How does acetylcholine transmit the nervous impulse across a synapse?

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An action potential from the axon arrives at the presynaptic neuron, stimulating voltage-gated calcium (Ca2+) ion channels to open. This allows calcium ions to diffuse into the synaptic knob, causing synaptic vesicles containing acetylcholine (the neurotransmitter) to fuse with the presynaptic membrane and release into the synaptic cleft: exocytosis. Acetylcholine then diffuses across the synaptic cleft and binds to cholinergic receptors on the postsynaptic membrane, causing ligand-gated sodium (Na+) channels to open. This results in the facilitated diffusion of sodium into the postsynaptic membrane - causing an action potential if threshold is reached. Acetylcholinesterase then breaks down the neurotransmitter to prevent continuous stimulation. 

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