A-Level Biology AQA Notes

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Cholinergic Synapse

Structure of a synapse:
At a cholinergic synapse (acetylcholine is the neurotransmitter), an action potential arrives at the pre-synaptic knob, depolarising the membrane and causes voltage-gated calcium ion channels to open. The influx of Ca2+ ions causes the synaptic vesicles to fuse with the membrane, releasing the neurotransmitter into the synaptic cleft. The neurotransmitter diffuses and binds receptors on the post synaptic membrane, causing an action potential.

Acetylcholinesterase breaks down acetyl choline in the cleft.
The synapses can be excitatory if the neurotransmitter opens Na+ channels or inhibitory if the neurotransmitter opens chloride or potassium channels causing hyperpolarisation.
Spatial summation is when action potentials from multiple presynaptic neurones are added together in a post-synaptic neurone
Temporal summation is when multiple action potentials from a single presynaptic neurone are added together in a post-synaptic neurone over time.

Neuromuscular Junction

When an action potential reaches the junction, voltage-gated calcium channels open, causing calcium ions to diffuse into the neurone. Synaptic vessels to fuse with the presynaptic membrane and release acetylcholine into the synapse. It diffuses across the synapse and binds with receptors on the muscle cell surface membrane, opening sodium channels. The muscle fibre depolarisation causes an action potential and muscle contraction.
Acetylcholinesterase breaks down acetyl choline
Neuromuscular junction & cholinergic synapse differences

Neuromuscular Junction	Cholinergic Synapse
Only excitatory	Can be excitatory or inhibitory
Links neurones to muscle	Links either neurones to neurones or neurones to other effectors
The action potential ends here	Another action potential may be generated along the post-synaptic neurones
Only motor neurones are involved	Intermediate, motor and sensory neurones may be involved
Acetylcholine binds to receptors on the membrane of the muscle fibre	Acetylcholine binds to receptors on membrane of post-synaptic neurone

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