Final answer:
Ca2+ ions trigger the fusion of synaptic vesicles with the presynaptic cell's membrane, initiating exocytosis. This occurs when action potentials open voltage-gated calcium channels, allowing Ca2+ to enter the neuron and induce neurotransmitter release. The correct option for the student's question is 1) Ca2+ ions, exocytosis.
Step-by-step explanation:
The trigger for the fusion of synaptic vesicles with the synaptic membranes of the presynaptic cell is the influx of Ca2+ ions. When an action potential reaches the axon terminal, it causes depolarization of the membrane, subsequently opening voltage-gated Na+ channels. This further depolarization prompts the opening of voltage-gated Ca2+ channels, allowing Ca2+ to enter the neuron.
The entry of Ca2+ ions into the presynaptic neuron initiates a signaling cascade that causes synaptic vesicles filled with neurotransmitter to merge with the presynaptic membrane. This process is known as exocytosis, the release of neurotransmitters into the synaptic cleft where they can then bind to receptors on the postsynaptic neuron. This cascade is crucial for communication between neurons at chemical synapses. The correct option for the student's question is 1) Ca2+ ions, exocytosis.