In the context of synaptic transmission, which protein is responsible for sensing calcium influx and triggering the fusion of neurotransmitter vesicles with the presynaptic membrane?

Solution

1. Action potentials reach the presynaptic terminal, opening voltage-gated ${\text{Ca}}^{2+}$ channels.
2. The influx of ${\text{Ca}}^{2+}$ ions increases the local concentration near the active zone.
3. Synaptotagmin binds to ${\text{Ca}}^{2+}$ ions.
4. This binding triggers the SNARE complex to mediate vesicle fusion.
5. Hence the answer is (B).