Synaptic Structure and Function Revision Notes | A Level AQA Biology

1. Why Synapses Matter

Your nervous system carries electrical signals at extraordinary speed — but those signals cannot leap directly from one neurone to the next. At every junction, a chemical relay takes over. Understanding that relay is the key to understanding how the nervous system coordinates every response your body makes.

This lesson covers the two most important junctions in AQA A Level Biology: the cholinergic synapse between two neurones, and the neuromuscular junction (NMJ) between a motor neurone and a muscle fibre. Both use acetylcholine, but their structures and consequences differ in ways the exam tests directly.

2. Structure of a Cholinergic Synapse

Picture two neurones end-to-end, separated by a tiny fluid-filled gap. The neurone carrying the signal towards the gap is the presynaptic neurone; its swollen terminal is the synaptic knob (also called the presynaptic terminal). The neurone receiving the signal is the postsynaptic neurone. Between them lies the synaptic cleft — a gap of roughly 20 nm.

• Synaptic knob: contains many mitochondria (for ATP production) and synaptic vesicles loaded with acetylcholine (ACh)
• Synaptic vesicles: membrane-bound sacs, each containing several thousand ACh molecules
• Presynaptic membrane: contains voltage-gated calcium ion channels
• Synaptic cleft: ~20 nm wide; filled with tissue fluid; contains acetylcholinesterase
• Postsynaptic membrane: contains complementary (nicotinic) acetylcholine receptors and sodium ion channels

The abundance of mitochondria in the synaptic knob is not coincidental — synthesising ACh and actively pumping ions both require ATP. This is a detail examiners use to test whether you can link structure to function.

ACh is synthesised in the synaptic knob from choline and acetyl coenzyme A, then packaged into synaptic vesicles. The vesicles are held near the presynaptic membrane, ready for rapid release when a nerve impulse arrives.

3. Transmission Across a Cholinergic Synapse — Step by Step

Now that you can picture the structure, here is how the signal actually crosses the cleft. Each stage has a precise mechanism — and examiners award marks for each one, so the sequence matters.

Worked example: Sequence of Events at a Cholinergic Synapse

Describe, in order, how an action potential in the presynaptic neurone leads to depolarisation of the postsynaptic membrane.

1. Depolarisation of the presynaptic membrane reaches the synaptic knob. — The electrical signal travels along the presynaptic neurone by local currents. It cannot cross the cleft electrically — the chemical relay begins here.
2. Depolarisation causes voltage-gated calcium ion channels in the presynaptic membrane to open. — Calcium ions (Ca²⁺) diffuse down their electrochemical gradient from the synaptic cleft into the synaptic knob. This influx is the trigger for vesicle release.
3. Ca²⁺ ions cause synaptic vesicles to move to and fuse with the presynaptic membrane; ACh is released by exocytosis into the synaptic cleft. — The more action potentials arrive in rapid succession, the more vesicles fuse — this is the basis of summation. Exocytosis is the precise term; avoid writing 'diffuse out of the vesicle'.
4. ACh molecules diffuse across the synaptic cleft (~20 nm) down their concentration gradient. — Diffusion is rapid over this tiny distance. The cleft is too narrow for active transport to be necessary.
5. ACh binds to complementary nicotinic acetylcholine receptors on the postsynaptic membrane. — Binding is specific — the receptor has a complementary shape to ACh. This causes associated sodium ion channels to open.
6. Na⁺ ions diffuse into the postsynaptic cell down their electrochemical gradient, depolarising the postsynaptic membrane. — If depolarisation reaches the threshold at the axon hillock, a new action potential is generated in the postsynaptic neurone.
7. Acetylcholinesterase in the synaptic cleft hydrolyses ACh into choline and ethanoate (acetate). Choline is reabsorbed into the presynaptic knob and used to re-synthesise ACh. — Hydrolysis terminates the signal — without it, Na⁺ channels would remain open and the postsynaptic neurone would fire continuously. This step also prevents the synapse from being permanently stimulated.

Answer: Action potential → Ca²⁺ influx → exocytosis of ACh → diffusion across cleft → binding to receptors → Na⁺ influx → postsynaptic depolarisation → ACh hydrolysed by acetylcholinesterase → choline recycled.

