At a synapse, the first neurone releases a chemical called a neurotransmitter. It crosses the gap, binds to receptors on the next neurone, and starts a new impulse there.
This lesson follows comparing nervous and endocrine coordination in the SPM Biology coordination and response chapter.
What are the steps?
- An impulse arrives at the end of the first neurone.
- The neurone releases neurotransmitter into the synapse.
- The neurotransmitter diffuses across the gap.
- It binds to receptors on the next neurone.
- A new impulse starts in the next neurone.
The message changes from electrical to chemical and back to electrical.
Worked example: a message crossing one synapse
In a reflex, a sensory neurone meets a relay neurone at a synapse.
- The impulse reaches the end of the sensory neurone.
- Neurotransmitter is released into the synapse.
- It diffuses across the small gap and binds to receptors on the relay neurone.
- The relay neurone receives a signal and starts its own impulse.
The same process repeats at the next synapse to the motor neurone.
Why does it go one way?
Only the end of the first neurone holds neurotransmitter, and only the next neurone has receptors for it. The message therefore cannot travel backwards.
This one-way rule keeps impulses on the correct pathway in the reflex arc.
The mistake that costs marks
The common slip is to say that the electrical impulse jumps across the gap, or that the neurones touch.
| Wrong | Right | |
|---|---|---|
| Crossing the gap | The impulse jumps across | Neurotransmitter diffuses across |
| The gap | Neurones touch | A small gap, the synapse, separates them |
| Direction | Either way | One way only |
Use the word “neurotransmitter” in any synapse answer, because it is usually a mark.
Check yourself
Describe how a message crosses a synapse.
Answer
The impulse reaches the end of the first neurone, which releases neurotransmitter into the synapse.
The neurotransmitter diffuses across the gap and binds to receptors on the next neurone, which starts a new impulse.
What to study next
Finish the chapter with relating receptors, effectors and responses. Practise with the biological data interpretation trainer, and log slips in the mistake log and paper-error review.
If you would like a teacher to check your sequence, see online one-to-one Biology tuition.