language. The spinal cord carries messages between the brain and body, and can also coordinate very fast responses such as reflexes.

The CNS works with the peripheral nervous system (PNS), which includes the nerves outside the brain and spinal cord. The PNS carries sensory information into the CNS and motor commands out to muscles and glands.

Neurons: the cells that carry messages

Neurons are built for communication. They have several important parts:

• Dendrites receive messages from other neurons.
• The cell body contains the nucleus and keeps the neuron alive.
• The axon carries an electrical impulse away from the cell body.
• The myelin sheath is a fatty insulating layer that speeds up transmission.
• Nodes of Ranvier are small gaps in the myelin sheath where the impulse effectively “jumps” along the axon.
• Axon terminals are the branches at the end of the neuron that pass messages to the next cell.

There are different types of neuron. Sensory neurons carry information from receptors to the CNS. Relay neurons, also called interneurons, connect neurons within the CNS. Motor neurons carry messages from the CNS to muscles or glands.

The message inside a neuron is mainly electrical. The message between neurons is mainly chemical. That distinction is very useful in exam answers.

The synapse: the gap between neurons

At the end of one neuron, the axon terminal does not usually touch the next neuron directly. Instead, there is a small gap called the synaptic cleft.

The neuron sending the message is the presynaptic neuron. The neuron receiving the message is the postsynaptic neuron.

Synaptic transmission happens in a sequence:

1. An action potential reaches the presynaptic axon terminal.
2. Small sacs called synaptic vesicles release neurotransmitter into the synaptic cleft.
3. The neurotransmitter diffuses across the gap.
4. The neurotransmitter binds to receptors on the postsynaptic neuron.
5. This makes the postsynaptic neuron more or less likely to fire its own action potential.
6. The neurotransmitter is removed by reuptake, enzyme breakdown, or diffusion away from the synapse.

Excitation and inhibition

Neurotransmitters do not all have the same effect. Some are mainly excitatory, meaning they make the postsynaptic neuron more likely to fire. Others are mainly inhibitory, meaning they make it less likely to fire.

This matters because behaviour depends on patterns of neural activity. A thought, movement or emotional response is not caused by one single neuron firing. It depends on many neurons communicating in networks.

Common examples include:

Reuptake, breakdown and drug effects

After neurotransmitters bind to receptors, they need to be cleared from the synaptic cleft. If they stayed there indefinitely, the postsynaptic neuron could be stimulated for too long.

Two key removal processes are:

• Reuptake: the neurotransmitter is taken back into the presynaptic neuron.
• Enzyme breakdown: enzymes break the neurotransmitter down into inactive substances.

Some drugs affect synaptic transmission. For example, some antidepressants called selective serotonin reuptake inhibitors (SSRIs) reduce the reuptake of serotonin. This leaves more serotonin available in the synaptic cleft for longer.

Linking the CNS and neurotransmitters to behaviour

In psychology, biological explanations often argue that behaviour is influenced by the activity of the brain, spinal cord, neurons and neurotransmitters.

For example:

• A fear response may involve activity in brain areas linked to threat detection and arousal.
• Movement depends on communication between the CNS and muscles through motor neurons.
• Mood and motivation can be affected by neurotransmitter systems such as serotonin and dopamine.
• Reflexes show that some behaviours can be coordinated rapidly by the spinal cord without conscious planning.

This does not mean behaviour is only biological. Experiences, learning, cognition and social context also matter. In a strong essay, you can show that biological processes are important but not complete explanations by themselves.

AO1, AO2 and AO3: how to write about this topic

AO1: description

For AO1, describe the process accurately and in order. A strong description of synaptic transmission might say:

The action potential travels along the axon to the presynaptic terminal. Neurotransmitters are released from vesicles into the synaptic cleft. They diffuse across the gap and bind to receptors on the postsynaptic membrane. This affects whether the postsynaptic neuron fires. The neurotransmitter is then removed by reuptake or enzyme breakdown.

AO2: application

For AO2, connect the biology to a scenario. If a question describes someone reacting quickly to danger, you could apply the CNS by explaining how sensory information reaches the spinal cord or brain, is processed, and leads to a motor response.

If a question describes a drug that changes mood, you could apply neurotransmission by explaining how the drug affects release, receptor binding, reuptake or breakdown.

AO3: evaluation

A strength of biological explanations is that they are scientific and measurable. Researchers can investigate brain activity, neurotransmitter systems and drug effects using controlled methods. Classic biological work such as Loewi (1921) helped demonstrate chemical transmission between nerve cells, while later brain-imaging research such as Raine et al. (1997) showed how CNS activity could be studied in relation to behaviour.

Another strength is real-world application. Understanding synaptic transmission has helped develop drug treatments, such as antidepressants that affect serotonin reuptake, and treatments for disorders involving dopamine or acetylcholine.

However, biological explanations can be reductionist. They may reduce complex behaviours such as depression, aggression or addiction to neurotransmitters alone, ignoring childhood experience, cognition, culture and social context.

There are also issues of cause and effect. If a person has unusual neurotransmitter activity, this may contribute to behaviour, result from behaviour, or be linked through another factor such as stress, sleep, medication or genes.

Ethically, biological explanations can be helpful because they may reduce blame and stigma. But they can also encourage biological determinism, where people assume behaviour is fixed by the brain and cannot change. Good psychology avoids that oversimplification.