3.4.4Additional lessons
Synapses — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 64 of 142
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.
Part 1 of 5Structure of a Synapse
Presynaptic neurone
- Presynaptic neurone Its terminal contains many mitochondria and vesicles filled with neurotransmitter,
- and voltage-gated calcium channels in its membrane.
Synaptic cleft
- Synaptic cleft This narrow fluid-filled gap, about 20 nm wide, separates the two cells so the electrical impulse cannot jump across directly.
Postsynaptic membrane
- Postsynaptic membrane It carries receptor proteins that bind neurotransmitter and ligand-gated ion channels that open when the neurotransmitter attaches.
Neurotransmitter
- Neurotransmitter A chemical messenger such as acetylcholine that is released by exocytosis, crosses the cleft by diffusion and binds a specific receptor.
Part 2 of 5Transmission Across an Excitatory Synapse
Each step depends on the one before it, so the order is a common exam question.
1
An action potential arrives at the presynaptic terminal and depolarises the membrane there.
2
Voltage-gated calcium channels open and Ca²⁺ ions diffuse into the terminal, moving down their concentration gradient.
3
The rise in Ca²⁺ makes synaptic vesicles fuse with the presynaptic membrane, releasing neurotransmitter by exocytosis.
4
Neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane, which opens ligand-gated sodium channels.
5
Na⁺ entry depolarises the postsynaptic membrane, and if the threshold is reached an action potential starts in the postsynaptic neurone.
6
Neurotransmitter is then removed, for example acetylcholine is broken down by the enzyme acetylcholinesterase, so the signal ends.
Part 3 of 5Excitation, Inhibition and Summation
Excitation, Inhibition and Summation
Excitatory synapse
- Excitatory synapse Neurotransmitter causes depolarisation of the postsynaptic membrane, moving it closer to the threshold for an action potential.
Inhibitory synapse
- Inhibitory synapse Neurotransmitter opens channels that make the inside more negative, so the postsynaptic membrane is less likely to fire.
Threshold
- Threshold A single small depolarisation is often too weak, so the postsynaptic neurone fires only if the total input reaches the threshold.
Summation
- Summation Inputs from many synapses are added together, which allows one neurone to integrate excitatory and inhibitory signals.
Part 4 of 5Why Synapses Matter
Why Synapses Matter
Unidirectional
- Unidirectional Only the presynaptic terminal has vesicles and only the postsynaptic membrane has receptors, so impulses pass in one direction.
Delay
- Delay Diffusion and vesicle release take time, so a synapse adds a small delay compared with conduction along an axon.
Control and integration
- Control and integration Many inputs converge on one neurone, so the nervous system can process and filter information.
Drug targets
- Drug targets Some drugs and toxins block receptors or enzymes at synapses, so they change transmission without affecting the axon itself.
Part 5 of 5Exam Checklist & Tips
3.4.4 – Key Points for the Exam
- Synapse A junction between two neurones, or a neurone and an effector, where information passes by chemical signalling.
- Neurotransmitter A chemical released by a presynaptic neurone that binds receptors on the postsynaptic cell.
- Exocytosis The fusion of vesicles with the cell surface membrane to release their contents.
- Acetylcholinesterase An enzyme in the cleft that breaks down acetylcholine to end the signal.
- Threshold The membrane potential that must be reached to trigger an action potential.
Common Mistakes
- Stating that the action potential itself crosses the cleft;
- the signal is carried by neurotransmitter diffusing across.
- Forgetting that calcium ions enter the presynaptic terminal, and writing that sodium triggers vesicle fusion.
- Saying neurotransmitter is actively transported across the cleft; it crosses by diffusion.
- Leaving out removal of neurotransmitter, which is why a single impulse produces a short, controlled response.
Last updated:
\n