At a chemical synapse, what sequence leads from presynaptic action potential to neurotransmitter release?

• A. AP closes Ca2+ channels; vesicle uptake.
• B. AP opens voltage-gated Ca2+ channels; Ca2+ influx triggers vesicle fusion via SNAREs; neurotransmitter released into synaptic cleft.
• C. AP triggers endocytosis of vesicles without Ca2+.
• D. AP opens Na+ channels; Na+ influx; vesicle release.

Answer: (B)

Calcium entry is the trigger for neurotransmitter release. When the presynaptic action potential arrives, it depolarizes the terminal and opens voltage-gated calcium channels. Calcium ions rush into the nerve ending and bind to calcium-sensitive proteins on the vesicles (the SNARE fusion machinery, with a calcium sensor like synaptotagmin). This interaction drives the vesicles to fuse with the presynaptic membrane and exocytose their neurotransmitter into the synaptic cleft. The transmitter then diffuses across the gap to receptors on the postsynaptic cell, modulating its activity.

The sequence described is essential—calcium influx is required for vesicle fusion and release. Closing calcium channels would prevent release, not cause it. Endocytosis is a recycling process that occurs after release, not the mechanism that causes neurotransmitter to be released. Sodium influx from initial depolarization helps trigger the action potential, but the actual triggering signal for vesicle fusion and release is the calcium influx.