A synapse is the junction between two neurones, and the signal crosses it as a chemical, not as electricity. The steps are: impulse arrives, neurotransmitter is released, it diffuses across the gap, it binds to receptors, and a new impulse starts.
This lesson follows tracing a reflex pathway, where the relay neurone connects two other neurones through synapses. It is part of coordination and responses.
What happens at a synapse, in order?
- An electrical impulse arrives at the end of the first neurone.
- Vesicles in that neurone release neurotransmitter into the gap.
- The neurotransmitter diffuses across the gap. The gap is very small, so this is quick.
- The neurotransmitter binds to receptors on the membrane of the next neurone.
- This triggers a new impulse in the next neurone.
Step 3 links to something you already know: diffusion is movement from a higher to a lower concentration. There is more neurotransmitter near the first neurone than near the second, so it moves in that direction.
Worked example
Explain why an impulse travelling along a sensory neurone can reach a relay neurone, and why it cannot go backwards.
Step 1: the impulse reaches the end of the sensory neurone. The two neurones do not touch, so the impulse itself cannot cross.
Step 2: vesicles release neurotransmitter into the synapse.
Step 3: the neurotransmitter diffuses across the gap and binds to receptors on the relay neurone.
Step 4: this starts an impulse in the relay neurone.
Why not backwards: the receptors are only on the relay neurone’s side, and the neurotransmitter is only released from the sensory side. So the signal can only pass in one direction.
Model answer in one line: a neurotransmitter is released, diffuses across the gap and binds to receptors on the next neurone, starting a new impulse, and this works only in one direction.
The mistake to watch for
Students often write that “the electrical impulse jumps across the synapse”. That is tempting, because the same signal appears on both sides.
Mistaken answer: the impulse jumps the gap and carries on along the next neurone.
This ignores the chemical step, which is the whole point of the structure.
The correction is to name the chemical messenger. An impulse arrives, a chemical crosses, and a new impulse starts. Use the word “neurotransmitter” and the word “diffuses” whenever the question asks you to explain.
Check yourself
1. Name the chemical that crosses a synapse.
Show answer
A neurotransmitter.
2. Put in order: neurotransmitter binds to receptors, impulse arrives, neurotransmitter diffuses across the gap, new impulse starts, neurotransmitter is released.
Show answer
Impulse arrives, neurotransmitter is released, neurotransmitter diffuses across the gap, neurotransmitter binds to receptors, new impulse starts.
3. A substance blocks the receptors on the second neurone. What would you expect to happen to the signal, and why?
Show answer
The signal would not pass to the second neurone, because the neurotransmitter cannot bind to receptors and so cannot start a new impulse.
Where this leads next
Next, compare this chemical step with a different kind of signal in nervous and hormonal signalling. You will meet substances that change signalling later in how a substance can alter signalling.
If writing a clear sequence is the hard part, a teacher in online one-to-one Biology tuition can check your written explanations line by line.