OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(d) the structure and roles of synapses in neurotransmission.
Full specification wording
Neuronal communication: (d) the structure and roles of synapses in neurotransmission.
A synapse is the junction between a presynaptic neurone and a postsynaptic cell, separated by the synaptic cleft (about 20 nm).
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A synapse is the junction between a presynaptic neurone and a postsynaptic cell, separated by the synaptic cleft (about 20 nm). The presynaptic knob contains vesicles of neurotransmitter, mitochondria and voltage-gated Ca²⁺ channels. When an action potential arrives, Ca²⁺ channels open and Ca²⁺ enters; vesicles fuse with the presynaptic membrane and release neurotransmitter by exocytosis into the cleft. The transmitter diffuses across and binds receptors on the postsynaptic membrane, opening ion channels. This may depolarise the postsynaptic membrane (excitatory, EPSP) or hyperpolarise it (inhibitory, IPSP). Synapses ensure one-way transmission, allow summation of signals, and let one neurone connect to many cells. Acetylcholinesterase breaks down acetylcholine to stop the signal.
What earns the marks
- The synaptic cleft separates the presynaptic and postsynaptic membranes and is the gap across which neurotransmitter diffuses.
- An arriving action potential opens voltage-gated Ca²⁺ channels, and Ca²⁺ entry triggers vesicle fusion and exocytosis of neurotransmitter.
- Neurotransmitter binds specific receptors on the postsynaptic membrane, opening ion channels.
- Excitatory synapses depolarise (EPSP); inhibitory synapses hyperpolarise (IPSP) the postsynaptic membrane.
- Synapses ensure unidirectional transmission and allow summation of impulses.
- Acetylcholinesterase hydrolyses acetylcholine, ending the signal and preventing continuous stimulation.
Common mistakes and how to put them right
Saying the electrical impulse crosses the cleft.
Put it right: A chemical neurotransmitter diffuses across; the electrical signal is regenerated postsynaptically.
Thinking Ca²⁺ enters the postsynaptic cell.
Put it right: Ca²⁺ enters the presynaptic knob to trigger vesicle release.
Ignoring inhibition: correction — inhibitory synapses hyperpolarise the postsynaptic membrane, raising the threshold.
Forgetting to remove transmitter.
Put it right: Acetylcholinesterase breaks down acetylcholine so the signal stops.
Exam tips
- Write the transmission sequence in order: impulse arrives → Ca²⁺ in → vesicles fuse → exocytosis → diffusion → receptor binding → ion channels open.
- Distinguish EPSP (depolarisation) from IPSP (hyperpolarisation) when explaining summation.
- Name acetylcholinesterase and state its role in terminating the signal.
Try the lesson
The start of the Neuronal communication: (d) the structure and roles of synapses in neurotransmission. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the main role of the synaptic cleft in neurotransmission?
Listen
A synapse is the junction between a presynaptic neurone and a postsynaptic cell, separated by the synaptic cleft (about 20 nm). The presynaptic knob contains vesicles of neurotransmitter, mitochondria and voltage-gated Ca²⁺ channels. When an action potential arrives, Ca²⁺ channels open and Ca²⁺ enters; vesicles fuse with the presynaptic membrane and release neurotransmitter by exocytosis into the cleft. The transmitter diffuses across and binds receptors on the postsynaptic membrane, opening ion channels. This may depolarise the postsynaptic membrane (excitatory, EPSP) or hyperpolarise it (inhibitory, IPSP). Synapses ensure one-way transmission, allow summation of signals, and let one neurone connect to many cells. Acetylcholinesterase breaks down acetylcholine to stop the signal. A strong answer earns marks for these points. The synaptic cleft separates the presynaptic and postsynaptic membranes and is the gap across which neurotransmitter diffuses. An arriving action potential opens voltage-gated Ca²⁺ channels, and Ca²⁺ entry triggers vesicle fusion and exocytosis of neurotransmitter. Neurotransmitter binds specific receptors on the postsynaptic membrane, opening ion channels. Excitatory synapses depolarise (EPSP); inhibitory synapses hyperpolarise (IPSP) the postsynaptic membrane. A common slip: Saying the electrical impulse crosses the cleft. Instead, a chemical neurotransmitter diffuses across; the electrical signal is regenerated postsynaptically. Exam tip: Write the transmission sequence in order: impulse arrives → Ca²⁺ in → vesicles fuse → exocytosis → diffusion → receptor binding → ion channels open.
Key words
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Synaptic cleft
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Calcium-triggered exocytosis
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Excitatory and inhibitory postsynaptic effects
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Unidirectional transmission and signal termination
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See every definition
- Synaptic cleft
- The gap, about 20 nm wide, that separates the presynaptic and postsynaptic membranes and across which neurotransmitter diffuses.
- Calcium-triggered exocytosis
- An arriving action potential opens voltage-gated Ca²⁺ channels, and Ca²⁺ entry into the presynaptic knob triggers vesicle fusion and release of neurotransmitter into the cleft.
- Excitatory and inhibitory postsynaptic effects
- Depolarisation of the postsynaptic membrane gives an excitatory effect (EPSP), while hyperpolarisation gives an inhibitory effect (IPSP).
- Unidirectional transmission and signal termination
- Synapses ensure one-way transmission and allow summation of impulses, and acetylcholinesterase breaks down acetylcholine to stop the signal.
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