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The synapse: transmission, and why it runs one way questions

The structure of a cholinergic synapse, the sequence from arriving impulse through calcium influx, vesicle fusion, diffusion and receptor binding to acetylcholinesterase clearing the cleft, why transmission is unidirectional, spatial and temporal summation, inhibitory synapses, and how named drugs act.

6 original questions · 21 marks · the synapse: transmission, and why it runs one way notes · Nervous coordination, receptors, muscles and behaviour

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• 5 marks · Describe

  Describe what happens at a cholinergic synapse from the moment an action potential arrives at the presynaptic knob to the moment the cleft is cleared again.

  Mark scheme

  1. B1 the arriving action potential depolarises the membrane of the knob, so voltage-gated calcium channels open and calcium ions diffuse in
  2. B1 the influx of calcium causes vesicles of acetylcholine to move to the presynaptic membrane and fuse with it, releasing their contents by exocytosis
  3. B1 acetylcholine diffuses across the 20 nm cleft down its concentration gradient and binds to receptor sites on the postsynaptic membrane
  4. B1 the receptors are sodium channels, so binding opens them, sodium ions diffuse into the postsynaptic neurone and depolarise it, and a new action potential begins if threshold is reached
  5. A1 acetylcholinesterase in the cleft hydrolyses the acetylcholine, and the products are reabsorbed into the knob and recombined using ATP from its mitochondria

• 4 marks · Explain

  Explain how an inhibitory synapse reduces the chance that the postsynaptic neurone produces an action potential, and explain why inhibition is useful rather than a fault in the system.

  Mark scheme

  1. B1 the receptors at an inhibitory synapse open chloride channels, letting negative chloride ions in, or potassium channels, letting positive potassium ions out
  2. B1 either movement makes the inside of the postsynaptic membrane more negative, so it is hyperpolarised rather than depolarised
  3. B1 a larger excitatory input is now needed to bring the membrane to the threshold of about −55 mV
  4. A1 this lets a postsynaptic neurone weigh positive against negative inputs, so signals are sharpened, unwanted movements are suppressed and the whole network does not fire at once

• 4 marks · Suggest

  A toxin from a marine snail prevents the voltage-gated calcium channels in presynaptic knobs from opening. Suggest the effect of this toxin on transmission at a neuromuscular junction, and suggest why an injection of extra acetylcholine into the cleft would not restore normal control of the muscle.

  Mark scheme

  1. B1 calcium ions could not diffuse into the synaptic knob when an action potential arrived
  2. B1 without the calcium influx the vesicles would not move to the presynaptic membrane and fuse with it, so no acetylcholine would be released into the cleft
  3. B1 no receptors on the postsynaptic membrane would be occupied, so no sodium channels would open, the muscle fibre membrane would not depolarise and the muscle would be paralysed
  4. A1 injected acetylcholine would bind and depolarise the fibre, but the amount present would no longer depend on the impulses arriving, so the contraction could not be graded or timed by the nervous system

• 3 marks · Explain

  Explain why transmission across a synapse can happen in one direction only, and explain why this matters for a reflex arc.

• 3 marks · Compare

  Compare spatial summation with temporal summation at a postsynaptic neurone.

• 2 marks · Give

  Give the name of the neurotransmitter used at a cholinergic synapse, and give the name of the enzyme that removes it from the synaptic cleft.

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