Final answer:
The correct answer is calcium (Ca²⁺). It causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane when it enters the synaptic knob due to action potential induced depolarization.
Step-by-step explanation:
An influx of calcium (Ca²⁺) into the synaptic knob results in the fusing of vesicles containing neurotransmitters with the knob's cellular membrane. When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated Na⁺ channels. This further depolarization causes voltage-gated Ca²⁺ channels to open, allowing Ca²⁺ ions to enter the cell. The entry of calcium ions initiates a series of events that result in the fusion of synaptic vesicles with the presynaptic membrane, thereby releasing neurotransmitters into the synaptic cleft through exocytosis. The neurotransmitters then diffuse across the synaptic cleft and may bind to ligand-gated ion channels in the postsynaptic membrane, causing a response in the next cell.