# **Detailed Temporal Dynamics of Presynaptic Neurotransmitter Release**

*With concentration changes, calcium clearance mechanisms, and multi-timescale modifications of release probability (Pr)*

---

## **Baseline State (Resting Condition)**

**Time:** Continuous resting state  
**Presynaptic [Ca²⁺]:** ~100 nM (resting cytosolic concentration)  
**Pr baseline:** 0.3 (30% release probability per action potential)  
**Ready vesicles:** 5-10 vesicles in readily releasable pool (RRP)  
**ATP/GTP levels:** Normal  
**Phosphorylation state:** Baseline kinase/phosphatase balance

---

## **PHASE 1: FAST TIMESCALE (0-100 ms) - SINGLE SPIKE CYCLE**

### **0.0-0.5 ms: Action Potential Arrival & Calcium Influx**

```
AP depolarization (+30mV) reaches terminal
    ↓
VGCCs (P/Q-type) open with ~50% probability
    ↓
Ca²⁺ enters through ~20 channels per active zone
    ↓
**Local [Ca²⁺] at release site: 100 nM → 10-50 µM** (peak in nanodomain)
    ↓
Calcium diffuses ~20-30 nm to docked vesicles
    ↓
**PROBABILISTIC DECISION POINT:**
Time window for decision: ~0.2-0.5 ms
```

### **0.5-2.0 ms: Release Decision & Execution**

```
├── **PATH A: RELEASE OCCURS (Pr = 0.3)**
│       ↓
│   [Ca²⁺] at Synaptotagmin > 10 µM
│       ↓
│   3-5 Ca²⁺ ions bind to C2 domains of Synaptotagmin-1
│       ↓
│   Synaptotagmin inserts into membrane (Kd ~5-10 µM)
│       ↓
│   **SNARE complex completes zippering** (t ≈ 0.8 ms)
│       ↓
│   Fusion pore opens (diameter ~1 nm initially)
│       ↓
│   **~5000 glutamate molecules released** (t = 1-2 ms)
│       ↓
│   Fusion pore expands → full fusion
│       ↓
│   Vesicle membrane incorporated into plasma membrane
│
└── **PATH B: NO RELEASE (1-Pr = 0.7)**
        ↓
    [Ca²⁺] at Synaptotagmin < 5 µM (insufficient binding)
        ↓
    Calcium buffers (calbindin, parvalbumin) bind Ca²⁺
        ↓
    Vesicle remains docked/primed
        ↓
    No fusion → **silent spike**
```

### **2.0-50 ms: Calcium Clearance & Fast Recovery**

```
**Primary clearance mechanisms:**
1. Plasma Membrane Ca²⁺ ATPase (PMCA): 
   - Kd ~100-200 nM
   - Rate: 30 Ca²⁺/sec per pump
   - **Clears 90% of Ca²⁺ in 10-20 ms**

2. Na⁺/Ca²⁺ exchanger (NCX):
   - Lower affinity (Kd ~1 µM) but higher capacity
   - Important for bulk clearance

3. Mitochondrial uptake:
   - MCU (mitochondrial Ca²⁺ uniporter)
   - Kd ~10-20 µM
   - Slower but provides long-term buffering

4. Endoplasmic reticulum uptake (SERCA):
   - Sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase
   - Kd ~0.5 µM

**Result:** [Ca²⁺] returns to ~500 nM by 50 ms
**Residual [Ca²⁺]:** ~200-300 nM persists for 100-500 ms
**Vesicle retrieval:** Clathrin-mediated endocytosis begins at ~1 sec
```

---

## **PHASE 2: SHORT-TERM PLASTICITY (100 ms - 10 sec)**

### **Example: Spike Train at 50 Hz (20 ms interval)**

```
Spike 1: Pr = 0.3 → Release probability
Spike 2 (20 ms later): 
   Residual [Ca²⁺] = 300 nM
   Pr increases to 0.45 (facilitation)
   Ca²⁺ influx adds to residual Ca²⁺
   
Spike 3 (40 ms):
   Residual [Ca²⁺] accumulates to 400 nM
   Pr = 0.55
   But RRP depletion begins (STD component)
   
Spike 4 (60 ms):
   RRP depleted to 60%
   Effective Pr = 0.5 × 0.6 = 0.3 (balance facilitation/depletion)
   
Spike 5-10:
   Steady-state: Pr ~0.25, RRP ~40% of baseline
```

