Neurones never quite touch. There is always a gap, and an electrical impulse cannot cross it. So the signal changes form: for a fraction of a second it stops being electrical and becomes chemical instead.
📚 What you need to know
Two neurones are separated by a tiny gap, the synaptic cleft. The two neurone endings plus the cleft form a synapse.
Impulses cannot jump the cleft, so the signal is carried across chemically by a neurotransmitter.
Depolarisation of the presynaptic membrane opens calcium ion channels; Ca2+ flooding in makes vesicles fuse with the membrane.
The neurotransmitter diffuses across and binds to receptors, opening sodium ion channels on the postsynaptic membrane.
If enough binds, the threshold is reached and a new action potential is generated.
Acetylcholine (ACh) is broken down by acetylcholinesterase into acetate and choline, which are reabsorbed and reused.
Synapses are unidirectional: transmitter is released on one side and receptors are on the other.
What a synapse is
Where two neurones meet they do not come into physical contact. A very small gap — the synaptic cleft — separates them, and the whole junction is called a synapse.
Synapses are the junctions between any cells in the nervous system, not just neurone-to-neurone. In a sense organ there are synapses between sensory receptor cells and sensory neurones; at a muscle there are synapses between motor neurones and muscle fibres.
Notice that the receptors are only on the postsynaptic side and the vesicles only on the presynaptic side. Every other feature of a synapse follows from that.
Synaptic transmission, step by step
The sequence below is worth learning as an ordered list. Exam questions frequently ask you to describe it, and the marks are given for the steps in the right order.
Steps 1 to 4 are electrical becoming chemical; steps 5 to 7 are chemical becoming electrical again. Step 8 resets everything ready for the next impulse.
The same sequence in sentences
An impulse arrives at the end of the axon and depolarises the presynaptic membrane.
That opens calcium ion channels, so calcium ions flood into the presynaptic knob.
The calcium ions cause vesicles of neurotransmitter to move to the presynaptic membrane and fuse with it.
Neurotransmitter — commonly acetylcholine — is released into the synaptic cleft.
It diffuses across the cleft and binds to receptor molecules on the postsynaptic membrane.
Binding opens sodium ion channels, so sodium ions diffuse into the postsynaptic cell.
If enough molecules bind and the threshold is reached, an action potential is generated and travels down the postsynaptic axon.
The neurotransmitter is then broken down, which stops the postsynaptic neurone being stimulated continuously.
The word diffuses is doing real work in step 5. The neurotransmitter is not pumped or pushed across — it moves down its concentration gradient, which is why the cleft has to be so narrow.
Acetylcholine and cholinergic synapses
There are over 40 known neurotransmitters, including dopamine and noradrenaline. The one you need in detail is acetylcholine (ACh), and synapses using it are called cholinergic synapses.
ACh is made in the presynaptic neurone by combining choline with an acetyl group.
It is stored in vesicles until an impulse arrives.
It binds to specific receptors on the postsynaptic membrane, opening the associated sodium ion channels.
Breaking it down again
If ACh stayed bound, the sodium channels would stay open and the postsynaptic membrane would be permanently depolarised — the signal would never stop. So it is destroyed almost immediately.
Catalysed by acetylcholinesterase
acetylcholine + water → acetate + choline
The enzyme acetylcholinesterase catalyses the hydrolysis of ACh into acetate and choline.
Both products are reabsorbed into the presynaptic neurone.
There they are recombined, using ATP, to reform active ACh ready for the next impulse.
This is why the enzyme name is worth learning precisely. Acetylcholine is the transmitter; acetylcholinesterase is the enzyme that destroys it. One letter cluster apart, opposite jobs.
Why synapses only work one way
Impulses can travel in either direction along an axon, but a synapse forces the signal one way. The reason is structural, and you saw it in the first diagram:
Neurotransmitter is released on one side only — the presynaptic side has the vesicles.
The receptors are on the other side only — the postsynaptic membrane.
Chemical transmission therefore cannot happen in reverse, so impulses cannot travel backwards.
That one-way rule is what keeps the nervous system organised: signals from receptors always head towards the CNS, and instructions always head out towards effectors.
Worked examples
WORKED EXAMPLE
Describe how an impulse is transmitted across a cholinergic synapse. [4]
Step 1: what the arriving impulse does
Depolarises the presynaptic membrane, opening calcium ion channels.
Step 2: what calcium triggers
Vesicles fuse with the presynaptic membrane and release ACh into the cleft.
Step 3: crossing the gap
ACh diffuses across and binds to receptors on the postsynaptic membrane.
Step 4: the new impulse
Sodium ion channels open, Na⁺ enters, and if the threshold is reached an action potential is generated.
Depolarisation → Ca²⁺ in → vesicles fuse → ACh diffuses and binds → Na⁺ in → new impulsefour marks, four stages — write them in order and name the ions
WORKED EXAMPLE
Some insecticides inhibit acetylcholinesterase. Predict the effect on the postsynaptic neurone and explain why. [3]
Step 1: what the enzyme normally does
It hydrolyses ACh into acetate and choline, clearing it from the cleft.
Step 2: what happens if it is blocked
ACh stays bound to the receptors, so the sodium ion channels remain open.
Step 3: the consequence
The postsynaptic membrane stays depolarised and keeps firing action potentials.
Continuous stimulation of the postsynaptic neurone, so it cannot be switched offthis is exactly why such compounds are toxic — muscles are held contracted
WORKED EXAMPLE
Explain why a nerve impulse can only travel in one direction across a synapse. [2]
Step 1: where the transmitter is
Vesicles of neurotransmitter are only in the presynaptic neurone.
Step 2: where the receptors are
Receptor molecules are only on the postsynaptic membrane.
Transmitter can only be released on one side and only detected on the otheranswer with structure, not with “because that is the way the signal goes”
💡 Exam tip
Name all three ions in their right places: Ca2+ into the presynaptic knob, Na+ into the postsynaptic cell.
Use diffuses for the neurotransmitter crossing the cleft. It is not active transport.
Say vesicles fuse with the presynaptic membrane — that phrase is usually a mark on its own.
Mention the threshold. A few molecules binding is not enough to fire the next neurone.
Spell acetylcholinesterase carefully and say it causes hydrolysis.
Explain unidirectionality using the positions of vesicles and receptors.
⚠ Common mix-up
Saying the impulse jumps the synaptic cleft. It cannot — a chemical carries the signal across.
Confusing acetylcholine with acetylcholinesterase. One is the transmitter; the other destroys it.
Swapping the ions. Calcium enters the presynaptic side; sodium enters the postsynaptic side.
Describing the neurotransmitter as being pumped across. It diffuses down a concentration gradient.
Forgetting the breakdown step. Without it the postsynaptic neurone would fire continuously.
Thinking every binding event fires the next neurone. Enough must bind to reach the threshold.
Saying synapses slow the signal down for no reason. They allow one-way transmission and control over which pathways fire.
That completes Nerves & Signalling. Up next: Hormones & Homeostasis — the body’s other messaging system, slower than nerves but reaching every cell at once.
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