IB Biology HLNerves & SignallingPaper 1 & 2~13 min read
Synaptic Transmission
A synapse is not just a gap to be crossed. It is a decision point — it can add signals up, cancel them out, or be hijacked entirely by a chemical that was never meant to be there.
📚 What you need to know
Neonicotinoids are synthetic pesticides similar to nicotine. They bind irreversibly to acetylcholine receptors in insects, blocking transmission and causing paralysis and death.
They affect insects far more than mammals, which is why they are used — but they are controversial because of their effect on pollinators such as bees.
Cocaine blocks the reuptake of dopamine by binding to the dopamine transporter protein, so dopamine builds up in the synapse.
Some neurotransmitters are inhibitory: they open potassium ion channels, the postsynaptic neurone becomes hyperpolarised, and threshold cannot be reached.
Inhibitory synapses stop random impulses and allow specific pathways to be stimulated. These pathways develop over time.
Summation is the adding together of several impulses so that threshold is reached.
Temporal summation — impulses arriving in quick succession at one synaptic knob.
Spatial summation — impulses arriving at the same time at several different synaptic knobs.
Chemicals from outside the body
Neonicotinoids
Neonicotinoids are synthetic compounds similar to nicotine, and they are common in pesticides. They block synaptic transmission at cholinergic synapses in insects by binding to the acetylcholine receptors.
The binding is irreversible, because acetylcholinesterase cannot break neonicotinoids down.
With the receptors blocked, acetylcholine cannot bind, so impulses stop being transmitted across the synapse.
The insect cannot send signals to its muscles, so this leads to paralysis and death.
They are considered suitable as pesticides because they are not toxic to humans and other mammals, for two reasons:
A much larger proportion of synapses in insects are cholinergic compared with mammals.
Neonicotinoids bind much more strongly to acetylcholine receptors in insects.
There is a great deal of controversy over their use because of the effect they are thought to have on essential pollinators such as bees.
Notice the difference between this and an acetylcholinesterase inhibitor. Blocking the enzyme leaves the synapse permanently switched on. Blocking the receptor switches it permanently off. Both cause paralysis, but for opposite reasons — examiners like testing that.
Cocaine
Cocaine works on a completely different neurotransmitter and a completely different step.
Cocaine blocks the reuptake of neurotransmitters into the presynaptic knob.
It mainly affects dopamine, by binding to the dopamine transporter protein.
Dopamine can no longer bind to its transporter, so it cannot be moved back through the membrane into the presynaptic neurone.
As a result dopamine builds up in the synapse, which can lead to feelings of pleasure.
Cocaine also blocks serotonin and norepinephrine, which enhances feelings of confidence and energy.
In regular users, the brain responds by increasing the number of dopamine receptors to cope with the high dopamine levels. Once levels return to normal, having all those extra receptors leads to increased sensitivity and depression.
Chemical
Where it acts
Result
Neonicotinoid
Binds irreversibly to ACh receptors on the postsynaptic membrane in insects
ACh cannot bind, transmission stops, paralysis and death
Cocaine
Binds to the dopamine transporter protein on the presynaptic membrane
Dopamine is not reabsorbed, builds up in the synapse, feelings of pleasure
Inhibitory postsynaptic potentials
Not every neurotransmitter makes the next neurone fire. Some do the opposite.
Some neurotransmitters result in an action potential in the postsynaptic neurone — these are excitatory.
Others prevent one. This is inhibition, and the impulse stops at the synapse.
The commonest way of inhibiting an impulse is by opening gated potassium ion channels in the postsynaptic membrane, so potassium ions diffuse out of the cell body.
The postsynaptic neurone becomes even more negatively charged, or hyperpolarised.
Because it starts further from threshold, the threshold will not be reached when the neurone is stimulated, so an action potential cannot be triggered.
When both arrive at once
If a motor neurone cell body is receiving from an excitatory synapse and an inhibitory synapse at the same time:
Sodium ions enter the cell body from the excitatory synapse, making the inside less negative.
Potassium ions leave the cell body because of the inhibitory synapse, making it more negative.
The two cancel each other out.
Threshold is not reached, so no action potential is generated.
Both neurones received exactly the same excitatory input. The only difference is that the second one also had an inhibitory synapse firing, pulling it further from threshold.
Why inhibition is useful
Inhibitory synapses play a vital role in the nervous circuit.
They prevent random impulses being sent around the body.
They allow specific pathways to be stimulated. A reflex should be rapid but also specific — if you grab a plank with a nail in it, you want your arm muscles to pull your hand away, not your leg muscles to move your foot.
