What is the synaptic cleft?

The gap of about 20 nm between the presynaptic and postsynaptic membranes.

Why are there many mitochondria in the presynaptic knob?

They supply ATP to resynthesise acetylcholine, move vesicles and reabsorb the breakdown products.

What happens when an action potential reaches the presynaptic knob?

Voltage-gated calcium ion channels open and calcium ions diffuse into the knob.

What do calcium ions do in the presynaptic knob?

They cause vesicles containing acetylcholine to move to and fuse with the presynaptic membrane.

How is acetylcholine released from the knob?

By exocytosis from synaptic vesicles.

What does acetylcholine bind to?

Receptors on the postsynaptic membrane, which are linked to sodium ion channels.

What happens to the postsynaptic membrane when sodium ion channels open?

Sodium ions diffuse in and depolarise it; if the threshold is reached an action potential is generated.

What does acetylcholinesterase do?

Hydrolyses acetylcholine in the cleft into choline and ethanoic acid, ending stimulation and freeing receptors.

What happens to choline and ethanoic acid after hydrolysis?

They are reabsorbed into the presynaptic knob and used, with ATP, to resynthesise acetylcholine.

Why is synaptic transmission unidirectional?

Vesicles and calcium ion channels are only in the presynaptic knob; receptors are only on the postsynaptic membrane.

What is the effect of an acetylcholinesterase inhibitor?

Acetylcholine is not broken down, so the postsynaptic membrane is stimulated continuously.

SynapsesEdexcel A-Level Biology A: Flashcards

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Name the neurotransmitter at a cholinergic synapse.

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Name the neurotransmitter at a cholinergic synapse.: Acetylcholine (ACh).
What is the synaptic cleft?: The gap of about 20 nm between the presynaptic and postsynaptic membranes.
Why are there many mitochondria in the presynaptic knob?: They supply ATP to resynthesise acetylcholine, move vesicles and reabsorb the breakdown products.
What happens when an action potential reaches the presynaptic knob?: Voltage-gated calcium ion channels open and calcium ions diffuse into the knob.
What do calcium ions do in the presynaptic knob?: They cause vesicles containing acetylcholine to move to and fuse with the presynaptic membrane.
How is acetylcholine released from the knob?: By exocytosis from synaptic vesicles.
How does acetylcholine cross the synaptic cleft?: By diffusion.
What does acetylcholine bind to?: Receptors on the postsynaptic membrane, which are linked to sodium ion channels.
What happens to the postsynaptic membrane when sodium ion channels open?: Sodium ions diffuse in and depolarise it; if the threshold is reached an action potential is generated.
What does acetylcholinesterase do?: Hydrolyses acetylcholine in the cleft into choline and ethanoic acid, ending stimulation and freeing receptors.
What happens to choline and ethanoic acid after hydrolysis?: They are reabsorbed into the presynaptic knob and used, with ATP, to resynthesise acetylcholine.
Why is synaptic transmission unidirectional?: Vesicles and calcium ion channels are only in the presynaptic knob; receptors are only on the postsynaptic membrane.
What is the effect of an acetylcholinesterase inhibitor?: Acetylcholine is not broken down, so the postsynaptic membrane is stimulated continuously.

Exam questions on Synapses

A farm worker is exposed to an organophosphate pesticide, a chemical that inhibits acetylcholinesterase. Within hours she develops uncontrollable muscle twitching, because the cholinergic synapses at her neuromuscular junctions are being stimulated continuously.
Explain why the pesticide causes the muscle fibres to be stimulated continuously.2 marks
Botulinum toxin is injected in very small doses to treat painful muscle spasm. It prevents the vesicles in the presynaptic knobs of motor neurones from fusing with the presynaptic membrane, so acetylcholine is not released.
Explain how the toxin causes the muscle to relax.2 marks
Researchers investigating a neuromuscular junction removed calcium ions from the fluid surrounding an isolated motor neurone and muscle fibre. When the neurone was stimulated, an action potential still reached the end of the neurone but the muscle fibre no longer contracted. Contraction returned when calcium ions were restored to the fluid.
Explain why removing calcium ions from the fluid prevented the muscle fibre from contracting.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).