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This lesson covers synaptic transmission as required by the Edexcel A-Level Biology specification (9BI0), Topic 9 -- Control Systems. You need to understand the structure of a cholinergic synapse, the mechanism of transmission across it, and the roles of different types of synapses.
A synapse is the junction between two neurones, or between a neurone and an effector cell (muscle or gland). There is a small gap -- the synaptic cleft (approximately 20-30 nm wide) -- between the two cells.
The neurone transmitting the signal is the presynaptic neurone; the neurone (or effector) receiving the signal is the postsynaptic neurone.
Signals cross the synaptic cleft via chemical messengers called neurotransmitters. This is therefore called chemical synaptic transmission.
A cholinergic synapse uses acetylcholine (ACh) as its neurotransmitter. This is the type you are expected to know in detail for the Edexcel specification.
| Structure | Location | Function |
|---|---|---|
| Synaptic knob | End of presynaptic neurone | Contains synaptic vesicles filled with ACh |
| Synaptic vesicles | Inside the synaptic knob | Membrane-bound sacs containing ACh molecules |
| Mitochondria | Abundant in the synaptic knob | Provide ATP for ACh synthesis and vesicle transport |
| Voltage-gated Ca²+ channels | Presynaptic membrane | Open when an action potential arrives, allowing Ca²+ influx |
| Synaptic cleft | Gap between neurones (~20 nm) | Space across which ACh diffuses |
| Receptor proteins | Postsynaptic membrane | Specific to ACh; contain Na+ ion channels |
| Acetylcholinesterase (AChE) | In the synaptic cleft / on postsynaptic membrane | Enzyme that hydrolyses ACh to stop the signal |
Exam Tip: Examiners frequently ask you to label a synapse diagram. You must know the specific names: 'synaptic vesicles' (not 'bags'), 'voltage-gated calcium ion channels' (not just 'channels'), and 'acetylcholinesterase' (not just 'enzyme').
The sequence of events at a cholinergic synapse is:
An action potential arrives at the synaptic knob of the presynaptic neurone.
The depolarisation causes voltage-gated Ca²+ channels in the presynaptic membrane to open. Ca²+ ions flood into the synaptic knob down their concentration gradient.
The influx of Ca²+ causes synaptic vesicles containing ACh to move towards and fuse with the presynaptic membrane. ACh is released into the synaptic cleft by exocytosis.
ACh molecules diffuse across the synaptic cleft (the cleft is very narrow, so diffusion is rapid).
ACh binds to specific receptor proteins on the postsynaptic membrane. These receptors have a shape that is complementary to ACh. The receptors are nicotinic cholinergic receptors -- they are ligand-gated Na+ ion channels.
Binding of ACh causes the receptor to change shape, opening the Na+ ion channel. Na+ ions flow into the postsynaptic neurone, causing depolarisation.
If sufficient Na+ enters and the threshold is reached, an action potential is generated in the postsynaptic neurone.
The following diagram summarises the key steps of synaptic transmission:
graph TD
A["Action Potential<br/>arrives at synapse"] --> B["Ca²⁺ channels open"]
B --> C["Vesicles fuse<br/>with membrane"]
C --> D["Neurotransmitter<br/>released into cleft"]
D --> E["Binds to receptors<br/>on post-synaptic membrane"]
E --> F["Na⁺ channels open"]
F --> G["New action potential<br/>(if threshold reached)"]
The enzyme acetylcholinesterase (AChE), located in the synaptic cleft, rapidly hydrolyses ACh into choline and ethanoic acid (acetate). This stops the signal.
Choline is reabsorbed into the presynaptic knob by active transport. It is then recombined with acetyl CoA (from mitochondria) by the enzyme choline acetyltransferase to reform ACh, which is repackaged into vesicles.
| Step | Event | Key Molecules |
|---|---|---|
| 1 | Action potential arrives | Na+, K+ |
| 2 | Ca²+ enters synaptic knob | Ca²+, voltage-gated Ca²+ channels |
| 3 | Vesicles fuse, ACh released | ACh, synaptic vesicles |
| 4 | ACh diffuses across cleft | ACh |
| 5 | ACh binds to receptors | ACh, nicotinic receptors |
| 6 | Na+ channels open | Na+ |
| 7 | Postsynaptic depolarisation | Na+ |
| 8 | AChE hydrolyses ACh | AChE, choline, acetate |
| 9 | Choline recycled, ACh resynthesised | Choline, acetyl CoA |
Exam Tip: The sequence of synaptic transmission is one of the most commonly examined topics. Practise writing out all nine steps in order. Key words the examiner looks for: 'voltage-gated calcium channels', 'exocytosis', 'diffusion', 'complementary receptors', 'acetylcholinesterase hydrolyses ACh'.
