Synapses - cholinergic
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of synapses from GCSE. You can test your knowledge on this below.
What is the function of a synapse?
It allows communication between one neurone and another (or between a neurone and an effector).
What chemical is typically used to transmit impulses across synapses?
A neurotransmitter such as acetylcholine.
How does the neurotransmitter cross the synaptic cleft?
By diffusion, from a region of higher concentration to a region of lower concentration.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains synaptic transmission or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Is a Cholinergic Synapse?
A cholinergic synapse is a type of synapse that uses acetylcholine (ACh) as its neurotransmitter. These are found in the CNS and at neuromuscular junctions.
Sequence of Transmission Across a Cholinergic Synapse
1. Arrival of action potential
- An action potential reaches the presynaptic neurone terminal.
- This causes voltage-gated Ca²⁺ channels to open, and calcium ions diffuse into the presynaptic knob.
2. Release of neurotransmitter (ACh)
- Calcium ions cause synaptic vesicles to move toward and fuse with the presynaptic membrane.
- The vesicles release acetylcholine into the synaptic cleft by exocytosis.
3. Diffusion and binding
- Acetylcholine diffuses across the synaptic cleft.
- It binds to complementary ACh receptors found on sodium ion channels in the postsynaptic membrane.
4. Depolarisation of postsynaptic membrane
- Binding of ACh causes Na⁺ channels to open, and sodium ions diffuse in to the post-synaptic neurone.
- If the threshold is reached, this triggers a new action potential in the postsynaptic neurone.
5. Breakdown of acetylcholine
- An enzyme called acetylcholinesterase breaks ACh into acetate and choline.
- This prevents ACh from remaining bound to its receptors. If it did so, the sodium ion channels would remain open and sodium ions would continue to diffuse into the post synaptic neurone.
- This prevents continuous depolarisation of the postsynaptic membrane.
6. Recycling
- Acetate and choline are reabsorbed into the presynaptic neurone and used to resynthesise acetylcholine, using ATP.
Summary Table
| Step | Description |
| Action potential arrives | Ca²⁺ channels open, Ca²⁺ enters presynaptic knob by diffusion. |
| Neurotransmitter release | Synaptic vesicles fuse with pre-synaptic membrane, ACh released by exocytosis. |
| ACh diffuses | Binds to ACh receptors on Na⁺ channels of postsynaptic membrane. |
| Na⁺ enters | Depolarisation occurs; possible action potential if threshold is reached. |
| ACh broken down | By acetylcholinesterase into acetate + choline. |
| Recycled | Products reabsorbed and ACh is resynthesised in presynaptic knob using ATP. |
Why Synaptic Transmission Is Unidirectional
Transmission at synapses is only ever in one direction (from the presynaptic neurone to the postsynaptic neurone) for these key biological reasons:
Structural & Functional Specificity:
- Neurotransmitters are only made and released by the presynaptic neurone: The synaptic vesicles containing neurotransmitter (e.g. acetylcholine) are only present in the presynaptic knob.
- Receptors for the neurotransmitter are only found on the postsynaptic membrane: So only the postsynaptic neurone can respond to the neurotransmitter.
Key Terms
- Cholinergic synapse: A synapse that uses acetylcholine as its neurotransmitter.
- Acetylcholine (ACh): A neurotransmitter involved in muscle movement and many CNS synapses.
- Acetylcholinesterase: Enzyme that breaks down ACh in the synaptic cleft.
- Synaptic cleft: The small gap between the presynaptic and postsynaptic neurone.
- Exocytosis: Process by which synaptic vesicles release neurotransmitters into the cleft.
No answer provided.
Exam Tips
Be clear whether you are referring to events happening in the pre-synaptic membrane, the presynaptic neurone terminal, or on the post-synaptic membrane.
Precision = marks. Examiner reports frequently highlight vague terms as a cause of lost marks. For example, saying ‘movement’ instead of ‘diffusion’.
No answer provided.
Describe how transmission occurs across a cholinergic synapse. (6 marks)
- An action potential arrives at the presynaptic neurone, causing Ca²⁺ channels to open.
- Calcium ions diffuse in, causing synaptic vesicles to fuse with the presynaptic membrane.
- Acetylcholine is released into the synaptic cleft via exocytosis.
- ACh diffuses across the synaptic cleft and binds to ACh receptors on the postsynaptic membrane.
- Na⁺ channels open, sodium ions diffuse in
- The postsynaptic membrane is depolarised / an action potential is generated.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!