How Does Synaptic Transmission Work?
Learn how synaptic transmission works at a cholinergic synapse, from acetylcholine release to a new nerve impulse, with the key AQA exam mistake explained.
Synaptic transmission passes a signal from one neurone to another across a synapse. At a cholinergic synapse, the chemical messenger is acetylcholine. The signal crosses a tiny gap called the synaptic cleft rather than travelling directly from one neurone into the next.
What happens at the synapse?
- An action potential arrives at the end of the presynaptic neurone. Voltage-gated calcium ion channels open, and calcium ions enter the presynaptic neurone.
- Calcium ions cause vesicles containing acetylcholine to fuse with the presynaptic membrane. Acetylcholine is released into the synaptic cleft by exocytosis, which means a vesicle releases its contents outside the cell.
- Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane. This opens sodium ion channels, so sodium ions enter and the membrane depolarises: its electrical potential becomes less negative.
- If depolarisation reaches the threshold needed to trigger an impulse, a new action potential is generated in the postsynaptic neurone.
- Acetylcholinesterase breaks down acetylcholine in the cleft, stopping it from repeatedly stimulating the postsynaptic neurone. Choline is taken back into the presynaptic neurone and used to make more acetylcholine.
Why does the signal travel one way?
The vesicles that release acetylcholine are in the presynaptic neurone, while the receptors it binds to are on the postsynaptic membrane. This arrangement makes transmission across the synapse one-way.
A single release of acetylcholine may not produce enough depolarisation to reach the threshold. Summation means the effects of signals combine: impulses arriving close together at one synapse can add up (temporal summation), as can signals arriving from different synapses (spatial summation). Inhibitory synapses can make the postsynaptic neurone less likely to reach the threshold.
A common exam mistake is to say that the action potential itself crosses the cleft. It does not: acetylcholine diffuses across the cleft, and its binding may trigger a new action potential.
