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SynapsesEdexcel A-Level Biology A: Revision notes

Section 1

Structure of a cholinergic synapse

A synapse is a junction between two neurones, or between a neurone and an effector such as a muscle fibre. A cholinergic synapse uses acetylcholine (ACh) as its neurotransmitter.

The parts to know are:

  • Presynaptic knob (end of the presynaptic neurone): contains many mitochondria (ATP supply), synaptic vesicles full of acetylcholine, and voltage-gated calcium ion channels in its membrane.
  • Synaptic cleft: a gap of about 20 nm between the two cells. Acetylcholine crosses it by diffusion.
  • Postsynaptic membrane: has receptors for acetylcholine, each linked to a sodium ion channel that opens when acetylcholine binds (a chemically gated, or ligand-gated, channel).
  • Acetylcholinesterase: an enzyme in the cleft that breaks down acetylcholine.
Key termssynapseneurotransmitteracetylcholinesynaptic cleft

Section 2

Transmission across the synapse

The sequence at a cholinergic synapse is:

  1. An action potential arrives and depolarises the presynaptic membrane.
  2. Voltage-gated Ca²⁺ channels open and calcium ions diffuse in to the knob.
  3. Calcium ions cause vesicles to move to, and fuse with, the presynaptic membrane, releasing acetylcholine by exocytosis.
  4. Acetylcholine diffuses across the synaptic cleft.
  5. It binds to receptors on the postsynaptic membrane, opening Na⁺ channels.
  6. Sodium ions diffuse in and depolarise the postsynaptic membrane.
  7. If the depolarisation reaches the threshold, an action potential is generated in the postsynaptic neurone (or the muscle fibre contracts).
Key termsvoltage-gated channelexocytosisdepolarisationthreshold
Exam tip

Name the type of channel and the ion every time: calcium ions enter the presynaptic knob, sodium ions enter the postsynaptic cell.

Common mistake

Acetylcholine does not cross the membrane or enter the postsynaptic cell. It binds to a receptor on the outside of the postsynaptic membrane.

Section 3

Ending the signal and recycling

If acetylcholine stayed bound, the postsynaptic membrane would be stimulated continuously. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which stops the stimulation and frees the receptors.

The products diffuse back into the presynaptic knob, where ATP from the mitochondria is used to resynthesise acetylcholine, which is repackaged into vesicles. Recycling means the synapse can respond to many impulses.

Key termsacetylcholinesterasehydrolysis

Section 4

Why synapses are one-way

Transmission is unidirectional. Vesicles of acetylcholine and voltage-gated calcium ion channels occur only in the presynaptic knob, and acetylcholine receptors occur only on the postsynaptic membrane. So acetylcholine can be released from one side and detected on the other only.

The many mitochondria in the presynaptic knob supply the ATP needed to make acetylcholine, move vesicles and reabsorb the breakdown products.

Key termsunidirectional

Section 5

Applying the model: toxins and inhibitors

Exam questions often change one step and ask for the effect. Work through the sequence:

  • Acetylcholinesterase inhibited (e.g. organophosphate pesticides): acetylcholine persists, receptors are stimulated again and again, so the muscle shows continuous contraction.
  • Vesicle fusion blocked (e.g. botulinum toxin): no acetylcholine released, no depolarisation, so the muscle relaxes.
  • No calcium ions: vesicles do not fuse, so no release.
  • Acetylcholine applied directly to the postsynaptic membrane: bypasses the knob and gives a response.
Key termsinhibitor
Exam tip

Say which step is affected, then follow the consequence through to the postsynaptic membrane.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Synapses

  1. 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
  2. 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
  3. 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
See the full worksheet

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).