### **Molecular Mechanisms of Short-Term Changes:**

```
**Facilitation (0-500 ms):**
Residual Ca²⁺ (~200-500 nM) → binds to Calmodulin
    ↓
Ca²⁺-Calmodulin binds to Munc13
    ↓
Munc13 increases priming rate 3-5×
    ↓
**Pr increases for next spike**

**Depression (0-2 sec):**
Vesicle fusion → RRP depletion
    ↓
Recovery requires:
1. Vesicle recycling (endocytosis: 1-10 sec)
2. Vesicle repriming (2-30 sec)
3. Reserve pool mobilization (seconds)
```

---

## **PHASE 3: MEDIUM-TERM ADAPTATION (10 sec - 10 min)**

### **Example: LTP Induction at 100 Hz for 1 sec (Tetanus)**

```
**0-1 sec: High-frequency stimulation**
- 100 APs delivered
- Massive Ca²⁺ accumulation in terminal
- [Ca²⁺] builds to sustained 1-2 µM
- Complete RRP depletion
- Strong glutamate release

**1-30 sec: Retrograde signaling arrives**
Postsynaptic spine produces:
1. Nitric Oxide (NO) - diffuses in seconds
2. Brain-Derived Neurotrophic Factor (BDNF) - released in minutes
3. Endocannabinoids (eCBs) - for LTD case

NO diffuses into presynaptic terminal (t ≈ 5-10 sec)
    ↓
**NO activates soluble guanylyl cyclase (sGC)**
    ↓
sGC produces cGMP from GTP
    ↓
**cGMP increases from ~1 nM to 100 nM**
    ↓
cGMP activates Protein Kinase G (PKG)
```

### **30 sec - 5 min: PKG-Mediated Pr Enhancement**

```
PKG phosphorylates multiple targets:

1. **VGCCs (P/Q-type):**
   - Phosphorylation at specific serine residues
   - Open probability increases from 0.5 → 0.7
   - More Ca²⁺ enters per AP

2. **Munc18:**
   - Enhanced interaction with Syntaxin
   - Vesicle priming rate increases 2×

3. **Synaptotagmin-1:**
   - Increased Ca²⁺ sensitivity (Kd decreases from 10→5 µM)
   - Faster binding kinetics

4. **RIM proteins:**
   - Enhanced vesicle tethering
   - Better VGCC-vesicle coupling

**Net effect by 5 min:**
- Pr increases from 0.3 → 0.5
- Baseline Ca²⁺ sensitivity increased
- Readily Releasable Pool size increases 30%
```

---

## **PHASE 4: SLOW CONSOLIDATION (10 min - 2 hours)**

### **Local Protein Synthesis (Presynaptic)**

```
**30 min - 2 hours:**
BDNF binds to TrkB receptors on presynaptic terminal
    ↓
Activation of PI3K/mTOR pathway
    ↓
**Local translation of presynaptic mRNAs:**
1. VGCC subunits (α1A, β4)
2. Synaptotagmin-1
3. Munc13-1
4. SNARE proteins

**Result by 2 hours:**
- 50% more VGCCs clustered at active zone
- 40% more Synaptotagmin molecules per vesicle
- Pr stabilizes at 0.6
```

### **Metabolic Support System**

```
**Astrocyte coordination:**
1. Glutamate uptake → converted to glutamine
2. Glutamine exported to presynaptic terminal
3. Presynaptic mitochondria increase oxidative phosphorylation
4. ATP production increases 2× to support enhanced release

**Energy requirements:**
- Vesicle recycling: ~10,000 ATP/vesicle
- Ca²⁺ clearance: ~1 ATP/2 Ca²⁺ ions
- Protein synthesis: ~4 ATP/amino acid
```

---

## **PHASE 5: STRUCTURAL CONSOLIDATION (2 hours - 24 hours)**

### **Nuclear Signaling & Gene Expression**

```
**2-6 hours:**
Persistent kinase activity (PKG, PKA, MAPK)
    ↓
CREB phosphorylation in presynaptic nucleus
    ↓
**Gene expression changes:**
1. Structural proteins (Bassoon, Piccolo)
2. Active zone components
3. Vesicle cycle proteins
4. Metabolic enzymes

**12-24 hours:**
New proteins arrive via axonal transport
    ↓
**Active zone remodeling:**
- Active zone area increases 30-50%
- More docked vesicles (RRP size doubles)
- VGCC-vesicle distance decreases to 15 nm
- **Pr stabilizes at 0.7-0.8**
```

