Final answer:
Calcium ions (Ca²+) initiate the release of neurotransmitter contents from vesicles upon an action potential reaching the synaptic knob.
Step-by-step explanation:
When an action potential passes over the surface of a synaptic knob, the contents of the vesicles are released in response to the presence of Calcium ions (Ca²+). An action potential reaching the axon terminal triggers the depolarization of the membrane, which opens voltage-gated sodium (Na+) channels. This further depolarization leads to the opening of voltage-gated calcium channels. The influx of calcium ions into the cell causes synaptic vesicles filled with neurotransmitters to fuse with the presynaptic membrane and release their contents into the synaptic cleft through a process known as exocytosis.