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6.2.7Organisms Respond to Changes in their Environments (A Level only)

Synapse Structure — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 22 of 44 1 quiz

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

6.2.7 Synapse Structure — AQA A Level Biology 7402 notes: Describe the structure of a cholinergic synapse and explain how it is adapted for transmission.A synapse is a junction between two neurones, or between a neurone and an effector cell.Action potentials cannot cross the gap …

  • neurotransmitter
  • acetylcholine
  • Presynaptic neurone
  • synaptic knob
  • voltage-gated calcium ion channels
  • Synaptic cleft

Section 1 of 9What is a synapse?

A synapse is a junction between two neurones, or between a neurone and an effector cell.

Neurotransmitter
Action potentials cannot cross the gap between cells, so the signal is passed chemically by a neurotransmitter.
Cholinergic synapse
In a cholinergic synapse the neurotransmitter is acetylcholine (ACh), which is also used at neuromuscular junctions.
Other synapses use different transmitters, such as noradrenaline or dopamine, but the AQA specification uses the cholinergic synapse as its model.

Section 2 of 9Structure

Structure
Presynaptic neurone
  • Presynaptic neurone its end is swollen into a synaptic knob containing many mitochondria, smooth endoplasmic reticulum,
    • and many vesicles containing ACh.
    • (ATP for making and releasing neurotransmitter)
  • The presynaptic membrane contains voltage-gated calcium ion channels.
Synaptic cleft
  • Synaptic cleft a narrow gap, about 20 nm wide, between the two cells, across which ACh diffuses.
Postsynaptic neurone
  • Postsynaptic neurone its membrane has receptor proteins specific to ACh, which are linked to sodium ion channels.
  • The enzyme acetylcholinesterase is present in the cleft and attached to the postsynaptic membrane, and it breaks down ACh.

The postsynaptic membrane may also contain other types of channel, but the ligand-gated Na+ channel linked to the ACh receptor is the key one for the specification.

Section 3 of 9Adaptations for function

FeatureAdaptation
Many mitochondria in the knobATP for synthesising ACh and for exocytosis and recycling
Vesicles with AChStore neurotransmitter ready for rapid release
Voltage-gated Ca2+ channelsLink the arrival of an action potential to vesicle release
Narrow cleftShort diffusion distance, so fast transmission
Specific receptorsEnsure that only ACh triggers the response
AcetylcholinesteraseRemoves ACh so the response is brief and can be repeated

Section 4 of 9Properties of synapses

Synapses have several important properties that explain their role in the nervous system:

Unidirectional
They are unidirectional, because only the presynaptic neurone contains vesicles of neurotransmitter and only the postsynaptic membrane has receptors.
  • Impulses therefore travel along defined pathways.
Integration
They allow integration of signals, since inputs from several neurones can be added together by summation.
They allow one neurone to be connected to many others (divergence) so one impulse can influence several pathways,
  • or many neurones to one (convergence).
Inhibitory
They can be inhibitory as well as excitatory, giving control and coordination.
Filter
They act as a filter, as low-level background stimulation may not release enough neurotransmitter to trigger a response.
They cause a slight delay in transmission, because chemical steps are slower than conduction in an axon.

Section 5 of 9Context and interpretation

Synapses make the nervous system flexible.

1
They can be modified by drugs and by changes in the number of receptors, which underlies learning and memory.
2
Synapses are also a point where fatigue occurs,
  • because the supply of neurotransmitter in the knob can run out if the synapse is stimulated repeatedly.
3
In diagrams, look for the side with vesicles and mitochondria: this is the presynaptic side.
4
If asked how impulses cross, always say that a chemical crosses and that a new action potential is generated in the postsynaptic neurone only if the threshold is reached.
5
The next lesson explains the sequence of events during transmission.

Section 6 of 9Drawing and labelling a synapse

In exam diagrams of a synapse, the presynaptic knob is drawn as a bulb with circles for vesicles and ovals for mitochondria, a narrow gap, and a flat postsynaptic membrane with small gaps representing channel proteins.

1
When asked to label, give the specific name rather than a general one: use presynaptic membrane, synaptic cleft, vesicle, postsynaptic membrane, receptor and sodium channel.
2
When asked to explain an adaptation, you need two linked statements: what the structure is and how it helps the function.
3
For example, mitochondria are present, therefore ATP is available for exocytosis and for resynthesis of ACh from choline and ethanoate.
4
Electron micrographs show a synapse as two membranes separated by a gap about 20 nm wide, with dark vesicles in the knob; if given a magnification, you can use the scale bar to calculate the actual width of the cleft by measuring it and dividing by the magnification.

Link this to the next lessonit is because Ca2+ channels are voltage-gated that the arrival of an action potential causes release, and because the receptors are specific that only ACh produces a response.

Section 7 of 9Exam-style questions

  1. Describe the structure of a synapse. [4]4 marks

  2. Explain why synapses make impulses travel in only one direction. [2]2 marks

  3. Explain the importance of the large number of mitochondria in the presynaptic knob. [2]2 marks

Section 8 of 9Key terms

TermMeaning
SynapseJunction between neurones or between a neurone and an effector
NeurotransmitterChemical released by a presynaptic neurone to signal across a synapse
AcetylcholineNeurotransmitter at cholinergic synapses
Synaptic cleftGap between presynaptic and postsynaptic membranes
ReceptorProtein on the postsynaptic membrane which binds neurotransmitter
AcetylcholinesteraseEnzyme that hydrolyses acetylcholine
VesicleMembrane-bound sac storing neurotransmitter

Section 9 of 9Exam technique

1
Writing that the action potential crosses the synapse; it is the neurotransmitter that crosses.
2
Saying that the vesicles are on the postsynaptic side.
3
Stating that acetylcholine is used at all synapses; some use other neurotransmitters.
4
Forgetting that mitochondria provide ATP for release and re-synthesis of neurotransmitter.
5
Confusing the synaptic cleft with the neuromuscular junction, which is a specific synapse with a muscle fibre.

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