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6.2.8Organisms Respond to Changes in their Environments (A Level only)

Synaptic Transmission — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 23 of 44 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

6.2.8 Synaptic Transmission — AQA A Level Biology 7402 notes: Explain the sequence of events at a cholinergic synapse, including summation and inhibition.An action potential arrives at the presynaptic knob and depolarises the membrane.Voltage-gated Ca2+ channels open, and Ca2+ di…

  • exocytosis
  • diffuses across the cleft
  • Na+ channels
  • depolarisation
  • Acetylcholinesterase
  • Spatial summation

Section 1 of 9Sequence of transmission

1
An action potential arrives at the presynaptic knob and depolarises the membrane.
2
Voltage-gated Ca2+ channels
Voltage-gated Ca2+ channels open, and Ca2+ diffuses in from the synaptic cleft, where its concentration is higher.
3
Vesicles
Ca2+ causes vesicles containing ACh to move to, and fuse with, the presynaptic membrane, releasing ACh by exocytosis.
4
Diffuses across the cleft
ACh diffuses across the cleft and binds to specific receptors on the postsynaptic membrane.
5
Depolarisation
This causes Na+ channels to open and Na+ diffuses into the postsynaptic neurone,
  • causing depolarisation, a small change called an excitatory postsynaptic potential.
6
If the threshold is reached, an action potential is generated in the postsynaptic neurone.
7
Acetylcholinesterase
Acetylcholinesterase hydrolyses ACh into choline and ethanoate (acetate),
  • which diffuse back into the presynaptic neurone and are recombined into ACh using ATP from mitochondria.

Section 2 of 9Summation

A single release of ACh may not depolarise the postsynaptic membrane to threshold. Two types of summation allow the effects of several inputs to be added together:

Spatial summation
  • Spatial summation several presynaptic neurones release neurotransmitter at the same time onto one postsynaptic neurone,
    • so that together they reach threshold.
Temporal summation
  • Temporal summation one presynaptic neurone fires many times in quick succession,
    • so the effect of repeated releases accumulates before the neurotransmitter is removed.

Summation means that weak, random stimuli are filtered out and only sufficient input causes a response.

Section 3 of 9Inhibitory synapses

Some synapses are inhibitory.

Inhibitory synapses
Their neurotransmitters cause Cl- channels to open so that Cl- moves in, and K+ channels to open so that K+ moves out.
The inside becomes more negative (hyperpolarisation),
  • so it is harder for the threshold to be reached and an action potential generated in the postsynaptic neurone.
Inhibition allows control and coordination, for example preventing a muscle and its antagonist from contracting at the same time,
  • and it prevents over-stimulation of the brain.

Section 4 of 9Why remove the neurotransmitter?

If ACh stayed in the cleft it would keep binding to receptors, causing continuous stimulation of the postsynaptic neurone.

Why remove the neurotransmitter?
Rapid breakdown of ACh by acetylcholinesterase ensures the response is brief and discrete, and that the synapse is ready to respond to a new impulse.
Recycling of choline and ethanoate saves energy and raw materials.
Interfering with this step, for example with nerve gases, has dramatic effects, as the next lesson shows.

Section 5 of 9Comparison with axonal conduction

AxonSynapse
Electrical (action potential)Chemical (neurotransmitter)
Fast, continuousShort delay (about 0.5 ms)
Can be both directions if stimulated artificiallyOne direction only
All-or-nothingGraded; summation possible
Not easily fatiguedCan fatigue when neurotransmitter is depleted

Section 6 of 9Applying the model to data and unusual situations

Questions often ask what would happen if one step were changed.

1
Use the sequence as a checklist.
2
If calcium channels are blocked, vesicles do not fuse and no ACh is released,
  • so there is no response in the postsynaptic neurone even though the presynaptic action potential is normal.
3
If the calcium concentration outside is lowered, less Ca2+ enters and less transmitter is released.
4
If the postsynaptic receptors are blocked, ACh is released normally but cannot trigger depolarisation.
5
If acetylcholinesterase is inhibited, the postsynaptic membrane stays depolarised and repeated stimulation can occur.
6
When interpreting a graph of postsynaptic potential, a small rise that does not reach threshold shows that a single input is insufficient,
  • and a higher frequency of stimulation producing a larger rise shows temporal summation.
7
Spatial summation is shown when two inputs together reach threshold but each alone does not.
8
Always state the evidence from the graph, then name the type of summation.
Applying the model to data and unusual situations
Synaptic delay can also be examined: the time between arrival of the impulse at the knob and the start of the response in the postsynaptic neurone is about 0.5 ms,
  • which is much longer than conduction over the same distance in an axon.
In summary, transmission is a chain: depolarisation, calcium entry, vesicle fusion, diffusion, receptor binding, sodium entry, threshold,
  • and finally removal of ACh.
  • Marks are awarded for the correct ion at each step and for stating that the process is passive diffusion across the cleft, not active transport.
Remember that the synapse gives the nervous system its ability to decide,
  • as a postsynaptic neurone fires only when the combined input is large enough.

Section 7 of 9Exam-style questions

  1. Describe how an action potential in a presynaptic neurone leads to an action potential in the postsynaptic neurone. [5]5 marks

  2. Explain the difference between spatial and temporal summation. [2]2 marks

  3. Explain why there is a short delay in transmission across a synapse. [2]2 marks

Section 8 of 9Key terms

TermMeaning
ExocytosisRelease of vesicle contents by fusion with the plasma membrane
Spatial summationCombined effect of several presynaptic neurones at one time
Temporal summationCombined effect of repeated impulses from one neurone
Excitatory postsynaptic potentialSmall depolarisation caused by neurotransmitter
Inhibitory synapseSynapse that makes the postsynaptic membrane more negative
HydrolysisBreakdown of a molecule using water

Section 9 of 9Exam technique

1
Saying that Na+ enter the presynaptic neurone to cause release; it is Ca2+.
2
Saying that ACh enters the postsynaptic neurone; it binds to receptors and remains outside.
3
Confusing spatial and temporal summation.
4
Writing that acetylcholinesterase destroys receptors; it breaks down ACh.
5
Forgetting that inhibitory synapses cause hyperpolarisation.

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