Synaptic Properties and Drug Effects Revision Notes | A Level AQA Biology

1. Why Synapses Are More Than Simple Relay Stations

Picture a busy junction on a motorway. Traffic can merge from several roads, some lanes speed things up, and some lanes slow everything down. A synapse works in exactly the same way — it is not a passive gap but an active decision-making point that integrates competing signals before deciding whether to pass a message on.

Three properties make synapses powerful: they enforce a one-way direction of travel, they can add signals together over time or space, and they can be actively switched off by inhibitory inputs. Understanding all three is central to AQA A Level questions on nervous coordination — and to making sense of how drugs hijack these mechanisms.

2. Unidirectionality — Why Signals Only Travel One Way

At a chemical synapse, the presynaptic neurone releases neurotransmitter molecules into the synaptic cleft. Those molecules diffuse across and bind to specific receptor proteins on the postsynaptic membrane. The key structural point is that the vesicles containing neurotransmitter exist only on the presynaptic side, and the complementary receptors exist only on the postsynaptic side.

Because of this asymmetry, the signal cannot travel backwards. Even if neurotransmitter were somehow released from the postsynaptic side, there are no receptors for that neurotransmitter on the presynaptic membrane to detect it. This structural arrangement is the entire reason for unidirectionality — it is not a rule imposed from outside; it is built into the anatomy of the synapse itself.

There is a second mechanism that reinforces this one-way flow: after neurotransmitter binds and produces its effect, it is rapidly removed from the cleft. It is either broken down by enzymes (for example, acetylcholinesterase breaks down acetylcholine) or reabsorbed by the presynaptic neurone via reuptake transporters. This removal prevents continuous stimulation and resets the synapse for the next signal.

3. Summation — How the Postsynaptic Neurone Integrates Signals

A single action potential arriving at one presynaptic terminal rarely releases enough neurotransmitter to depolarise the postsynaptic membrane all the way to threshold. The postsynaptic neurone must therefore add up, or summate, multiple inputs before it fires. This is where the synapse truly earns its role as an integration centre.

There are two distinct forms of summation, and AQA expects you to distinguish between them clearly. Think of them as two different ways of accumulating enough votes to pass a motion: you can collect votes quickly from one source, or collect them simultaneously from many sources.

Temporal summation depends on timing. If the presynaptic neurone fires repeatedly and rapidly, the postsynaptic membrane does not have time to return to its resting potential between each EPSP. The small depolarisations stack on top of one another until the combined effect crosses the threshold of approximately -55 mV, triggering an action potential in the postsynaptic cell.

Spatial summation depends on location rather than timing. Imagine several presynaptic neurones all synapsing onto the same postsynaptic cell. If they all fire at once, each contributes a small EPSP from a different point on the postsynaptic membrane. These local depolarisations spread and add together. If their combined effect reaches threshold at the axon hillock, an action potential is generated.

Both forms of summation reflect the same underlying principle: the postsynaptic neurone acts as a biological integrator. It weighs up all incoming signals — excitatory and inhibitory — and only generates an action potential when the net depolarisation reaches threshold. This is what allows the nervous system to produce graded, context-sensitive responses rather than firing indiscriminately.

4. Inhibitory Synapses — Putting the Brakes On

Not every synapse is excitatory. Some presynaptic neurones release inhibitory neurotransmitters that make the postsynaptic membrane less likely to fire. This is not a failure of the system — it is a deliberate and essential feature of neural control.

When an inhibitory neurotransmitter binds to its receptor on the postsynaptic membrane, it typically causes ion channels to open that allow either chloride ions (Cl⁻) to flow in or potassium ions (K⁺) to flow out. Both movements make the inside of the cell more negative than the resting potential — a state called hyperpolarisation. The membrane potential moves further away from threshold, so even if excitatory inputs arrive at the same time, they have a larger deficit to overcome before an action potential can be generated.

The result is an inhibitory postsynaptic potential (IPSP). An IPSP subtracts from any EPSPs that are simultaneously arriving. If the inhibitory input is strong enough, it can completely prevent the postsynaptic neurone from reaching threshold, effectively silencing it. This is how the brain suppresses unwanted signals, coordinates antagonistic muscle groups, and maintains precise control over movement and sensation.

