3.5.8Inquiry Process
Synaptic Transmission — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 139 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Events at the Synapse
1
An action potential arrives at the presynaptic terminal and opens voltage-gated calcium channels, so Ca²⁺ enters.
2
Calcium triggers vesicles carrying neurotransmitter to fuse with the presynaptic membrane and release it by exocytosis.
3
The neurotransmitter diffuses across the cleft and binds to receptors that are ligand-gated ion channels in the postsynaptic membrane.
4
The channels open, ions move across and the postsynaptic membrane changes potential.
5
The transmitter is then removed, for example acetylcholine is hydrolysed by acetylcholinesterase and the products are reabsorbed.
Section 2 of 5Excitatory and Inhibitory Signals
Excitatory
- Excitatory Acetylcholine opens sodium channels, depolarising the postsynaptic membrane towards threshold and so making a new impulse more likely.
Inhibitory
- Inhibitory GABA opens chloride channels, hyperpolarising the membrane so it moves further from threshold and an impulse is less likely.
Spatial summation
- Spatial summation Inputs from several presynaptic neurones arriving together add up, and together may reach threshold when none could alone.
Temporal summation
- Temporal summation Repeated inputs from one neurone in rapid succession add up before the earlier effect has faded away.
| Type | Ion channel | Effect |
|---|---|---|
| Excitatory | Na⁺ channels open | Depolarisation |
| Inhibitory | Cl⁻ channels open | Hyperpolarisation |
Section 3 of 5Applications
Applications
Threshold decision
- Threshold decision The postsynaptic neurone sums all excitatory and inhibitory inputs and fires only if threshold is reached at the axon hillock.
Neonicotinoids
- Neonicotinoids These insecticides bind acetylcholine receptors in insects, and acetylcholinesterase does not break them down.
Resulting paralysis
- Resulting paralysis The receptors remain permanently stimulated, then the synapse is blocked, so the insect is paralysed and dies.
Selectivity
- Selectivity They bind insect receptors more tightly than vertebrate ones, but they can still harm beneficial insects such as bees.
Section 4 of 5Worked Example
Question
- Neuron X receives excitatory inputs of +8 mV from each of three presynaptic neurones and an inhibitory input of −10 mV from one.
- Resting potential is −70 mV and threshold is −55 mV.
- Does it fire when all four are active?
- Sum of excitatory inputs = 3 × 8 = +24 mV.
- Net change = +24 + (−10) = +14 mV.
- New potential = −70 + 14 = −56 mV, which is more negative than −55 mV.
Answer
- No, because −56 mV does not reach the −55 mV threshold;
- one more excitatory input would trigger firing.
Section 5 of 5Exam Checklist & Tips
3.5.8 – Key Points for the Exam
- Exocytosis Release of vesicle contents by fusion with the cell membrane.
- Summation Addition of postsynaptic potentials from different synapses or successive impulses.
- Acetylcholinesterase Enzyme that hydrolyses acetylcholine in the cleft.
- Inhibitory neurotransmitter A transmitter that makes the postsynaptic membrane more negative, such as GABA.
Common Mistakes
- Saying neurotransmitter enters the postsynaptic cell; it binds a receptor at the surface.
- Mixing up the ionscalcium enters presynaptically, sodium postsynaptically at excitatory synapses.
- Forgetting transmitter removal, without which the synapse would keep firing.
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