---

## **CALCIUM HOMEOSTASIS TIMELINE SUMMARY**

| Time   | \[Ca²⁺\] at Release Site | Clearance Mechanism      | Residual Effect            |
|--------|--------------------------|--------------------------|----------------------------|
| 0 ms   | 100 nM (baseline)        | \-                       | \-                         |
| 0.5 ms | 10-50 µM (peak)          | Diffusion only           | Fusion decision            |
| 5 ms   | 1-5 µM                   | Fast buffers (calbindin) | Ca²⁺-calmodulin activation |
| 20 ms  | 500 nM                   | PMCA pumps active        | Facilitation of next spike |
| 100 ms | 300 nM                   | NCX contributes          | Augmentation phase         |
| 1 sec  | 200 nM                   | Mitochondrial uptake     | Potentiation               |
| 10 sec | 150 nM                   | Steady-state clearance   | LTP induction possible     |
| 1 min  | 120 nM                   | Full homeostasis         | \-                         |
| 1 hour | 100 nM                   | Normal resting state     | \-                         |

---

## **EXAMPLE: COMPLETE LTP TIMELINE**

### **Day 1: Induction Phase**

```
**T=0:** 100 Hz tetanus (1 sec)
**T=5 sec:** NO arrives at presynaptic terminal
**T=30 sec:** cGMP peaks, PKG activated
**T=5 min:** Pr increases to 0.45 (phosphorylation)
**T=30 min:** Local protein synthesis begins
**T=2 hours:** Pr = 0.55, structural proteins arriving
**T=6 hours:** Active zone remodeling begins
**T=24 hours:** Pr stabilizes at 0.65, active zone enlarged 40%
```

### **Day 2-7: Maintenance**

```
**Metabolic support ongoing:**
- Astrocyte supplies glutamine/lactate
- Mitochondrial density increases near active zone
- Vesicle recycling efficiency improves

**Structural stabilization:**
- New active zone material incorporated
- Cytoskeleton reorganizes
- Pr maintains at 0.65-0.70
```

---

## **KEY BIOLOGICAL INSIGHTS**

1. **Pr is multi-dimensional:**
   - **Fast component:** Vesicle availability × Ca²⁺ sensitivity
   - **Slow component:** Protein composition × active zone geometry
2. **Energy dependence:**
   - Ca²⁺ clearance requires constant ATP
   - Without ATP, [Ca²⁺] remains elevated → toxicity
   - Metabolic veto: Low ATP → release inhibition despite Ca²⁺
3. **Timescale hierarchy:**
   - **ms:** Existing protein modification (phosphorylation)
   - **min:** Local protein synthesis
   - **hours:** Nuclear transcription
   - **days:** Structural reorganization
4. **Homeostatic balance:**
   - Enhanced Pr increases metabolic demand
   - Requires coordinated astrocyte support
   - Long-term maintenance depends on energy availability

This detailed timeline shows how a single probabilistic event (vesicle release) is embedded in a complex, multi-timescale regulatory system that balances immediate communication needs with long-term information storage and metabolic sustainability.

---

---

---

Based on the provided document, here is a specification of the Short-Term Plasticity (STP/STD) and Long-Term Plasticity (LTP/LTD) mechanisms between the Presynapse and Postsynapse, detailing both internal processes and their interactions, with explicit timescales.