| Type | Neurotransmitter | Effect on Postsynaptic Membrane | Result |
|---|---|---|---|
| Excitatory | ACh, glutamate, noradrenaline | Opens Na+ channels → depolarisation (EPSP) | Makes an action potential more likely |
| Inhibitory | GABA, glycine | Opens Cl- channels or K+ channels → hyperpolarisation (IPSP) | Makes an action potential less likely |
EPSP = Excitatory Post-Synaptic Potential (depolarisation towards threshold) IPSP = Inhibitory Post-Synaptic Potential (hyperpolarisation away from threshold)
A single synaptic vesicle rarely releases enough neurotransmitter to reach the threshold in the postsynaptic neurone. Summation is the process by which multiple small EPSPs add together to reach threshold.
Multiple impulses arrive rapidly at the same presynaptic neurone. The EPSPs from successive transmissions add up before the previous ones decay.
Multiple presynaptic neurones converge on one postsynaptic neurone and fire simultaneously. The EPSPs from different synapses add together.
If the combined EPSPs minus any IPSPs reach the threshold, an action potential is generated.
Exam Tip: Summation explains how the nervous system integrates signals. In spatial summation, excitatory and inhibitory inputs from different neurones are 'summed' -- if net depolarisation reaches threshold, the postsynaptic neurone fires.
Synapses are not merely passive junctions -- they have important functional roles:
| Role | Explanation |
|---|---|
| Unidirectionality | Neurotransmitter is only released from the presynaptic side and receptors are only on the postsynaptic side, so impulses travel in one direction only |
| Integration | Summation of excitatory and inhibitory inputs allows the nervous system to process complex information |
| Amplification | One action potential can cause the release of many vesicles, amplifying the signal |
| Adaptation (fatigue) | Repeated stimulation can deplete neurotransmitter, reducing the response -- prevents over-stimulation |
| Memory and learning | Long-term changes in synaptic strength underpin learning (e.g. long-term potentiation) |
| Convergence and divergence | Allows complex neural pathways -- one neurone can synapse with many others (divergence) or many can synapse onto one (convergence) |
Many drugs and toxins exert their effects by interfering with synaptic transmission:
| Substance | Mechanism | Effect |
|---|---|---|
| Nerve agents (e.g. sarin) | Inhibit acetylcholinesterase | ACh accumulates → continuous stimulation → muscle paralysis |
| Curare | Blocks ACh receptors (competitive inhibitor) | Prevents depolarisation → muscle paralysis |
| Nicotine | Mimics ACh at nicotinic receptors (agonist) | Stimulates the postsynaptic neurone |
| Botulinum toxin (Botox) | Prevents vesicle fusion (inhibits Ca²+-mediated exocytosis) | No ACh release → prevents muscle contraction |
| SSRIs (e.g. fluoxetine) | Inhibit reuptake of serotonin | Serotonin remains in cleft longer → prolonged effect |
| Cocaine | Inhibits reuptake of dopamine | Dopamine accumulates in cleft → prolonged stimulation |
Exam Tip: Drug questions often ask you to predict the effect on synaptic transmission. Think about which step is affected: release, binding, breakdown, or reuptake. Then work through the consequences logically.
This lesson sits in Edexcel 9BI0 Topic 8 — Grey Matter (Coordination, Response and Gene Technology), specifically the sub-strand on chemical transmission across the synaptic cleft. The relevant content statements paraphrase to: describe a cholinergic synapse; explain the cascade from AP arrival to postsynaptic AP; distinguish excitatory and inhibitory synapses; explain temporal and spatial summation; and account for named drugs and toxins (refer to the official Pearson Edexcel 9BI0 specification document for exact wording). The material is examined on Paper 2 — Energy, Exercise and Coordination and reactivates synoptically through Lesson 2 (APs are the upstream trigger for vesicle fusion), Lesson 1 (synapses are the fast, short-range counterpart to slow long-range hormones), Topic 5 (mitochondrial ATP fuels vesicle recycling and the Na+/K+ pump), Topic 6 (the immunological synapse uses the same vesicle-fusion machinery in cytotoxic T cells) and the pharmacology of psychoactive drugs.
Question (8 marks): A motor neurone synapses with a skeletal muscle fibre at the neuromuscular junction. The presynaptic terminal contains acetylcholine.
(a) Describe the events at this cholinergic synapse from arrival of an action potential at the presynaptic terminal to generation of an action potential in the postsynaptic membrane. (5)
(b) Explain the difference between ionotropic and metabotropic receptors, with one named example of each. (3)
Solution with mark scheme:
(a) Step 1 — Ca²+ influx. The AP depolarises the presynaptic terminal, opening voltage-gated Ca²+ channels; Ca²+ flows in down its electrochemical gradient.
