A researcher records from a presynaptic terminal and blocks voltage-gated calcium channels with a specific toxin. Action potentials still invade the terminal normally, but neurotransmitter release ceases. The step lost by this blockade is:

• A Binding of calcium to synaptotagmin, triggering SNARE-mediated vesicle fusion with the presynaptic membrane ✓
• B Reuptake of transmitter into the terminal by vesicular transporters
• C Opening of postsynaptic ligand-gated ion channels
• D Activation of voltage-gated sodium channels at the terminal to propagate the signal

Explanation

Neurotransmitter release is triggered by calcium entry through voltage-gated calcium channels at the active zone. Calcium binds synaptotagmin, which removes the inhibition on the SNARE complex and allows vesicle fusion with the presynaptic membrane. Without calcium influx, vesicles remain docked but do not fuse. Option C describes a postsynaptic event that fails secondarily, not the lost step itself. Option B concerns transmitter loading, which is independent of terminal calcium channels.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.