The shape of the lesson
By the end of the lesson, learners can
- From impulse to chemical and back: Press Send an action potential and read the stage readout as it changes: action potential arrives, Ca 2+ channels open, vesicles fuse, ACh binds, postsynaptic depolarisation, ACh broken down.
- Why synapses matter: Run the synapse simulator again and notice where the vesicles are and where the receptors are.
- Ca2+ enters the knob and triggers exocytosis.
- ACh diffuses across the cleft and binds to receptors.
- Na+ enters the postsynaptic neurone: threshold?
- Acetylcholinesterase hydrolyses ACh; choline is recycled.
One picture every idea returns to
A cholinergic synapse uses acetylcholine (ACh) as its neurotransmitter. The presynaptic knob contains many mitochondria and vesicles of ACh; it is separated from the postsynaptic membrane by a synaptic cleft about 20 nm wide. When an action potential arrives, voltage-gated Ca2+ channels open and Ca2+ diffuses into the knob. This makes vesicles move to and fuse with the presynaptic membrane, releasing ACh by exocytosis. ACh diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane, opening Na+ channels; Na+ diffuses in and depolarises the membrane, and if threshold is reached an action potential is set up. Acetylcholinesterase in the cleft hydrolyses ACh to choline and ethanoic acid, which are taken back into the knob and recombined using ATP. Transmission is one-way because only the presynaptic side has vesicles and only the postsynaptic side has receptors.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Crossing the gap.. An action potential cannot jump the narrow gap between two neurones. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: From impulse to chemical and back | Press Send an action potential and read the stage readout as it changes: action potential arrives, Ca 2+ channels open, vesicles fuse, ACh binds, postsynaptic depolarisation, ACh broken down. Project tab 6 of the lab (Control and coordination lab, a cholinergic synapse) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why synapses matter | Run the synapse simulator again and notice where the vesicles are and where the receptors are. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why synapses matter, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Calcium ions do not cross the cleft and do not bind to the postsynaptic receptors. | Ca 2+ acts inside the presynaptic knob, triggering vesicle fusion. It is ACh that diffuses across the cleft. |
| Choline is recycled, but it is reabsorbed into the presynaptic knob, not the postsynaptic neurone. | The postsynaptic neurone does not store or release ACh at this synapse. |
The cholinergic synapse: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the structure of a cholinergic synapse.
Explain how an impulse is transmitted across a cholinergic synapse, including the role of calcium ions.
Explain why impulses pass across a synapse in one direction only.
Nicotine has a shape similar to ACh and binds to some ACh receptors, but is not broken down by acetylcholinesterase. Suggest and explain its effect on a postsynaptic neurone.
Suggest why transmission across a synapse is slower than conduction along the same length of axon.
Explain why acetylcholinesterase is needed at a cholinergic synapse.
Answer key and marking notes
- Presynaptic knob containing many mitochondria and vesicles of acetylcholine, with voltage-gated Ca2+ channels in its membrane.
- A synaptic cleft about 20 nm wide.
- Postsynaptic membrane with receptor proteins for ACh (ligand-gated Na+ channels) and acetylcholinesterase.
- The action potential arrives at the presynaptic knob and depolarises its membrane.
- Voltage-gated Ca2+ channels open and Ca2+ diffuses into the knob.
- Ca2+ causes vesicles to move to and fuse with the presynaptic membrane, releasing ACh by exocytosis.
- ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane.
- Na+ channels open; Na+ diffuses in and depolarises the postsynaptic membrane; an action potential is set up if threshold is reached.
- Acetylcholinesterase hydrolyses ACh to choline and ethanoic acid; choline is taken back into the knob and recycled.
- Vesicles containing the neurotransmitter are found only in the presynaptic knob.
- Receptors for the neurotransmitter are found only on the postsynaptic membrane.
- Nicotine binds to the ACh receptors because its shape is complementary to the binding site.
- The Na+ channels open, so the postsynaptic membrane is depolarised (action potentials may be set up).
- As it is not hydrolysed by acetylcholinesterase, it stays bound longer, so stimulation continues for longer than with ACh.
- Transmission relies on diffusion of ACh across the cleft (and on exocytosis and binding), which takes time.
- Conduction along an axon is by local circuits / ion movement, which is faster than these chemical steps.
- It hydrolyses ACh so the receptors are freed and the postsynaptic membrane can repolarise; otherwise it would be stimulated continuously.
- The products (choline) can be taken back into the knob and recycled to make more ACh.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What are the products when acetylcholinesterase hydrolyses acetylcholine?
- Acholine and ethanoic acid
- Bcholine and ATP
- Cacetyl coenzyme A and water
- Dglycerol and ethanoic acid
Which event directly causes vesicles in the presynaptic knob to fuse with the presynaptic membrane?
