Questions: Synaptic Vesicle Release and Exocytosis

Question 1

A drug blocks voltage-gated calcium channels at the axon terminal. Action potentials still fire and propagate normally. What effect does this have on neurotransmitter release?

• A. Release is unaffected because the action potential itself causes vesicle fusion
• B. Release is eliminated because calcium influx is the trigger that activates synaptotagmin and initiates SNARE-mediated fusion
• C. Release is reduced by 50% because only some vesicles require calcium
• D. Release is delayed but eventually occurs as calcium leaks through other channels

Answer: (B)

Calcium influx through voltage-gated channels is the essential trigger. When the action potential depolarizes the terminal, it opens these channels specifically; calcium floods in and binds synaptotagmin, which releases the complexin clamp on the SNARE complex and catalyzes membrane fusion. Without calcium entry, SNARE proteins remain primed but blocked — the action potential provides no alternative route to fusion. Option A reflects a common misconception that depolarization alone drives release.

Question 2

At a typical central synapse, what happens when a single action potential arrives at the axon terminal?

• A. All docked vesicles fuse and release their neurotransmitter
• B. Vesicle release is triggered only if the action potential frequency exceeds a threshold
• C. Most docked vesicles do not release — each has only a 10–30% probability of fusing on any single action potential
• D. Exactly one vesicle fuses, as release is controlled by an all-or-none mechanism

Answer: (C)

Synaptic release is probabilistic, not deterministic. Any given docked vesicle has only a 10–30% chance of fusing per action potential, depending on local calcium concentration and channel proximity. This means most vesicles remain docked after any single action potential. This probabilistic nature is not a flaw — it is the basis of short-term synaptic plasticity, allowing release probability to be tuned up or down by modulatory signals.

Question 3

The amount of neurotransmitter released by a single vesicle fusion event varies continuously depending on how much calcium enters the axon terminal.

• T. True
• F. False

Answer: False

Each vesicle contains a fixed 'quantum' of approximately 5,000 neurotransmitter molecules, and fusion releases the entire contents. What varies with calcium concentration is the *probability* that a given vesicle will fuse — not the amount released per fusion event. This quantal nature of release was a key discovery in synaptic physiology. The calcium-dependent variable is release probability, not quantum size.

Question 4

The SNARE complex drives membrane fusion by generating mechanical force: as synaptobrevin, syntaxin, and SNAP-25 zipper together into a four-helix bundle, they pull the vesicle and plasma membranes into close enough apposition to fuse.

• T. True
• F. False

Answer: True

This is correct. The SNARE proteins form a coiled-coil bundle (analogous to twisting ropes together) that generates mechanical force overcoming the natural electrostatic repulsion between lipid bilayers. Without this force, the two membranes would not come close enough to fuse spontaneously. The calcium-sensing step (synaptotagmin) gates the final assembly, but the SNARE complex itself provides the physical driving force for fusion.

Question 5

Why is calcium specifically — rather than sodium or potassium ions that also flow during the action potential — the trigger for synaptic vesicle release?

Model answer: Calcium is the trigger because of two specific features: first, voltage-gated calcium channels are concentrated precisely near docked vesicles, creating a high-calcium microdomain right where it's needed; second, the vesicle protein synaptotagmin is a calcium sensor that specifically binds Ca2+ ions and undergoes the conformational change that releases the SNARE clamp. Sodium and potassium channels are not concentrated near vesicle docking sites, and there is no sodium- or potassium-sensing machinery on vesicles. The specificity is architectural (where the channels are) and molecular (what the sensor detects).

The steep extracellular-to-intracellular calcium gradient (roughly 10,000-fold) means that even brief channel opening floods the local area with calcium. The coupling between channel location and vesicle docking ensures that calcium reaches synaptotagmin within microseconds. This spatial co-localization is why the action potential-to-release delay is less than 1 millisecond.