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."**

1. **Number of Release Sites & Vesicles:** Determines the **spatial pattern and concentration profile** of neurotransmitter in the cleft.
2. **Diffusion & Uptake:** Determine how far that neurotransmitter spreads and how long it lasts.
3. **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.