How is a nerve impulse passed from one neurone to the next across a gap?
The detailed structure of a synapse and of a neuromuscular junction. The sequence of events involved in transmission across a cholinergic synapse. The roles of summation, both spatial and temporal, and the importance of synapses in ensuring unidirectional transmission. Predicting and explaining the effects of specific drugs on synaptic transmission.
A focused answer to the AQA 3.6 dot point on synaptic transmission. Details the cholinergic synapse step by step, compares it with the neuromuscular junction, explains spatial and temporal summation, and shows why transmission is unidirectional.
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What this dot point is asking
AQA wants you to describe the structure of a synapse and neuromuscular junction, give the step-by-step events at a cholinergic synapse, explain spatial and temporal summation, justify unidirectional transmission, and predict the effects of drugs.
The answer
Structure of a synapse
A synapse is the junction between two neurones. The presynaptic neurone ends in a swelling (synaptic knob) containing synaptic vesicles full of neurotransmitter and many mitochondria. A small gap, the synaptic cleft, separates it from the postsynaptic membrane, which carries receptor proteins.
Transmission across a cholinergic synapse
A cholinergic synapse uses the neurotransmitter acetylcholine (ACh). The sequence of events is:
- An action potential arrives and depolarises the presynaptic membrane, opening voltage-gated calcium ion channels.
- Ca2+ diffuses into the synaptic knob down its concentration gradient.
- Calcium causes synaptic vesicles to move to and fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft by exocytosis.
- Acetylcholine diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
- This opens sodium ion channels; Na+ diffuses in and the postsynaptic membrane depolarises.
- If threshold is reached, an action potential is generated in the postsynaptic neurone.
- Acetylcholinesterase in the cleft hydrolyses acetylcholine into choline and ethanoic acid, stopping the response and preventing continuous stimulation.
- The breakdown products are reabsorbed into the presynaptic knob and reformed into acetylcholine using ATP from the mitochondria.
The neuromuscular junction
A neuromuscular junction is the synapse between a motor neurone and a skeletal muscle fibre. It works in the same basic way (acetylcholine is released and binds to receptors), but with key differences from a synapse between two neurones:
| Feature | Cholinergic synapse (neurone to neurone) | Neuromuscular junction |
|---|---|---|
| Postsynaptic cell | Another neurone | Muscle fibre |
| Effect | May excite or inhibit the next neurone | Always excitatory (causes contraction) |
| Outcome | May start a new action potential | Causes the muscle to contract |
| Receptors | On the postsynaptic neurone | On the muscle (motor end plate) |
At the neuromuscular junction, acetylcholine binding depolarises the muscle membrane, the depolarisation spreads down the T-tubules, and the muscle fibre contracts.
Summation
A single impulse often releases too little neurotransmitter to reach threshold. Summation is the way several small depolarisations add together.
- Spatial summation. Several different presynaptic neurones release neurotransmitter onto the same postsynaptic neurone at the same time; together they reach threshold.
- Temporal summation. One presynaptic neurone fires repeatedly in quick succession, so the neurotransmitter accumulates and reaches threshold.
Summation allows synapses to act as decision points, integrating many inputs before deciding whether to fire.
Unidirectional transmission
Transmission is one-way only because:
- Neurotransmitter is only made and released by the presynaptic neurone.
- Receptors are only found on the postsynaptic membrane.
So the impulse can only cross from presynaptic to postsynaptic, ensuring impulses travel in the correct direction through the nervous system.
Effects of drugs
You may be asked to predict a drug's effect from a description of how it acts:
- Agonists mimic the neurotransmitter or stimulate its release, increasing transmission (for example nicotine binds acetylcholine receptors).
- Antagonists block receptors or prevent neurotransmitter release, decreasing transmission.
- Enzyme inhibitors that block acetylcholinesterase (for example some organophosphate insecticides and nerve agents) leave acetylcholine in the cleft, causing continuous stimulation of the postsynaptic membrane.
Examples in context
Example 1. Organophosphate poisoning. Organophosphate insecticides inhibit acetylcholinesterase, so acetylcholine is not broken down in the synaptic cleft. Postsynaptic membranes and neuromuscular junctions are stimulated continuously, causing uncontrolled muscle contraction, cramps and, in severe cases, paralysis of the respiratory muscles. This illustrates the essential role of acetylcholinesterase in stopping transmission.
Example 2. Curare and muscle relaxation in surgery. Curare-derived drugs block acetylcholine receptors at the neuromuscular junction. Acetylcholine released by the motor neurone cannot bind, so the muscle membrane does not depolarise and the muscle cannot contract, causing relaxation or paralysis. Anaesthetists use this controlled paralysis during surgery, a direct application of receptor antagonism.
Try this
Q1. Describe the role of calcium ions in synaptic transmission. [2 marks]
- Cue. Depolarisation of the presynaptic membrane opens voltage-gated calcium channels; Ca2+ diffuses in and causes synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine by exocytosis.
Q2. Explain why transmission across a synapse occurs in one direction only. [2 marks]
- Cue. Neurotransmitter is made and released only by the presynaptic neurone; receptors are present only on the postsynaptic membrane.
Q3. Distinguish between spatial and temporal summation. [2 marks]
- Cue. Spatial: several presynaptic neurones release neurotransmitter onto one postsynaptic neurone at the same time. Temporal: one presynaptic neurone fires repeatedly in quick succession so neurotransmitter builds up to threshold.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA5 marksDescribe how an action potential arriving at the presynaptic membrane leads to an action potential in the postsynaptic neurone.Show worked answer →
A 5-mark answer needs the calcium trigger, vesicle fusion, diffusion, receptor binding and postsynaptic depolarisation.
- The action potential depolarises the presynaptic membrane, opening voltage-gated calcium channels, so Ca2+ diffuses in.
- Calcium causes synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft by exocytosis.
- Acetylcholine diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
- This opens sodium ion channels, so Na+ diffuses into the postsynaptic neurone, causing depolarisation.
- If enough receptors are activated the membrane reaches threshold and an action potential is generated in the postsynaptic neurone.
Markers reward Ca2+ entry, exocytosis of acetylcholine, diffusion across the cleft, binding to receptors, Na+ influx and reaching threshold.
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