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A2 · Practice questions · The cholinergic synapse

Electrical, chemical, electrical.

Six original Cambridge-style questions on cholinergic synapses, from labelling the structure to predicting the effects of drugs and toxins.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Calcium in, ACh across.

01
Recall
[3 marks]

Describe the structure of a cholinergic synapse.

  • 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. ✓
02
Explain
[6 marks]

Explain how an impulse is transmitted across a cholinergic synapse, including the role of calcium ions.

  • 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. ✓
03
Explain
[2 marks]

Explain why impulses pass across a synapse in one direction only.

  • Vesicles containing the neurotransmitter are found only in the presynaptic knob. ✓
  • Receptors for the neurotransmitter are found only on the postsynaptic membrane. ✓
04
Analysis
[3 marks]

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.

  • 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. ✓
05
Analysis
[2 marks]

Suggest why transmission across a synapse is slower than conduction along the same length of axon.

  • 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. ✓
06
Explain
[2 marks]

Explain why acetylcholinesterase is needed at a cholinergic synapse.

  • 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. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.