At a chemical synapse, what ionic trigger directly causes synaptic vesicles to fuse with the presynaptic terminal membrane and exocytose neurotransmitters into the synaptic cleft?

Answer and explanation

Correct answer: A. Influx of calcium ions (Ca2+) through voltage-gated calcium channels opened by the arriving action potential

  1. Step 1: When an action potential reaches the axon terminal (synaptic knob), the localized depolarization triggers the opening of voltage-gated channels.
  2. Step 2: Because extracellular is thousands of times higher than intracellular levels, rushes into the presynaptic terminal.
  3. Step 3: Inward calcium binds to synaptotagmin and activates SNARE complex proteins, driving the fusion of synaptic vesicles with the presynaptic membrane and releasing neurotransmitter molecules into the synaptic cleft.
Answer
Influx of calcium ions (Ca2+) through voltage-gated calcium channels opened by the arriving action potential
Why not the others: 'Efflux of chloride ions...' produces hyperpolarization (IPSPs) on the postsynaptic membrane; 'Active export of potassium...' maintains resting potentials, not vesicle exocytosis; 'Influx of magnesium ions...' is incorrect; actually acts as a competitive antagonist of at presynaptic terminals.

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