Questions: Neuromodulation and Presynaptic Dynamics

Question 1

A synapse has a very high initial release probability — nearly every vesicle in the readily releasable pool fuses on the first action potential. When stimulated at high frequency, this synapse will most likely exhibit:

• A. Paired-pulse facilitation, because high release probability means more calcium will accumulate with each spike
• B. Paired-pulse depression, because the first stimulus depletes vesicles faster than they can be replenished, leaving fewer available for the second stimulus
• C. No change in response amplitude, because high release probability makes the synapse resistant to depletion
• D. Postsynaptic potentiation, because high-frequency depolarization increases AMPA receptor sensitivity

Answer: (B)

When initial release probability is high, the first action potential depletes most of the readily releasable pool. The second action potential arrives before vesicle recycling can replenish the pool, so fewer vesicles are available — the response is smaller. This is paired-pulse depression. Facilitation occurs at low initial release probability synapses, where ample vesicle reserve exists and residual calcium from the first stimulus can recruit extra fusion events from that reserve.

Question 2

A facilitating synapse responds weakly to isolated action potentials but strongly to rapid bursts. This makes it function as a:

• A. Low-pass filter — it selectively transmits slow, sustained signals while attenuating bursts
• B. High-pass filter — it selectively amplifies high-frequency burst signals while attenuating isolated spikes
• C. Band-pass filter — it responds only to a specific frequency range and ignores both very slow and very fast signals
• D. Notch filter — it selectively suppresses one specific frequency of presynaptic firing

Answer: (B)

Facilitating synapses have low initial release probability. A single spike releases little neurotransmitter. But rapid successive spikes allow residual calcium from each preceding spike to accumulate, progressively increasing release probability with each additional burst stimulus. The synapse becomes an increasingly strong signal only at high presynaptic firing frequency — the hallmark of high-pass filtering: low-frequency (isolated spikes) pass weakly, high-frequency (bursts) pass strongly.

Question 3

Paired-pulse depression at a synapse indicates a failure of vesicle recycling and represents a pathological breakdown in synaptic function.

• T. True
• F. False

Answer: False

Paired-pulse depression is a normal, functional property of high initial release probability synapses — not a malfunction. It serves as a low-pass filter: the synapse responds strongly to the onset of activity (first spike, large response) but attenuates sustained high-frequency input (subsequent spikes, smaller responses). This makes depressing synapses sensitive to changes in presynaptic firing rate, detecting the onset of activity rather than tracking its steady-state magnitude. This is computationally useful, not a sign of dysfunction.

Question 4

A neuromodulator that increases release probability at a synapse that previously showed paired-pulse facilitation could convert it to a synapse that shows paired-pulse depression.

• T. True
• F. False

Answer: True

The facilitation/depression distinction depends on initial release probability relative to vesicle pool size. A synapse that facilitated (low initial release probability, ample reserve) can be converted to a depressing synapse if a neuromodulator raises release probability sufficiently — now the first stimulus depletes the readily releasable pool rather than leaving ample reserve. Neuromodulators can thus shift a synapse's entire functional identity, changing how it filters information without changing its anatomical connections.

Question 5

Why do facilitating synapses function as high-pass filters and depressing synapses as low-pass filters? Explain using vesicle pool dynamics.

Model answer: At facilitating synapses (low initial release probability), a single action potential releases few vesicles — the readily releasable pool is far from depleted. Residual calcium from rapid successive spikes accumulates and recruits more vesicles per spike, so burst responses are disproportionately large relative to isolated spike responses. The synapse amplifies high-frequency input — a high-pass filter. At depressing synapses (high initial release probability), the first spike depletes most of the pool; subsequent rapid spikes find fewer vesicles available and produce diminishing responses. Only slow input (long inter-spike intervals allowing pool replenishment) produces sustained responses — a low-pass filter.

The computational logic maps directly onto vesicle pool dynamics: facilitation = reserve capacity that bursts can recruit; depression = pool depletion that high-frequency firing accelerates. This is why neuromodulators that shift release probability can fundamentally alter a synapse's information-filtering role without changing its anatomy — altering circuit response properties by changing the operating point of individual synapses.