Neurotransmitter release at a central chemical synapse is triggered when an action potential invades the presynaptic terminal. The immediate trigger for fusion of synaptic vesicles with the active zone membrane is:

• A Binding of the transmitter itself to receptors on the vesicle surface
• B Depolarization of the vesicle membrane by local circuit currents
• C Entry of Ca2+ through voltage-gated calcium channels, detected by synaptotagmin ✓
• D Activation of voltage-gated Na+ channels clustered at the active zone

Explanation

Depolarization opens voltage-gated Ca2+ channels concentrated at the active zone. The resulting microdomains of Ca2+ bind synaptotagmin on the vesicle, which displaces the complexin clamp and allows SNARE proteins (syntaxin, SNAP-25, synaptobrevin) to complete membrane fusion. Na+ channel activation is needed only to propagate the action potential into the terminal; it does not trigger exocytosis. This Ca2+ dependence accounts for the 0.5 ms synaptic delay.

Reference: Principles of Neural Science, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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