Neurotransmitter release is primarily triggered by the arrival of an action potential at the axon terminal of a neuron. When the electrical impulse travels down the axon and reaches the synaptic terminal, it causes voltage-gated calcium channels to open. This influx of calcium ions into the neuron is critical; it signals the synaptic vesicles, which contain neurotransmitter molecules, to fuse with the presynaptic membrane.
Once these vesicles merge with the membrane, they release their contents into the synaptic cleft through a process called exocytosis. This release enables neurotransmitters to travel across the gap between neurons and bind to specific receptors on the postsynaptic neuron, facilitating communication in the nervous system. Various factors, including the frequency of action potentials and the type of neurotransmitter involved, can influence the amount and rate of neurotransmitter release.