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8.1.5Topic 8 · Grey Matter

Synapses- Structure and Function — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 22 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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8.1.5 Synapses- Structure and Function — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the structure of a cholinergic synapse, explain how transmission occurs and how synapses allow summation, integration and inhibition.A synapse is the junction between two neurones, or b…

  • synaptic cleft
  • presynaptic neurone
  • postsynaptic membrane
  • unidirectional
  • acetylcholine (ACh)
  • Ca2+ channels

Section 1 of 8Structure of a synapse

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

Structure of a synapse
Synaptic cleft
The two cells are separated by a tiny gap of about 20 nm, the synaptic cleft.
Presynaptic neurone
The presynaptic neurone ends in a swelling, the synaptic knob, containing many mitochondria (to supply ATP), vesicles filled with neurotransmitter and voltage-gated calcium ion channels.
Postsynaptic membrane
The postsynaptic membrane carries specific receptor proteins that bind the neurotransmitter and are linked to ligand-gated ion channels.
Unidirectional
Because the receptors are only on the postsynaptic membrane, transmission is unidirectional.

Section 2 of 8Transmission at a cholinergic synapse

A cholinergic synapse uses acetylcholine (ACh) as its neurotransmitter:

An action potential arrives at the presynaptic knob and depolarises the membrane.
Voltage-gated Ca2+ channels open and Ca2+ diffuses into the knob.
Exocytosis
Ca2+ causes synaptic vesicles to fuse with the presynaptic membrane and release ACh into the cleft by exocytosis.
ACh diffuses across the cleft and binds to receptors on the postsynaptic membrane.
Sodium channels open, Na+ enters and the postsynaptic membrane depolarises. If the threshold is reached, an action potential starts in the postsynaptic neurone.
Acetylcholinesterase
Acetylcholinesterase in the cleft hydrolyses ACh into choline and ethanoate (acetate). These are reabsorbed by the presynaptic neurone and resynthesised into ACh using energy from ATP. This stops the response and allows the synapse to be reset.

Section 3 of 8Functions of synapses

Functions of synapses
One-way transmission
One-way transmission impulses move only from presynaptic to postsynaptic neurone.
Spatial summation
Spatial summation several presynaptic neurones release neurotransmitter on the same postsynaptic neurone; their combined effect can reach the threshold even though a single one cannot.
Temporal summation
Temporal summation a single presynaptic neurone fires repeatedly in rapid succession, and the neurotransmitter accumulates, bringing the postsynaptic membrane to threshold.
Inhibitory synapses
Inhibitory synapses some neurotransmitters, such as GABA, open chloride ion channels or potassium channels, causing hyperpolarisation (an inhibitory postsynaptic potential), making an action potential less likely.
Integration
Integration the postsynaptic neurone adds excitatory and inhibitory inputs and fires only if the net effect reaches threshold. This allows decisions and filters out weak background signals.
Divergence and convergence
Divergence and convergence one neurone can stimulate many others, or many can converge on one, coordinating complex responses.
Fatigue
Fatigue if a synapse is stimulated repeatedly, the vesicles of neurotransmitter run out, so transmission temporarily stops, which prevents overstimulation.

Section 4 of 8Neuromuscular junctions and drugs

A neuromuscular junction is a cholinergic synapse between a motor neurone and a muscle fibre; ACh depolarises the muscle fibre membrane and starts contraction.

Nicotine
Many drugs act on synapses. Nicotine mimics ACh and stimulates the receptors. Curare blocks the receptors, causing paralysis.
Organophosphate insecticides and nerve gases inhibit acetylcholinesterase so ACh accumulates and muscles stay contracted.
Some drugs, such as cocaine, block the reuptake of neurotransmitters, and antidepressants such as SSRIs prolong the action of serotonin.
In data questions, predict the effect by asking what part of the sequence is affected.
FeatureSynapseAxon
SignalChemicalElectrical
SpeedSlower, with synaptic delay of about 0.5 msFaster
DirectionOne wayBoth ways if stimulated in the middle

Synaptic transmission also needs a supply of calcium ions in the surrounding fluid: with too little Ca2+ fewer vesicles fuse, so less neurotransmitter is released and the postsynaptic response is smaller. Likewise, a drug that blocks the voltage-gated calcium channels would stop transmission even though action potentials still reach the knob.

Section 5 of 8Why synapses matter

Synapses give the nervous system its flexibility.

Why synapses matter
Without them, a nerve pathway would be a simple wire, and the same signal would always cause the same output.
Because the postsynaptic neurone sums many excitatory and inhibitory inputs, the system can filter out low-level background noise, combine information from several receptors, and allow learning: repeated use of a synapse can increase its strength, so a stimulus produces a larger response later, which is thought to underlie memory.
A weakness is the delay of about 0.5 ms for the chemical steps, which makes synaptic transmission slower than conduction along an axon, so a pathway with fewer synapses is quicker.

The nervous system uses several neurotransmitters in addition to ACh: noradrenaline at sympathetic synapses, dopamine, serotonin, GABA and glutamate in the brain. Parkinson's disease is linked to loss of dopamine-producing neurones, and drugs can be designed to mimic or block specific neurotransmitters, to increase them or to prevent their reuptake.

Section 6 of 8Exam-style questions

  1. Describe the sequence of events that occurs when an action potential arrives at a cholinergic synapse. [5]5 marks

  2. Explain the role of acetylcholinesterase. [2]2 marks

  3. Explain the difference between spatial and temporal summation. [3]3 marks

Section 7 of 8Key terms

TermMeaning
SynapseJunction between two neurones or between a neurone and an effector
NeurotransmitterChemical that carries a signal across a synapse
AcetylcholineNeurotransmitter at cholinergic synapses and neuromuscular junctions
AcetylcholinesteraseEnzyme that hydrolyses acetylcholine
Spatial summationCombined effect of impulses from several presynaptic neurones
Temporal summationCombined effect of repeated impulses from one presynaptic neurone
ExocytosisRelease of vesicle contents by fusion with the cell surface membrane
Inhibitory synapseSynapse that makes the postsynaptic cell less likely to fire

Section 8 of 8Exam technique

1
Saying neurotransmitter is pumped across the cleft: it diffuses.
2
Saying that the action potential crosses the synaptic cleft: a chemical signal does.
3
Forgetting that calcium ions enter the presynaptic neurone, not the postsynaptic one.
4
Saying acetylcholinesterase removes ACh from the receptors by recycling it: it hydrolyses it.
5
Forgetting the need for mitochondria in the synaptic knob.

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