IB Biology HLNerves & SignallingPaper 1 & 2~12 min read
Synapses
Neurones never actually touch. There is always a tiny gap, and an electrical impulse cannot jump it. So the signal changes form — from electrical to chemical and back again — millions of times a day inside your head.
📚 What you need to know
Where two neurones meet they do not touch. A tiny gap called the synaptic cleft separates them.
The two neurone endings plus the cleft together form a synapse.
Synapses join any cells in the nervous system — receptor to sensory neurone, neurone to neurone, motor neurone to muscle fibre.
Electrical impulses cannot jump the cleft, so the signal is carried across by a chemical called a neurotransmitter.
An arriving impulse depolarises the presynaptic membrane, letting calcium ions in, which makes vesicles fuse with the membrane and release neurotransmitter.
A common neurotransmitter is acetylcholine (ACh); synapses using it are called cholinergic synapses.
ACh binds to receptors on the postsynaptic membrane, opening sodium ion channels. If enough binds, threshold is reached and an action potential is generated.
Acetylcholinesterase breaks ACh down into acetate and choline, which are reabsorbed and recycled.
Synapses make transmission one way only.
What a synapse is
Where two neurones meet, they do not come into physical contact. A very small gap called the synaptic cleft sits between them. The end of the first neurone, the cleft, and the start of the second neurone together make up the synapse.
Synapses are not just between neurones. They act as the junctions between any cells in the nervous system:
In sense organs, between sensory receptor cells and sensory neurones
In muscles, between motor neurones and muscle fibres
The membrane on the neurone before the gap is the presynaptic membrane. The one after the gap is the postsynaptic membrane. Get those two words the right way round and half the topic becomes easier.
The receptor proteins are the reason a synapse is selective. Only a molecule with a complementary shape fits, so the wrong chemical arriving in the cleft does nothing.
How the message gets across
Acetylcholine is made in the presynaptic neurone by joining choline to an acetyl group. It is stored in vesicles until it is needed. Here is the whole sequence.
Read down the green column first, then down the orange one. Steps 9 and 10 are the ones students leave out, and they are usually worth a mark each.
Why the neurotransmitter has to be destroyed
If acetylcholine stayed stuck to the receptors, the sodium ion channels would stay open and the postsynaptic membrane would stay permanently depolarised. The neurone could never reset, so it could never fire again.
So the enzyme acetylcholinesterase catalyses the hydrolysis of ACh into acetate and choline. These products are absorbed back into the presynaptic neurone, where ACh is rebuilt. Nothing is wasted, and the synapse is ready for the next impulse.
This is a lovely place for a "suggest" question. Anything that blocks acetylcholinesterase leaves ACh sitting on the receptors, so the muscle stays contracted. That is exactly how some nerve agents and insecticides work.
One way only
Synapses make sure impulses can travel in one direction only. The reason is beautifully simple:
Neurotransmitter is only released on one side of the cleft — the presynaptic side, because that is where the vesicles are.
Receptors are only on the other side — the postsynaptic membrane.
So chemical transmission simply cannot happen in the opposite direction. There is nothing to release, and nothing to receive.
This stops impulses wandering back the way they came and keeps signals travelling along proper pathways.
Two reasons, not one. Impulses travel one way for two separate reasons, and exams sometimes ask for both. Along an axon it is the refractory period. At a synapse it is the one-sided arrangement of vesicles and receptors.
Worked examples
WE 1
Describe transmission across a cholinergic synapse
Describe how an impulse is transmitted across a cholinergic synapse. (5 marks)
Point 1: the trigger
The action potential depolarises the presynaptic membrane, so calcium ion channels open and calcium ions diffuse in.
Point 2: release
Calcium causes vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.
Point 3: crossing
ACh diffuses across the cleft and binds to complementary receptor proteins on the postsynaptic membrane.
Point 4: the new impulse
Sodium ion channels open, Na⁺ diffuses in, and if enough binds the membrane reaches threshold and a new action potential is generated.
Point 5: the reset
Acetylcholinesterase hydrolyses the ACh into acetate and choline, which are reabsorbed and recycled.
Calcium in, vesicles fuse, ACh across, sodium in, enzyme clears updescribe questions want the steps in order. Numbering them in your answer costs nothing and keeps you from skipping one
WE 2
Suggest the effect of blocking an enzyme
A drug binds to acetylcholinesterase and stops it working. Suggest the effect on a synapse between a motor neurone and a muscle fibre. (3 marks)
Step 1: what the enzyme normally does
It hydrolyses ACh in the cleft, clearing it off the receptors.
Step 2: what happens without it
ACh is not broken down, so it stays bound to the receptors on the postsynaptic membrane.
Step 3: the consequence
Sodium ion channels stay open, so the membrane stays depolarised and action potentials keep being generated in the muscle fibre.
Continuous stimulation — the muscle stays contracted and cannot relaxfor "suggest" questions, follow the chain right through to an effect you can actually see, such as paralysis
💡 Exam tips
Learn the order of ions: calcium in first, then sodium in on the other side. Mixing these up is the most common error in the whole topic.
Say ACh binds to receptors, not "enters the postsynaptic neurone". It never crosses that membrane.
Name the enzyme in full: acetylcholinesterase. Spelling matters here.
Mention diffusion across the cleft — it is a passive process and a free mark.
For unidirectionality, quote vesicles on one side, receptors on the other.
⚠ Common mistakes
Saying the impulse jumps across the synapse. It cannot — that is the whole point of a chemical synapse.
Confusing acetylcholine with acetylcholinesterase. One is the messenger, the other is the enzyme that destroys it.
Saying calcium ions cross the synaptic cleft. They enter the presynaptic knob from outside the cell.
Forgetting the threshold. Neurotransmitter binding does not guarantee an action potential; enough of it must bind.
Leaving out recycling. Choline is reabsorbed and used again.
Writing "presynaptic" when you mean "postsynaptic". Read your answer back and check every one.
Up next: Action Potentials — a proper look at the spike itself, and why it is all-or-nothing.
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