What event is triggered by calcium entry into the axon terminal?

The correct answer is that synaptic vesicles fuse to the plasma membrane of the axon terminal in response to calcium entry into the axon terminal. When an action potential reaches the axon terminal, it causes voltage-gated calcium channels to open, allowing calcium ions to flow into the terminal. This influx of calcium ions acts as a crucial signal that triggers the synaptic vesicles, which contain neurotransmitters, to move toward and fuse with the presynaptic membrane. The fusion process leads to the release of neurotransmitters into the synaptic cleft, allowing communication between neurons.

This process is fundamental to neurotransmission, as the binding of neurotransmitters to receptors on the postsynaptic neuron's membrane can initiate a new action potential, continuing the transmission of signals in the nervous system. Understanding this mechanism is essential for elucidating how neuronal communication occurs at synapses.