Neurons and Synaptic Transmission
Types of Neuron
The nervous system works because of specialised cells called neurons. There are three main types, each with a different role in passing information around the body. Psychologists study neurons because understanding how signals travel between them helps explain how we learn, react and behave.
Sensory neurons carry information from the body’s sense organs — the eyes, ears, nose, skin and taste receptors — to the central nervous system. When you touch a hot surface, sensory neurons fire a signal towards the spinal cord and brain before you are even consciously aware of the danger.
Relay neurons act as connectors within the CNS. They receive signals from sensory neurons, process them, and pass them on to motor neurons or other relay neurons. Most of the neurons in the brain are relay neurons, making them central to thinking, memory and decision-making.
Motor neurons carry instructions from the CNS to the muscles and glands. When you decide to move your hand away from the heat, motor neurons carry that instruction from your spinal cord to the muscles in your arm, triggering contraction.
Three neuron types: sensory (sense organs → CNS), relay (connects within CNS), motor (CNS → muscles/glands). The typical sequence when you respond to a stimulus is sensory → relay → motor.
Synaptic Transmission
Neurons do not touch each other. The tiny gap between them is called a synapse. When an electrical signal (called an action potential) reaches the end of a neuron, it triggers the release of neurotransmitters — chemical messengers stored in small packages called vesicles at the end of the sending (pre-synaptic) neuron.
The neurotransmitters drift across the synapse and bind to receptor sites on the receiving (post-synaptic) neuron. This binding either starts a new electrical signal in the receiving neuron or prevents one from firing. Once the signal is passed, the neurotransmitters are either broken down or drawn back into the pre-synaptic neuron in a process called reuptake.
When a neurotransmitter binds to the post-synaptic neuron and makes it more likely to fire an action potential, this is called excitation. Excitatory neurotransmitters “turn up the volume” on neural activity.
When binding makes the receiving neuron less likely to fire, this is called inhibition. Inhibitory neurotransmitters act as “brakes,” preventing unwanted signals from spreading. The nervous system depends on a balance between excitation and inhibition — too much excitation can cause a seizure; too much inhibition can slow responses dangerously.
Neurotransmitters and Behaviour
Different neurotransmitters have different effects depending on which neurons they act upon. Dopamine is involved in the brain’s reward system — it is released during pleasurable experiences and reward-seeking behaviour, reinforcing the actions that led to the reward. This mechanism also underlies addiction. Serotonin contributes to feelings of well-being and emotional stability. These neurotransmitters are relevant to psychology because many mental health conditions — including depression and anxiety — are associated with imbalances in neurotransmitter levels, and many psychiatric medications work by altering the availability of neurotransmitters at the synapse.
For example, a class of antidepressants known as SSRIs (Selective Serotonin Reuptake Inhibitors) works by blocking the reuptake of serotonin. Because serotonin is not drawn back into the pre-synaptic neuron as quickly, it remains in the synapse for longer and continues to stimulate the post-synaptic receptor. This illustrates how a change at the synapse level can have wide-reaching effects on mood and behaviour — demonstrating the direct link between neurochemistry and psychological experience.
Hebb’s Theory of Learning and Neuronal Growth
In 1949, the psychologist Donald Hebb proposed that learning changes the connections between neurons. His core idea is often summarised as: “neurons that fire together, wire together.” This is known as Hebbian learning.
According to Hebb, every time two neurons fire together, the synapse between them is strengthened — the pre-synaptic neuron becomes better at triggering the post-synaptic one. Over repeated activation, new synaptic connections form and existing ones grow stronger. This process, known as synaptic plasticity, is the biological basis of learning and memory. A practical example is learning a musical instrument: initially, playing a scale requires conscious effort as the relevant neural pathways are weak. With practice, the pathways strengthen and the movement becomes fluent and automatic — Hebb’s mechanism in action.
Hebb (1949): when neurons fire repeatedly together, the synapse between them strengthens. This “neurons that fire together, wire together” principle explains how practice and repetition create durable memories and skills.
A strength of Hebb’s theory is that it is supported by later research. The discovery of long-term potentiation (LTP) — a process by which repeated neural activity strengthens synaptic connections — provides direct physiological evidence for Hebb’s mechanism. Brain scanning research confirms that practising a skill increases neural connectivity in the relevant areas.
A weakness is that Hebb’s theory was proposed before the technology to observe synapses in living brains existed, so it was initially theoretical. Additionally, the theory focuses on a “bottom-up” strengthening of connections but does not fully explain how we deliberately choose what to learn or how some memories are forgotten despite repeated practice.