Notice that the synapse only works in one direction: vesicles are on the presynaptic side and receptors are on the postsynaptic side. This unidirectionality is a structural consequence, not a coincidence — it ensures signals travel in a controlled pathway through the nervous system.

4. Structure of the Neuromuscular Junction

With the cholinergic synapse clear in your mind, the neuromuscular junction (NMJ) will feel familiar — because it shares the same neurotransmitter and the same basic release mechanism. The differences lie in the postsynaptic cell and the scale of the response required.

At the NMJ, the presynaptic terminal is the motor neurone's synaptic knob. The postsynaptic cell is a skeletal muscle fibre. The postsynaptic membrane of the muscle fibre is highly folded into junctional folds, which dramatically increase the surface area available for acetylcholine receptors.

• Motor neurone terminal (synaptic knob): contains mitochondria and synaptic vesicles loaded with ACh
• Synaptic cleft: present between the motor neurone terminal and the muscle fibre membrane
• Motor end plate: the specialised region of the muscle fibre membrane at the NMJ
• Junctional folds: deep infoldings of the motor end plate that increase receptor surface area
• Nicotinic ACh receptors: present in high density on the junctional folds
• Acetylcholinesterase: present in the synaptic cleft to hydrolyse ACh after binding

The junctional folds are a key structural feature unique to the NMJ. They ensure that a single motor neurone action potential can reliably trigger a muscle action potential — the NMJ is designed for reliable, one-to-one transmission, unlike neurone-to-neurone synapses where summation is often required.

5. Comparing the Cholinergic Synapse and the Neuromuscular Junction

AQA explicitly requires you to compare these two junctions — which means identifying both similarities and differences. A vague answer that only lists features of each separately will not score comparison marks. You need to make direct, paired statements.

The most important functional difference is reliability. At the NMJ, the large surface area of junctional folds and the high density of receptors mean that every motor neurone action potential produces a muscle action potential. At a neurone-to-neurone synapse, a single presynaptic impulse often produces only a subthreshold excitatory postsynaptic potential (EPSP) — summation of multiple inputs is needed to fire the postsynaptic cell.

Both junctions share the same core chemistry: ACh is the neurotransmitter, nicotinic receptors mediate the response, and acetylcholinesterase terminates the signal. This shared mechanism means that drugs or toxins targeting ACh — such as nerve agents (organophosphates) or neuromuscular blocking agents (e.g. curare) — affect both synapses and NMJs simultaneously.

6. Key Functional Properties of Synaptic Transmission

Beyond the sequence of events, AQA expects you to explain why synapses have certain properties. Three properties come up repeatedly in exam questions: unidirectionality, summation, and the role of acetylcholinesterase.

Unidirectionality arises because synaptic vesicles and Ca²⁺-sensitive release machinery are only on the presynaptic side, while receptors are only on the postsynaptic side. The signal can only travel from pre- to postsynaptic — this ensures information flows in a defined direction through neural circuits.

Summation occurs when multiple subthreshold EPSPs combine to reach the threshold for an action potential. Temporal summation occurs when repeated impulses arrive at the same synapse in rapid succession; spatial summation occurs when impulses arrive simultaneously from multiple presynaptic neurones. Both increase the likelihood of a postsynaptic action potential.

Acetylcholinesterase is essential for signal termination. Without it, ACh would remain bound to receptors, Na⁺ channels would stay open, and the postsynaptic cell would be locked in a state of continuous depolarisation. Hydrolysis of ACh into choline and ethanoate ends the signal; choline is then actively transported back into the presynaptic knob for ACh re-synthesis, conserving resources.

7. Exam Technique and Common Mistakes

Synapse questions are among the most mark-rich in the nervous system topic — and also among the most commonly answered poorly. The mistakes below appear repeatedly in mark schemes and examiner reports. Knowing them before the exam is a genuine advantage.

For extended-response questions on synaptic transmission, structure your answer as: ion movement → vesicle event → diffusion → receptor binding → postsynaptic ion movement → outcome → termination. Covering all seven stages with correct terminology will address every available marking point.