Final answer:
Calcium ions are crucial for vesicle fusion and neurotransmitter release in the axon terminal at a chemical synapse.
Explanation:
Calcium ions are responsible for the fusion of vesicles in the axon terminal to release neurotransmitters into the synaptic cleft. When an action potential depolarizes the presynaptic membrane, it opens voltage-gated Ca²+ channels, allowing Ca²+ to enter the cell. This influx of calcium triggers a signaling cascade that leads to the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters through exocytosis.
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