An action potential arrives at the terminal bouton of a presynaptic neuron. Which event directly triggers fusion of synaptic vesicles with the presynaptic membrane?

• A Influx of sodium ions through ligand-gated channels
• B Entry of calcium ions through voltage-gated channels ✓
• C Efflux of potassium ions through leak channels
• D Binding of glutamate to postsynaptic receptors

Explanation

Depolarization of the terminal opens voltage-gated Ca2+ channels; the resulting Ca2+ influx binds synaptotagmin and allows SNARE complex-mediated vesicle fusion and transmitter release. Removing extracellular calcium abolishes transmission. Sodium entry depolarizes the terminal but is not the direct trigger, and postsynaptic receptor binding occurs after release, so options A and D cannot initiate exocytosis.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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