AQA Syllabus focus:
'The process of synaptic transmission, including neurotransmitters, excitation and inhibition.'
Synaptic transmission explains how neurons communicate across tiny gaps using chemicals. Understanding this process is essential for explaining how the nervous system activates responses, regulates activity, and prevents uncontrolled firing.
Synaptic transmission
Neurons do not usually touch each other directly. Communication occurs at a synapse, where the end of one neuron is separated from the next by a tiny gap.
Synapse: The junction between neurons where signals pass from one neuron to another across a small gap.
At a synapse, the sending neuron is the presynaptic neuron and the receiving neuron is the postsynaptic neuron. The presynaptic terminal contains vesicles, which store a neurotransmitter.
Neurotransmitter: A chemical messenger released from a presynaptic neuron that carries a signal across a synapse to a postsynaptic neuron.
When an electrical impulse reaches the presynaptic terminal, it triggers the vesicles to release neurotransmitter molecules into the synaptic cleft.
This labeled diagram summarizes the core steps of chemical synaptic transmission. It shows how an arriving nerve impulse opens calcium ion channels, causing vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft. Neurotransmitter binding to receptor sites on the postsynaptic membrane then opens ion channels, producing a postsynaptic electrical change. Source
The molecules then diffuse across the gap and bind to specific receptor sites on the postsynaptic membrane. These receptor sites are selective, so only neurotransmitters with a complementary shape can attach successfully.
Stages in transmission
An action potential arrives at the terminal of the presynaptic neuron.
Synaptic vesicles release neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses across the cleft.
It binds to matching receptor sites on the postsynaptic neuron.
This produces a small electrical change in the postsynaptic neuron.
If the change is strong enough to reach threshold, a new action potential is generated.
Synaptic transmission is mainly chemical, even though communication within a neuron is electrical. This helps explain why transmission across a synapse is slightly slower than conduction along a neuron. Synapses are usually one-way because neurotransmitters are released from the presynaptic side, while receptor sites are located on the postsynaptic side.
Excitation and inhibition
Once neurotransmitters bind to receptors, one possible effect is excitation.
Excitation: The effect of a neurotransmitter that increases the likelihood that the postsynaptic neuron will fire an impulse.
An excitatory neurotransmitter makes the postsynaptic neuron more likely to fire by moving its electrical state closer to threshold. If enough excitatory input is received, the postsynaptic neuron will generate an action potential and the message will continue through the nervous system. Excitation therefore promotes the onward transmission of neural signals.
A second possible effect is inhibition.
Inhibition: The effect of a neurotransmitter that decreases the likelihood that the postsynaptic neuron will fire an impulse.
An inhibitory neurotransmitter makes the postsynaptic neuron less likely to fire by moving its electrical state further from threshold.
This means a new action potential is less likely to begin. Inhibition is just as important as excitation because nervous system activity has to be controlled and regulated rather than allowed to spread without limit.
Balancing signals
Most neurons receive many signals at the same time rather than only one input. Some synapses are excitatory and some are inhibitory. The neuron responds to the overall balance of these influences.
If excitatory influences are stronger than inhibitory influences, the neuron is more likely to reach threshold.
If inhibitory influences outweigh excitatory ones, the neuron is less likely to fire.
If the combined input does not reach threshold, no action potential is produced.
This balancing process allows the nervous system to respond selectively. Important messages can be passed on, while irrelevant or competing activity can be reduced. Without inhibition, neurons would fire too easily, making neural communication less precise and less stable.
Why both are necessary
Excitation and inhibition work together to create controlled neural activity. Excitation allows information to move through neural pathways, but inhibition prevents every signal from automatically producing more firing.
Excitation supports communication, response, and activation.
Inhibition filters activity and reduces unnecessary firing.
Together they make neural processing more accurate and better regulated.
This interaction is essential because behavior, thought, and perception depend on patterns of activity that are carefully coordinated rather than random or excessive.
The importance of neurotransmitters
Neurotransmitters are central to synaptic transmission because they convert one neuron's electrical signal into a chemical message that another neuron can detect. Different neurotransmitters interact with different receptor sites, which helps explain the complexity of communication in the nervous system.
The effect of a neurotransmitter is not determined only by the chemical itself. A key factor is the receptor site it binds to on the postsynaptic neuron. This means synaptic transmission is flexible: the same basic process can either increase or decrease the chance of firing, depending on how the postsynaptic cell responds.
Because neurotransmitters can produce either excitation or inhibition, synapses provide fine control over the flow of information through the nervous system.
Practice Questions
Briefly outline what happens after neurotransmitters are released into the synaptic cleft. (2 marks)
1 mark for stating that neurotransmitters diffuse across the synaptic cleft and bind to complementary receptor sites on the postsynaptic neuron.
1 mark for stating that this produces a postsynaptic effect, such as excitation or inhibition, and may start a new impulse if threshold is reached.
Explain how excitation and inhibition affect synaptic transmission. (6 marks)
1 mark for explaining that neurotransmitters are released from the presynaptic neuron and bind to receptor sites on the postsynaptic neuron.
1 mark for explaining that excitation increases the likelihood that the postsynaptic neuron will fire.
1 mark for explaining that excitatory input brings the postsynaptic neuron closer to threshold.
1 mark for explaining that inhibition decreases the likelihood that the postsynaptic neuron will fire.
1 mark for explaining that inhibitory input moves the postsynaptic neuron further from threshold.
1 mark for explaining that whether an action potential occurs depends on the balance of excitatory and inhibitory inputs.
FAQ
Synaptic delay is the very short pause that happens when a signal crosses a synapse.
It occurs because several steps are involved: neurotransmitter release, diffusion across the gap, and receptor binding. Even though the delay is tiny, pathways with many synapses are slower than pathways with fewer synapses.
They can be removed in several ways:
Reuptake: the presynaptic neuron takes the neurotransmitter back in.
Enzyme breakdown: enzymes in the synapse break it into inactive parts.
Diffusion: some molecules drift away from the synaptic cleft.
This matters because neurotransmitters must be cleared to stop the signal at the right time.
These terms describe substances that affect neurotransmitter action.
Agonists increase the effect of a neurotransmitter, either by copying it or by increasing its availability.
Antagonists reduce the effect, often by blocking receptor sites or preventing the neurotransmitter from working properly.
Many drugs act as agonists or antagonists, which is why they can strongly alter neural communication.
These terms describe how inputs build up in the postsynaptic neuron.
Temporal summation happens when one synapse fires repeatedly in a short period of time.
Spatial summation happens when several synapses fire at the same time.
Both processes affect whether the postsynaptic neuron reaches threshold. They help explain how many small inputs can combine to produce a larger overall effect.
Desensitization means receptor sites become less responsive after repeated or prolonged stimulation.
As a result, the same amount of neurotransmitter may produce a weaker effect than before. This can happen naturally in the nervous system and can also contribute to tolerance when certain drugs are used repeatedly.
It shows that synaptic transmission is not fixed; receptor sensitivity can change over time.
