7.5 KiB
Excellent question that gets to the core of how synaptic transmission works. Yes, it's fundamentally based on concentration, but with a critical biological twist: the concept of saturation.
Let's walk through the logic:
1. The Concentration Argument (Simplified View)
You are correct. Neurotransmitter molecules (e.g., glutamate) diffuse across the synaptic cleft and bind to receptors (e.g., AMPA receptors) on the postsynaptic density (PSD).
- 5000 molecules create a certain concentration in the cleft.
- Fewer molecules (e.g., 2500 from a depleted second spike) create a lower concentration.
- According to the law of mass action, a lower concentration of ligand (neurotransmitter) results in fewer receptors being bound at any given moment.
- Fewer bound receptors mean fewer open ion channels, leading to a smaller net ionic current (EPSC), and thus a smaller EPSP.
So, at first glance, it seems purely a concentration-driven, dose-response relationship: more transmitter → more receptor binding → bigger response.
2. The Critical Biological Reality: Receptor Saturation
Here's where it gets interesting. For a typical fast excitatory synapse, the cleft is extremely narrow (~20-40 nm), and the postsynaptic density is packed with receptors (thousands of AMPA receptors per PSD).
During a single vesicle release event, the local neurotransmitter concentration in the cleft skyrockets to a very high level (mM range) for about 1 ms before diffusion and uptake clear it.
This leads to a key phenomenon: The receptors under the release site are often temporarily saturated.
- What is Saturation? It means that at the peak of the release event, virtually all available receptors are bound by neurotransmitter. Adding more neurotransmitter molecules (e.g., from a second, simultaneous release at the same site) cannot open more channels because they're already all in use.
- Implication: Under saturated conditions, the size of the postsynaptic response is not linearly proportional to the number of transmitter molecules. Instead, it's determined by the number of postsynaptic receptors, not the amount of transmitter beyond what's needed to saturate them.
3. So Why Does Reducing Vesicle Release Reduce the EPSP?
Given receptor saturation, why does releasing fewer vesicles (during short-term depression) still cause a smaller EPSP? There are two main reasons:
A. Saturation is Local and Incomplete While receptors directly under a vesicle fusion site may be saturated, neurotransmitter diffuses. A single release event activates receptors in the immediate vicinity. If a second vesicle is released from a different release site within the same active zone (or if the first release is from a site with fewer local receptors), its glutamate diffuses to activate a spatially distinct, non-saturated population of receptors. Therefore:
- First Spike: Vesicle from Site A saturates Receptor Cluster A. Vesicle from Site B saturates Receptor Cluster B. Total Response = A + B.
- Second Spike (Depressed): Only Vesicle from Site A releases. It saturates Receptor Cluster A. Total Response = A only. The EPSP is smaller because it's missing the contribution from the spatially separate receptor cluster B.
B. The Role of Multi-Vesicular Release (MVR) and Transmitter Spillover At many synapses, a single action potential can cause the fusion of more than one vesicle (Multi-Vesicular Release). The glutamate from these multiple vesicles can summate in the cleft, increasing the concentration duration and potentially activating extra- or peri-synaptic receptors (like NMDA receptors or metabotropic receptors) that have lower affinity or are located farther away.
- First Spike: High Pr → MVR (e.g., 3 vesicles) → Very high, prolonged [Glutamate] → Activates local AMPARs + distal/extrasynaptic NMDARs/mGluRs.
- Second Spike (Low Pr): Low Pr → Single vesicle release → Lower, shorter [Glutamate] → Activates only local AMPARs (and maybe saturates them). The loss of this spillover component to more distant receptors contributes to the amplitude decrease.
The Integrated Picture: It's About the "Volume" of Activated Receptors
Think of it not as "5000 vs. 2500 molecules," but as "activating 200 receptors vs. activating 100 receptors."
- Number of Release Sites & Vesicles: Determines the spatial pattern and concentration profile of neurotransmitter in the cleft.
- Diffusion & Uptake: Determine how far that neurotransmitter spreads and how long it lasts.
- Receptor Distribution & Affinity: Determine how many receptors are recruited by that spatial/chemical profile.
During short-term depression:
- Fewer vesicles are released → The "cloud" of neurotransmitter is smaller and/or originates from fewer points.
- This smaller cloud activates a smaller total volume of the postsynaptic membrane (or activates the same volume but with a lower, non-saturating concentration).
- Result: Fewer receptor channels open overall, producing a smaller summed postsynaptic current and a smaller EPSP.
Summary: Why Less Neurotransmitter = Smaller EPSP
| Cause of Reduced Release | Effect on Neurotransmitter Profile | Effect on Postsynaptic Receptors | Final Effect on EPSP |
|---|---|---|---|
| Vesicle Depletion (fewer vesicles released) | Lower peak concentration, less spatial spread. | Fewer total receptors are activated (spatially restricted). A smaller area of the PSD is engaged. | Amplitude decreases. |
| Lower Release Probability (e.g., from G-protein inhibition) | Same as above. | Same as above. | Amplitude decreases. |
| Receptor Saturation (for a single vesicle) | Limits the effect of adding more transmitter at the exact same location. | Makes the synapse's response to a single release site binary (on/off). | Makes the synapse's output depend on the number of independent release sites activated, not the raw molecule count. |
So, you are right—it is based on concentration. But because of the tiny, structured environment of the synapse, the critical concentration is the local concentration at each receptor cluster, which dictates how many clusters are activated, not just how many molecules are floating around. Reducing the number of released vesicles reduces the spatial extent and intensity of the neurotransmitter signal, thereby recruiting fewer postsynaptic receptors and generating a smaller EPSP.