M1 (AO1) — name the voltage-gated Ca²+ channel; "calcium enters" without gating loses M1.
Step 2 — vesicle fusion. Ca²+ binds synaptotagmin on synaptic vesicles, triggering SNARE-mediated fusion with the presynaptic membrane. ACh is released into the cleft by exocytosis.
A1 (AO2) — explicit Ca²+-triggered exocytosis. "Vesicles release ACh" without exocytosis or trigger loses the mark.
Step 3 — diffusion across the cleft. ACh diffuses across the ~20 nm cleft down its concentration gradient (sub-millisecond).
M1 (AO1) — diffusion as the cleft-crossing mechanism. The AP itself does not cross.
Step 4 — receptor binding. ACh binds nicotinic acetylcholine receptors — ligand-gated Na+ channels with a complementary binding site. The conformational change opens the channel; Na+ flows in, producing a local EPSP.
A1 (AO2) — name the ligand-gated Na+ channel and link binding to depolarisation.
Step 5 — threshold. If summed EPSPs reach threshold (~-55 mV) at the axon hillock, a postsynaptic action potential is generated (or here, muscle contraction is triggered).
A1 (AO3) — explicit threshold integration, not "the muscle contracts".
(b) Ionotropic receptors are ligand-gated ion channels — binding directly opens the pore. Example: the nicotinic ACh receptor at the neuromuscular junction (~1 ms onset).
M1 (AO1) — ionotropic = ligand-gated ion channel + nicotinic example.
Metabotropic receptors are GPCRs — binding activates a G-protein and second-messenger cascade. Example: the muscarinic ACh receptor in cardiac pacemaker cells, where ACh slows the SAN via Gi-coupled signalling.
A1 (AO2) — metabotropic = GPCR + muscarinic example.
A1 (AO3) — synthesis: contrasting timescales (~1 ms vs 100 ms+) and tying speed to mechanism.
Total: 8 marks (5 + 3).
Question (6 marks): Botulinum toxin (Botox) cleaves SNARE proteins in presynaptic terminals. Explain why patients exposed to botulinum toxin develop muscle paralysis.
Mark scheme decomposition by AO:
| Mark | AO | Awarded for |
|---|---|---|
| 1 | AO1 | Identifying SNARE proteins as required for vesicle fusion with the presynaptic membrane. |
| 2 | AO1 | Identifying acetylcholine (ACh) as the neurotransmitter at the neuromuscular junction. |
| 3 | AO2 | Linking SNARE cleavage → no vesicle fusion → no exocytosis of ACh into the synaptic cleft. |
| 4 | AO2 | Linking absence of ACh in the cleft → no binding to nicotinic receptors → no Na+ influx → no postsynaptic depolarisation. |
| 5 | AO2 | Linking absence of postsynaptic AP in the muscle fibre → no Ca²+ release from sarcoplasmic reticulum → no actin–myosin cross-bridge cycling → no contraction. |
| 6 | AO3 | Synthesis / evaluation — explicit linking to clinical use, e.g. "controlled local injection of Botox produces targeted paralysis exploited cosmetically and to treat dystonia, blepharospasm and chronic migraine; tetanus toxin cleaves a different SNARE and produces opposite (spastic) paralysis by selectively blocking inhibitory interneurones." Equivalent: explaining recovery as growth of new presynaptic terminals over weeks. |
Total: 6 marks (AO1 = 2, AO2 = 3, AO3 = 1). A typical "explain why" Edexcel extended response — AO3 reserved for linking mechanism to clinical or evolutionary consequence rather than restating the cascade.
Connects to:
Synapse questions on 9BI0 typically split AO marks toward AO1 and AO2, with AO3 reserved for synthesis or evaluation:
| AO | Typical share | Earned by |
|---|---|---|
| AO1 (knowledge) | 35–45% | Naming voltage-gated Ca²+ channels, synaptic vesicles, ACh, nicotinic receptors, ligand-gated Na+ channels, acetylcholinesterase; recalling the ~20 nm cleft width |
| AO2 (application) | 40–50% | Sequencing the cascade from AP arrival to postsynaptic AP; linking Ca²+ influx to exocytosis; linking ACh binding to channel opening; explaining temporal vs spatial summation |
| AO3 (analysis / evaluation) | 10–20% | Predicting the effect of a named drug or toxin on each stage; comparing ionotropic and metabotropic timescales; evaluating unidirectionality, integration and amplification as functional roles |
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