A drug that changes behaviour usually does so by changing how messages cross synapses. A strong answer describes the normal route first, then says which step the substance alters and what follows.
This lesson uses invented substances. It builds on interpreting stimulant evidence by asking for the biology behind a faster or slower response, and on coordination and responses.
What is the normal route across a synapse?
Follow the message in order. Write each link as its own short statement.
- An impulse reaches the end of the first neurone.
- A chemical called a neurotransmitter is released into the gap.
- The neurotransmitter diffuses across the gap.
- It binds to receptors on the next neurone.
- This can start a new impulse in the next neurone.
How can a substance change the route?
A substance can affect one of the links. At curriculum level, four ideas cover most questions.
| Change | Effect on signalling |
|---|---|
| Blocks receptors | Neurotransmitter cannot bind, so fewer new impulses |
| Increases release of neurotransmitter | More receptors activated, so more impulses |
| Slows removal of neurotransmitter | Receptors stay activated longer |
| Mimics the neurotransmitter | Binds and activates receptors itself |
Your syllabus may name particular drugs and effects. Check it on the Cambridge page, and use the route above to structure any explanation.
Worked example
Substance Q is invented. It can bind to receptors on the second neurone without starting an impulse. A student takes it, and their reaction time gets longer. Explain why.
Step 1, normal route: neurotransmitter is released from the first neurone and diffuses across the gap.
Step 2, the change: Substance Q occupies the receptors on the second neurone, so less neurotransmitter can bind.
Step 3, the result at the synapse: fewer receptors are activated, so fewer new impulses start in the second neurone.
Step 4, the result in the body: the message reaches the muscle more slowly or more weakly, so the response takes longer and reaction time increases.
The chain is target, change, consequence at the synapse, consequence in the body. Notice that the answer names the gap, the receptors and the neurotransmitter.
The mistake to watch for
Mistaken answer: “Substance Q kills the neurone, so the impulse stops.”
The question did not say neurones were destroyed, and the claim skips the synapse entirely. The answer invents a mechanism that is not in the information given.
The correction is to use only the route stated in the question and to name each step at the gap: release, diffusion, binding to receptors, new impulse.
Check yourself
1. Put these in order: receptors on the next neurone are activated; impulse reaches the end of the first neurone; neurotransmitter diffuses across the gap; neurotransmitter is released.
Show answer
Impulse reaches the end of the first neurone, neurotransmitter is released, neurotransmitter diffuses across the gap, receptors on the next neurone are activated.
2. Invented Substance R makes the first neurone release more neurotransmitter. Predict the effect on the second neurone and on reaction time.
Show answer
More neurotransmitter binds to receptors, so more impulses start in the second neurone. Messages pass on more strongly, so reaction time is likely to decrease, meaning a faster response.
3. Why is “the drug stops the nerve working” an incomplete answer?
Show answer
It names no target, no change and no consequence. A better answer says which step at the synapse the drug affects, such as blocking receptors, and what follows from that.
Where this leads next
Next, recognise a correlation limitation in supplied data to practise judging claims about cause. The drugs and biological effects practice set mixes this lesson with the others. The inheritance model board offers a different kind of model practice, where simple assumptions produce predictions you can compare with reality.
Students often know the synapse steps but lose the order once the question adds a twist. Our teachers work on that in online one-to-one Biology tuition.