- Acalcium ions diffusing into the knob
- Bsodium ions diffusing out of the knob
- Cacetylcholine binding to receptors on the knob
- Dpotassium ions being pumped into the knob
The diagram shows a cholinergic synapse. Which labelled structure contains acetylcholine before it is released?
- AW
- BX
- CY
- DZ
Curare is a poison that binds to ACh receptors on the postsynaptic membrane without opening them. What is its effect?
- AACh cannot bind, so the postsynaptic membrane is not depolarised
- BACh is not released from the presynaptic knob
- CACh is not broken down, so the postsynaptic membrane stays depolarised
- Dthe presynaptic neurone cannot conduct action potentials
Why does a cholinergic synapse transmit impulses in one direction only?
- Aonly the presynaptic knob has vesicles of ACh, and only the postsynaptic membrane has ACh receptors
- Bthe synaptic cleft is wider on the postsynaptic side
- Cacetylcholinesterase is found only in the presynaptic knob
- Dthe postsynaptic neurone has no voltage-gated Na+ channels
A nerve-muscle preparation is placed in a solution with a much lower Ca2+ concentration than normal tissue fluid. Stimulating the nerve now causes a much weaker muscle response. Which is the best explanation?
- Aless Ca2+ diffuses into the presynaptic knob, so fewer vesicles release ACh
- Baction potentials cannot travel along the nerve without Ca2+
- CACh needs Ca2+ to diffuse across the synaptic cleft
- Dacetylcholinesterase is activated by low Ca2+ and breaks ACh down faster
A presynaptic neurone is stimulated at a very high frequency for several minutes. Transmission across the synapse gradually fails, although the presynaptic neurone keeps conducting impulses. What is the most likely reason?
- Avesicles of ACh are used up faster than ACh can be resynthesised
- Bthe postsynaptic receptors are destroyed by repeated binding of ACh
- CCa2+ channels in the presynaptic membrane have been permanently closed
- Dthe refractory period of the postsynaptic neurone has become permanent
One impulse arriving at a synapse depolarises the postsynaptic membrane from -70 mV by 6 mV. The threshold of the postsynaptic neurone is -55 mV. Assuming the effects add together, what is the smallest number of impulses that must arrive in quick succession to start an action potential?
- A3
- B2
- C15
- D1
Quiz key
Acetylcholinesterase breaks acetylcholine into choline and ethanoic acid (acetate); choline is reabsorbed into the presynaptic knob for recycling. ATP is used to resynthesise ACh, not produced by its breakdown. Acetyl coenzyme A is a reactant in ACh synthesis, not a product of hydrolysis. Glycerol is part of lipids, not ACh.
Depolarisation opens voltage-gated Ca2+ channels; the Ca2+ that diffuses in triggers vesicles to move to and fuse with the membrane. Na+ moves into, not out of, depolarising membranes. ACh receptors are on the postsynaptic membrane, not the knob. The pumping of K+ maintains the resting potential and does not trigger exocytosis.
W points to a vesicle, which stores ACh until Ca2+ triggers exocytosis. X is a mitochondrion, which supplies ATP but does not store ACh. Y is the synaptic cleft, which ACh crosses only after release. Z is a receptor protein on the postsynaptic membrane, where ACh binds after crossing the cleft.
Curare blocks the receptors, so ACh cannot bind and the Na+ channels stay closed; the postsynaptic cell is not stimulated. Release of ACh from the knob is not affected. Blocking breakdown is the action of acetylcholinesterase inhibitors, which have the opposite effect. Curare acts at the synapse, not on the presynaptic axon.
The synapse is asymmetric: ACh is stored and released on the presynaptic side and detected by receptors on the postsynaptic side. The cleft does not have sides of different width. Acetylcholinesterase acts in the cleft, at the postsynaptic membrane. The postsynaptic neurone does have voltage-gated Na+ channels, which it needs to fire its own action potentials.
Ca2+ enters the knob down its concentration gradient. With less Ca2+ outside, the gradient is smaller, less Ca2+ enters, and fewer vesicles fuse, so less ACh reaches the muscle. Action potentials along the axon depend on Na+ and K+, not Ca2+. ACh diffuses across the cleft by itself. There is no evidence that low Ca2+ activates acetylcholinesterase.
Each impulse releases ACh from vesicles; recycling choline into new ACh takes time and ATP, so at high frequency the supply runs out (synaptic fatigue). Receptors are not destroyed by ACh binding. The Ca2+ channels still open with each depolarisation. A refractory period lasts milliseconds and cannot become permanent.
From -70 mV to -55 mV is 15 mV. 15 ÷ 6 = 2.5, so 3 impulses are needed (18 mV). Two impulses give only 12 mV, short of threshold. 15 is the number of millivolts, not impulses. One impulse gives only 6 mV, which is why summation is needed.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.