Which ONE of the following statements about neurotransmitter release at chemical synapses is CORRECT?

Question

BlackTom AI · Accepted Answer

Calcium influx into the presynaptic terminal is the physiological trigger for vesicle fusion and transmitter release.

1. Relevant knowledge points and concepts
- Action potentials depolarize the presynaptic membrane and open voltage-gated calcium channels (VGCCs).
- Intracellular Ca2+ rises locally from ~50–100 nM at rest to micromolar concentrations near open channels.
- Synaptotagmin acts as a Ca2+ sensor; SNARE proteins mediate membrane fusion.
- Neurotransmitters are packaged in synaptic vesicles and are released by Ca2+-triggered exocytosis in quantal packets.

2. Step-by-step reasoning process
1) An action potential arrives at the presynaptic terminal and depolarizes the membrane.
2) Depolarization opens VGCCs; Ca2+ flows down its electrochemical gradient into the terminal.
3) Local Ca2+ concentration rapidly increases near docked vesicles.
4) Ca2+ binds to synaptotagmin and other Ca2+-sensing proteins, inducing conformational changes.
5) The Ca2+-bound sensors promote SNARE complex-driven fusion of the vesicle membrane with the presynaptic membrane.
6) The vesicle releases its neurotransmitter into the synaptic cleft; transmitter diffuses to postsynaptic receptors and is cleared by reuptake or enzymatic degradation.

3. Intermediate steps and values
- Resting [Ca2+] ~50–100 nM; microdomains near open VGCCs can reach several µM—sufficient to trigger synaptotagmin.
- A single vesicle often contains ~10^3–10^4 neurotransmitter molecules (quantal release).

4. Why the correct answer is correct
- The central causal link is well established: Ca2+ entry is necessary and rapid enough to couple electrical activity to vesicular exocytosis. Blocking VGCCs blocks evoked release; buffering intracellular Ca2+ prevents fusion.

5. Why the other options are incorrect
- “Released continuously regardless of presynaptic activity”: false—release is largely activity-dependent; spontaneous “mini” events occur but are not continuous bulk release.
- “Vesicles rupture in response to sodium influx”: false—Na+ influx causes depolarization but does not directly trigger fusion; Ca2+ is the direct trigger.
- “Fusion occurs only after neurotransmitter binds postsynaptic receptors”: impossible temporally—release must occur before postsynaptic binding; postsynaptic binding is the consequence, not the cause.

6. Conclusion
This reasoning confirms the provided correct answer: Calcium entry into the presynaptic terminal acts as the key signal triggering vesicle fusion and transmitter release.

Which ONE of the following statements about neurotransmitter release at chemical synapses is CORRECT?单项选择题

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