Final answer:
Neurotransmitters are released when synaptic vesicles fuse with the presynaptic membrane after being triggered by calcium ions, allowing them to diffuse across the synaptic cleft and potentially initiate a response in the postsynaptic neuron.
Step-by-step explanation:
After the synaptic vesicles fuse to the presynaptic membrane, it allows neurotransmitter to be released into the synaptic cleft. This process, known as exocytosis, involves the vesicles merging with the presynaptic membrane due to an influx of Ca2+ ions when an action potential reaches the axon terminal.
Once released, the neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane. This binding may cause a localized depolarization or hyperpolarization of the postsynaptic neuron, leading to the propagation or inhibition of the neural signal.