An action potential arrives at the presynaptic terminal of a central synapse. What is the immediate trigger for fusion of synaptic vesicles with the presynaptic membrane?

• A Depolarization-induced opening of transmitter-gated chloride channels
• B Activation of voltage-gated calcium channels in the active zone, raising intracellular calcium ✓
• C Reversal of the vesicular proton pump, alkalinizing the vesicle lumen
• D Direct mechanical coupling of the action potential to the SNARE complex without any ion flux

Explanation

Depolarization of the terminal opens voltage-gated Ca2+ channels clustered at the active zones. The rise in intracellular Ca2+ binds synaptotagmin on the vesicle, allowing SNARE-mediated fusion and transmitter release within microseconds. The key distractor is D because the action potential itself does not trigger exocytosis; it is the Ca2+ influx, not the voltage change directly, that gates release. Removing extracellular calcium abolishes transmission.

Reference: Ganong's Review of Medical Physiology, 27th ed.

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