Questions: Synaptic Transmission and Neurotransmitter Release

Question 1

A researcher applies a drug that blocks voltage-gated calcium channels in presynaptic terminals. An action potential arrives at the terminal. What happens to neurotransmitter release?

• A. Release is unaffected — the action potential directly opens vesicle fusion pores without requiring calcium
• B. Release is abolished or severely reduced — calcium influx is the required trigger for synaptotagmin activation and SNARE-mediated vesicle fusion
• C. Release increases — blocking calcium prevents inactivation of the release machinery
• D. Release is delayed but not reduced — vesicles can fuse spontaneously given sufficient time

Answer: (B)

Calcium influx through voltage-gated Ca²⁺ channels is the critical trigger between electrical depolarization and chemical release. Without calcium, synaptotagmin cannot activate the SNARE complex, and vesicle fusion does not occur. The action potential depolarizes the terminal membrane, but depolarization alone is insufficient — calcium is the essential intermediate. The misconception (option A) confuses the electrical trigger for calcium channel opening with a direct trigger for vesicle fusion.

Question 2

A neuron fires repeatedly at high frequency. After many action potentials, the postsynaptic response becomes progressively weaker, even though each action potential remains normal in amplitude and shape. What is the most likely presynaptic cause?

• A. The postsynaptic receptors have been permanently desensitized by excess neurotransmitter
• B. Voltage-gated calcium channels are being blocked by released neurotransmitter
• C. The readily-releasable pool of docked vesicles has been depleted faster than it can be replenished
• D. The action potential is no longer propagating to the axon terminal

Answer: (C)

Short-term synaptic depression results from presynaptic vesicle depletion. The readily-releasable pool near the active zone is finite; with rapid repeated firing, vesicles fuse and release transmitter faster than the replenishment machinery can dock and prime new ones. Each successive action potential releases less neurotransmitter. Option A is possible with some receptor types but is typically reversible and slower in onset; the question specifies a pattern that matches progressive presynaptic depletion.

Question 3

Calcium entering the presynaptic terminal through voltage-gated channels is required to trigger neurotransmitter release during normal synaptic transmission.

• T. True
• F. False

Answer: True

Calcium binds to synaptotagmin, the calcium sensor on synaptic vesicles, which then activates the SNARE complex to drive membrane fusion. This calcium requirement is universal across chemical synapses. The action potential opens voltage-gated Ca²⁺ channels by depolarizing the terminal; the resulting calcium influx is what bridges the electrical signal to chemical release.

Question 4

The strength of synaptic transmission between two neurons is fixed once the synapse has formed and can seldom be altered by neural activity.

• T. True
• F. False

Answer: False

Synaptic strength is dynamically regulated at three points: (1) presynaptic calcium entry — modulatory receptors on the terminal can amplify or reduce Ca²⁺ influx per action potential; (2) vesicle availability — the size of the readily-releasable pool changes with recent activity; (3) postsynaptic receptor density — more receptors produce a larger response to the same amount of transmitter. This dynamic regulation is the molecular basis of short-term synaptic plasticity and ultimately forms the substrate for learning and memory.

Question 5

Why is calcium influx — rather than membrane depolarization alone — the trigger for neurotransmitter release at chemical synapses?

Model answer: Synaptic vesicles are held in a primed but unfused state near the active zone by SNARE proteins. Membrane depolarization alone does not activate fusion; a specific calcium signal is required to complete it. Calcium binds to synaptotagmin, a protein on the vesicle membrane, which then undergoes a conformational change that activates the SNARE complex and drives lipid bilayer fusion. This calcium requirement provides precise temporal control: release occurs only milliseconds after the action potential opens the Ca²⁺ channels, ensuring that chemical signaling is tightly coupled to electrical signaling.

The calcium step also allows modulation: anything that changes the amount of calcium entering the terminal (autoreceptors, neuromodulators, plasticity mechanisms) directly scales the amount of transmitter released per action potential — giving synapses enormous dynamic range.