Which event triggers neurotransmitter release at a chemical synapse?

• A. Binding of neurotransmitter to postsynaptic receptors.
• B. Presynaptic action potential arriving and calcium influx.
• C. Reuptake of neurotransmitters.
• D. Hyperpolarization of the postsynaptic membrane.

Answer: (B)

The trigger is the arrival of an action potential at the presynaptic terminal, which opens voltage-gated calcium channels and allows Ca2+ to enter. The resulting rise in intracellular calcium signals synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft via exocytosis. After release, the neurotransmitter diffuses across the gap and binds to receptors on the postsynaptic cell, producing the postsynaptic response. Reuptake and clearance of transmitter terminate the signal, and hyperpolarization of the postsynaptic membrane is a downstream effect, not the trigger for release.