Neurons, Synapses and the Central Nervous System
Neurons and the Central Nervous System
Every thought, feeling and movement depends on a network of specialised cells called neurons. Neurons pass messages around the body using electrical signals inside the cell and chemical signals between cells. Psychologists study neurons because understanding how these messages travel helps explain how we sense the world, learn, and react to danger.
Sensory neurons carry information from the body’s sense organs — the eyes, ears, nose, skin and taste receptors — towards the central nervous system. When you touch a hot pan, 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 central nervous system. They receive signals from sensory neurons, process them, and pass instructions on to motor neurons or other relay neurons. Most of the neurons found in the brain are relay neurons, making them central to thinking, memory and decision-making.
Motor neurons carry instructions from the central nervous system out to the muscles and glands. When you decide to pull 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 carry information around the body: sensory (sense organs → CNS), relay (within the CNS), motor (CNS → muscles/glands). The typical sequence when responding to a stimulus is sensory → relay → motor.
The Role of the Central Nervous System
The central nervous system (CNS) is made up of the brain and the spinal cord. It is the body’s control centre: it receives signals carried in by sensory neurons, processes and interprets that information, and sends out instructions through motor neurons.
The spinal cord is not simply a cable connecting the brain to the body — it can also organise simple, fast responses on its own, called reflexes, without needing input from the brain. This is why pulling your hand away from something hot happens almost instantly: the relay neurons involved are in the spinal cord, so the signal does not have to travel all the way to the brain and back before the muscles respond. The brain remains responsible for more complex processing, such as recognising danger, remembering past experiences and planning deliberate movements.
Synapses and Synaptic Transmission
Neurons do not touch each other directly. The tiny gap between two neurons is called a synapse. When an electrical signal reaches the end of a neuron, it triggers the release of neurotransmitters — chemical messengers stored in small packages 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 encourages a new electrical signal to start in the receiving neuron or prevents one from firing. Once the message has been passed on, 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, this is called excitation. Excitatory neurotransmitters increase the chance that the signal continues along the pathway.
When binding makes the receiving neuron less likely to fire, this is called inhibition. Inhibitory neurotransmitters act as brakes, stopping unwanted signals from spreading. The nervous system depends on a constant balance between excitation and inhibition to work properly.
Neurotransmitters and Behaviour
Different neurotransmitters have different effects depending on which neurons they act upon. Dopamine is released during pleasurable experiences and reward-seeking behaviour, reinforcing the actions that led to the reward. Serotonin contributes to feelings of well-being and emotional stability.
These neurotransmitters matter to psychologists because many mental health conditions, including depression, are linked to imbalances in neurotransmitter levels. A class of drugs called SSRIs (Selective Serotonin Reuptake Inhibitors) works by blocking the reuptake of serotonin, so it remains in the synapse for longer and continues to stimulate the post-synaptic receptor. This shows how a change at the level of a single synapse can have wide-reaching effects on mood and behaviour.
Synaptic transmission sequence: an action potential arrives at the pre-synaptic neuron → neurotransmitters are released → they cross the synapse → they bind to post-synaptic receptors, causing excitation or inhibition → reuptake clears the synapse for the next signal.
A single neuron rarely acts alone. Each neuron can form synapses with thousands of other neurons, so a signal travelling through the nervous system passes through a whole network of sensory, relay and motor neurons, each connected by synapses. Whether a neuron fires depends on the combined effect of all the excitatory and inhibitory signals it receives at any moment — not just one synapse in isolation.
This network structure is why the CNS can process complex information: a single sensory signal, such as a painful stimulus, can be relayed through many synapses, compared against past experience, and used to produce a precise, coordinated motor response — all within a fraction of a second.