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15.1.914-15 · Homeostasis and Control

Cholinergic Synapses — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 24 of 31 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

15.1.9 Cholinergic Synapses — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how an impulse crosses a cholinergic synapse using acetylcholine.Synaptic cleft — a very small gap between two neurones, which do not touch.Synapse — the ends of the two neurones plu…

  • Neurotransmitter
  • Acetylcholinesterase
  • Presynaptic membrane
  • Postsynaptic membrane
  • Synapse — the ends of the two neurones plus the cleft
  • Impulses — electrical impulses cannot jump across the gap

Part 1 of 5Synapse structure

Synapse structure
Synaptic cleft
  • Synaptic cleft a very small gap between two neurones, which do not touch.
Synapse
  • Synapse the ends of the two neurones plus the cleft.
Impulses
  • Impulses electrical impulses cannot jump across the gap.

Part 2 of 5Basic mechanism

1
An impulse reaches the presynaptic axon end.
2
Neurotransmitters are released from vesicles at the presynaptic membrane.
3
They diffuse across the cleft and bind temporarily to receptors on the postsynaptic membrane.
4
The postsynaptic neurone generates an impulse.
5
The neurotransmitter is destroyed or recycled to stop repeated stimulation.

Part 3 of 5Detailed mechanism with acetylcholine

1
The action potential depolarises the presynaptic membrane.
2
Voltage-gated Ca2+ channels open.
3
Ca2+ diffuse in from the tissue fluid down an electrochemical gradient.
4
ACh-containing vesicles fuse with the presynaptic membrane, releasing ACh into the cleft.
5
ACh diffuses across and binds to receptor proteins on the postsynaptic membrane.
6
The receptors change shape and Na+ channels open, so Na+ enter the postsynaptic neurone.
7
Na+ depolarise the postsynaptic membrane, restarting the impulse.

Part 4 of 5Removal and recycling of ACh

1
Acetylcholinesterase hydrolyses ACh to acetate and choline.
2
This prevents the sodium channels staying permanently open.
3
Choline is absorbed back into the presynaptic neurone.
4
Choline reacts with acetyl coenzyme A to form ACh.
5
ACh is packaged into vesicles for the next action potential.

Part 5 of 5Facts about neurotransmitters

Number
  • Number over 40 different neurotransmitters are known.
ACh
  • ACh one of the key neurotransmitters; synapses that use it are cholinergic.

Key terms

TermMeaning
SynapseJunction between two neurones.
Synaptic cleftGap between the presynaptic and postsynaptic membranes.
NeurotransmitterChemical released across a synapse.
AcetylcholineNeurotransmitter at cholinergic synapses.
AcetylcholinesteraseEnzyme hydrolysing ACh.
Presynaptic membraneMembrane of the neurone sending the signal.
Postsynaptic membraneMembrane of the neurone receiving the signal.

Avoid these mistakes

1
Saying the impulse jumps the cleft — a chemical crosses it.
2
Saying ACh enters the postsynaptic neurone — it binds to receptors on the membrane.
3
Forgetting acetylcholinesterase — without it the postsynaptic membrane stays depolarised.
4
Saying Na+ cause vesicle fusion — Ca2+ cause it.

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