Final answer:
Neurotransmitters are released into the synaptic gap through a process known as exocytosis, which is triggered by the influx of calcium ions into the presynaptic neuron's axon terminal.
Step-by-step explanation:
The neurotransmitter is released into the synaptic gap by exocytosis.
When an action potential reaches the axon terminals, it triggers the opening of voltage-gated Ca2+ channels in the presynaptic membrane. The influx of calcium ions (Ca2+) causes the synaptic vesicles, which contain neurotransmitters, to merge with the presynaptic membrane and release their contents into the synaptic cleft through exocytosis.
This mechanism ensures that the chemical signal in the form of a neurotransmitter can bridge the gap between the neurons and continue the transmission of the nervous impulse. After their release, neurotransmitters diffuse across the synaptic cleft and bind to ligand-gated ion channels on the postsynaptic membrane, which can cause a response in the target cell.