By the end of this lesson
- Describe neurotransmitter transmission.
- Explain one-way synaptic signalling.
- Relate synapses to delay and integration.
BIG QUESTION
How does an electrical impulse cross a physical gap between two neurones?
The signal briefly becomes chemical at the synapse, then electrical again in the next cell.
An arriving impulse triggers release
When an impulse reaches a presynaptic ending, vesicles release a neurotransmitterA chemical released from the end of a neurone that diffuses across a synapse and triggers an impulse in the next neurone. into the synaptic cleftThe tiny gap between two neurones at a synapse, across which neurotransmitter diffuses.. This release is triggered by events including calcium-ion entry.
The neurotransmitter is a chemical messenger, not the electrical impulse itself.
Diffusion carries the chemical signal
Neurotransmitter molecules diffuse across the narrow cleft and bind to complementary receptors on the postsynaptic membrane.
Receptor activation changes ion movement and may trigger a new electrical impulse if threshold is reached.
Synapses are directional and adjustable
Vesicle release occurs mainly presynaptically and receptors are positioned postsynaptically, so transmission is normally one way. Neurotransmitter is then removed by breakdown or reuptake.
Synapses introduce delay but also allow signals to be amplified, inhibited or combined.
Across a synapse
The signal form changes across the junction.
| Location | Main event | Signal form |
|---|---|---|
| Presynaptic axon | Impulse arrives. | Electrical. |
| Presynaptic ending | Vesicles release neurotransmitter. | Electrical → chemical. |
| Synaptic cleft | Neurotransmitter diffuses. | Chemical. |
| Postsynaptic membrane | Receptors bind transmitter and alter ion flow. | Chemical → electrical. |
| After transmission | Breakdown or reuptake removes transmitter. | Signal terminates. |
Language that earns precision
- synapse
- Junction through which one neurone communicates with another cell.
- neurotransmitter
- Chemical messenger released from a neurone at a synapse.
- synaptic cleft
- Narrow gap between pre- and postsynaptic cells.
- reuptake
- Transport of released neurotransmitter back into the presynaptic cell.
Common misconceptions
Claim: The electrical impulse jumps directly through empty space.
Correction: A neurotransmitter normally diffuses across the cleft.
Claim: Neurotransmitter remains in the cleft permanently.
Correction: It is removed by breakdown, diffusion or reuptake.
Claim: Every synapse always triggers a new impulse.
Correction: Inputs may excite or inhibit, and threshold may not be reached.
Match the answer to the command word
Arrives → released → diffuses → binds → new impulse.
One-way transmission follows the arrangement of vesicles and receptors.
Recall, apply, then justify
Describe transmission across a synapse.
Hint
Keep the five events in order.
Suggested marking points
- An impulse reaches the presynaptic ending.
- Vesicles release neurotransmitter.
- The neurotransmitter diffuses across the synaptic cleft.
- It binds to receptors on the postsynaptic membrane.
- Ion movement can trigger a new electrical impulse.
Explain why synaptic transmission is normally one way.
Hint
Vesicles on one side, receptors on the other.
Suggested marking points
- Neurotransmitter vesicles release from the presynaptic ending.
- Matching receptors are positioned on the postsynaptic membrane.
Explain how a drug that blocks neurotransmitter reuptake could alter signalling.
Planning hint
The transmitter remains available longer.
What a strong design includes
- Reuptake normally removes neurotransmitter from the cleft.
- Blocking it leaves transmitter present for longer.
- Postsynaptic receptors may remain activated/repeatedly activated.
- The postsynaptic response may become stronger or prolonged.
- Effects depend on whether the transmitter is excitatory or inhibitory.
- Persistent exposure can also change receptor sensitivity.