It is worth pausing to connect this back to summation. The postsynaptic neurone receives both EPSPs and IPSPs simultaneously. The net membrane potential change at any moment is the algebraic sum of all these inputs. This integration — excitation minus inhibition — is what determines whether threshold is reached. Inhibitory synapses are therefore not 'off switches'; they are precision instruments that fine-tune the sensitivity of the postsynaptic cell.

5. Drug Effects at Synapses — Applying Your Knowledge

Now that you understand how synapses work normally, you are in a strong position to tackle one of the most rewarding question types in this topic: predicting and explaining drug effects. AQA will give you information about a drug's mechanism and ask you to apply your synaptic knowledge — you do not need to memorise specific drug names.

Every drug that acts at a synapse does so by interfering with one or more of the following steps: neurotransmitter synthesis, vesicle storage, release into the cleft, binding to postsynaptic receptors, or removal from the cleft. Once you identify which step is affected and whether the drug enhances or inhibits that step, the rest of the explanation follows logically from your knowledge of normal synaptic function.

• Drugs that mimic a neurotransmitter (agonists): bind to postsynaptic receptors and produce the same effect as the natural neurotransmitter, increasing postsynaptic depolarisation or hyperpolarisation depending on the receptor type.
• Drugs that block a receptor (antagonists): occupy the receptor without activating it, preventing the natural neurotransmitter from binding and reducing postsynaptic response.
• Drugs that inhibit neurotransmitter breakdown: prevent enzymes in the cleft from breaking down the neurotransmitter, so it remains in the cleft longer and continues to stimulate the postsynaptic receptor.
• Drugs that block reuptake: prevent the presynaptic neurone from reabsorbing the neurotransmitter, again prolonging its presence in the cleft and increasing postsynaptic stimulation.
• Drugs that block neurotransmitter release: prevent vesicle fusion with the presynaptic membrane, reducing or eliminating neurotransmitter release and therefore reducing postsynaptic response.

Worked example: Applying Drug Information to Predict Synaptic Effects

A drug blocks the enzyme that normally breaks down a neurotransmitter in the synaptic cleft. The neurotransmitter is excitatory. Predict and explain the effect of this drug on the postsynaptic neurone.

1. The drug blocks the enzyme responsible for breaking down the neurotransmitter in the cleft. — Normally, after neurotransmitter binds to postsynaptic receptors, enzymes rapidly hydrolyse it to terminate the signal. Blocking this enzyme means the neurotransmitter is not removed.
2. Neurotransmitter accumulates in the synaptic cleft because it is not broken down. — With no enzymatic removal, the neurotransmitter remains available to repeatedly bind to and activate postsynaptic receptors.
3. Prolonged receptor activation produces sustained or repeated EPSPs on the postsynaptic membrane. — Because the neurotransmitter is excitatory, each binding event depolarises the postsynaptic membrane. Sustained binding means the membrane is depolarised for longer.
4. The postsynaptic neurone is more likely to reach threshold and generate action potentials, and may do so more frequently than normal. — This is an excitatory drug effect — it amplifies the normal excitatory signal by preventing its termination.

Answer: The drug causes the excitatory neurotransmitter to accumulate in the synaptic cleft, producing prolonged or repeated EPSPs on the postsynaptic membrane. The postsynaptic neurone is therefore more likely to reach threshold and generate action potentials more frequently than normal.

Notice the logical chain in that worked example: drug mechanism → effect on neurotransmitter concentration → effect on postsynaptic membrane potential → effect on action potential generation. This four-step reasoning chain works for any drug scenario AQA presents. Practise applying it to different mechanisms until it becomes automatic.

6. Exam Technique — Writing for Full Marks

Questions on synaptic properties and drug effects tend to reward students who write in precise mechanistic chains rather than vague generalisations. The examiner is looking for cause-and-effect logic at the molecular and cellular level. Here is how to structure your answers to maximise marks.

For 'explain' questions about summation, always state the type (temporal or spatial), describe what happens to the membrane potential, and link explicitly to whether threshold is reached. For drug questions, always start by identifying which step of synaptic transmission the drug affects, then trace the consequences through to the postsynaptic response.

Every concept in this lesson — unidirectionality, temporal summation, spatial summation, and inhibitory synapses — feeds directly into your ability to answer drug-effect questions. When you see an unfamiliar drug in an exam, map its mechanism onto the normal sequence of synaptic events and trace the consequences step by step. That logical chain, written with precise terminology, is what separates a top-band answer from a mid-band one.