Inhibitory pathways develop over time. They matter for skills such as painting and drawing, and young children struggle with these because their inhibitory pathways have not yet developed enough to refine uncontrolled movements.
Summation
When an impulse arrives at a synapse it does not always cause an impulse in the next neurone. A single impulse at one synaptic knob is often not enough, because:
Only a small amount of acetylcholine is released into the cleft.
Only a small number of gated ion channels open.
An insufficient number of sodium ions cross the membrane.
The threshold potential is not reached.
The small amount of ACh on the receptors is broken down rapidly by acetylcholinesterase before any more can arrive.
The effect of several impulses can be added together to get past this. That is summation, and it comes in two forms.
Type
What arrives
How it works
Temporal
Several impulses at one synaptic knob in quick succession
Each impulse adds more ACh before the previous lot has been cleared, so a large amount builds up, many gated channels open, enough sodium enters and threshold is reached
Spatial
Impulses at several different synaptic knobs at the same time
Several knobs release ACh onto the same cell body at once, giving a large amount in one go and generating an action potential
In the left graph each impulse starts from a slightly higher point than the last, because the previous one has not fully decayed. That stacking is the whole idea of temporal summation.
Why summation is worth having
It allows the effect of a stimulus to be magnified.
A combination of different stimuli can trigger a response.
It stops the nervous system being overwhelmed — synapses act as a barrier, only passing signals on when there has been enough input from other neurones and receptors.
Worked examples
WE 1
Explain the selective toxicity of a neonicotinoid
Explain why a neonicotinoid pesticide kills insects but has little effect on mammals. (4 marks)
Point 1: how it acts
It binds to acetylcholine receptors on the postsynaptic membrane, so ACh cannot bind and transmission is blocked.
Point 2: why it is permanent
The binding is irreversible because acetylcholinesterase cannot break neonicotinoids down.
Point 3: the effect on the insect
Impulses cannot reach the muscles, causing paralysis and death.
Point 4: why mammals cope
A much larger proportion of insect synapses are cholinergic, and neonicotinoids bind far more strongly to insect ACh receptors than to mammalian ones.
Blocked receptors, and insects have far more of the vulnerable synapsesthe last point is the one people miss. "Insects are smaller" is not an answer
WE 2
Explain the effect of an inhibitory synapse
A motor neurone cell body receives input from an excitatory synapse and an inhibitory synapse at the same time. Explain why no action potential is generated. (3 marks)
Step 1: the excitatory side
Sodium ions enter the cell body, making the inside less negative.
Step 2: the inhibitory side
Gated potassium channels open, so K⁺ diffuses out and the membrane becomes hyperpolarised.
Step 3: the two together
The potassium leaving cancels out the sodium entering, so the membrane potential never reaches −55 mV.
Threshold is not reached, so no action potentialuse the word hyperpolarised and name both ions and their directions
WE 3
Distinguish between the two types of summation
Distinguish between temporal and spatial summation. (3 marks)
Point 1: temporal
Several impulses arrive in quick succession at a single synaptic knob.
Point 2: spatial
Impulses arrive at the same time at several different synaptic knobs on the same cell body.
Point 3: what they share
Both release a large enough total amount of acetylcholine to open many gated channels, so enough sodium enters to reach threshold.
Temporal = one knob over time. Spatial = many knobs at one moment."distinguish" means give the contrast in the same sentence structure for both — not two unconnected descriptions
💡 Exam tips
For neonicotinoids, say irreversible and say why — the enzyme cannot break them down.
For cocaine, name the exact target: the dopamine transporter protein, and the exact process: reuptake.
Inhibition works through potassium leaving, not sodium being blocked.
Temporal is about time; spatial is about space. The names tell you the answer.
If a graph shows small bumps that do not reach threshold, the word the examiner wants is summation.
⚠ Common mistakes
Saying neonicotinoids block acetylcholinesterase. They block the receptors; the enzyme simply cannot destroy them.
Saying cocaine increases dopamine release. It stops dopamine being taken back up.
Saying an inhibitory synapse stops sodium entering. It opens potassium channels and hyperpolarises the membrane.
Describing summation as making a bigger action potential. It makes threshold more likely to be reached; the action potential is the same size.
Mixing up the two summations. One knob repeatedly is temporal; many knobs at once is spatial.
Forgetting that inhibitory pathways develop with age. It is an easy mark on a "suggest why children struggle" question.
Up next: Neurones in the Brain — how pain is detected and perceived, and what happens when you connect billions of these cells together.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