### **Summary: Primary Plasticity Mechanisms & Timescales**

| Mechanism                         | Primary Locus    | Key Internal Trigger                          | Key Interactive Signal                          | Timescale                          | Functional Role                                            |
|-----------------------------------|------------------|-----------------------------------------------|-------------------------------------------------|------------------------------------|------------------------------------------------------------|
| **Short-Term Depression (STD)**   | **Presynaptic**  | Vesicle pool depletion                        | Reduced glutamate release                       | **Fast (<100ms)**                  | Filters high-frequency bursts; transient synaptic fatigue. |
| **Short-Term Potentiation (STP)** | **Presynaptic**  | Residual Ca²⁺ buildup                         | Increased glutamate release probability ($P_r$) | **Fast to Medium (<100ms to 10s)** | Facilitates temporal summation; augments recent activity.  |
| **Long-Term Depression (LTD)**    | **Postsynaptic** | Moderate, sustained Ca²⁺ influx (~1-5 µM)     | Retrograde endocannabinoids (eCBs)              | **Slow (Seconds to Minutes)**      | Weakens ineffective connections; homeostatic adjustment.   |
| **Long-Term Potentiation (LTP)**  | **Postsynaptic** | Strong, coincident Ca²⁺ influx (>10 µM)       | Retrograde NO/BDNF                              | **Slow (Seconds to Minutes)**      | Strengthens correlated pre- and postsynaptic activity.     |
| **Structural LTP/LTD**            | **Both**         | Persistent molecular tags & protein synthesis | Trophic factors & homeostatic scaling           | **Structural (Days+)**             | Embeds memory persistently via physical changes.           |

---

### **Detailed Breakdown by Timescale**

#### **1. Fast Timescale (<100 ms): STP & STD Internal Mechanisms**

* **Presynaptic Internal (STD):** Rapid vesicle fusion and release depletes the readily releasable pool. This is a **presynaptic, internal** mechanism causing a transient decrease in synaptic strength.
* **Presynaptic Internal (STP):** Residual Ca²⁺ from a preceding action potential lingers, increasing the release probability ($P_r$) for the next spike. This is a **presynaptic, internal** facilitatory mechanism.
* **Interaction (Fast Signaling):** The presynapse releases **glutamate** (outgoing signal). The postsynapse receives it and, if sufficiently depolarized, opens NMDA receptors, allowing a **Ca²⁺ influx**. This **Ca²⁺ transient** is the **postsynaptic, internal** coincidence detector signal that initiates the cascade for slower plasticity.

#### **2. Medium Timescale (100 ms – 10 s): Augmentation & Modulation**

* **Presynaptic Internal:** Augmentation via Munc13 proteins modifies $P_r$ based on Ca²⁺ sensing. This is a **presynaptic, internal** continuation of STP.
* **Postsynaptic Internal:** Metabotropic glutamate receptor (mGluR) activation modulates local spine excitability and prepares plasticity pathways. This is a **postsynaptic, internal** modulatory state.
* **Interaction:** Largely an extension of fast signaling, setting the stage for slower decisions. The **pattern of glutamate release** interacts with the **postsynaptic voltage state**.

#### **3. Slow Timescale (Seconds – Minutes): LTP & LTD Decision & Expression**

This is the critical window for **bidirectional interaction** that establishes long-term change.

* **Postsynaptic Internal (The Decision):**
  * **LTP Trigger:** **High, localized Ca²⁺** (from strong NMDA activation + back-propagating AP) activates **CaMKII**, creating a **plasticity tag**.
  * **LTD Trigger:** **Moderate, sustained Ca²⁺** (from isolated glutamate release or low-frequency stimulation) activates phosphatases (e.g., calcineurin).
  * This decision is **postsynaptic and internal**.
* **Interaction (Retrograde Messaging):**
  * For **LTP:** The postsynapse synthesizes and releases **retrograde signals (NO, BDNF)**. These diffuse to the presynapse.
  * For **LTD:** The postsynapse releases **endocannabinoids (eCBs)**.
  * These are **interactive signals from postsynapse to presynapse**.
* **Presynaptic Internal (Expression):**
  * **For LTP:** NO/BDNF activate enzymatic cascades that **persistently increase the baseline** $P_r$.
  * **For LTD:** eCBs bind to CB1 receptors, inhibiting VGCCs and **persistently decreasing** $P_r$ (Direct Synaptic Depression, DSE).
  * This change in baseline $P_r$ is the **presynaptic, internal** expression of the long-term change.

#### **4. Metabolic Timescale (Minutes – Hours): Consolidation**

* **Interaction (Astrocyte Bridge):** The astrocyte supplies **D-serine** (co-agonist for NMDAR) and **lactate** (energy). This **external, interactive support** is required for stable consolidation.
* **Postsynaptic Internal:** The spine initiates **local protein synthesis**, using the "tag" to capture newly made proteins, transitioning early-LTP/LTD to a more stable state. This is a **postsynaptic, internal** consolidation process.

#### **5. Structural Timescale (Days+): Embodiment**

* **Presynaptic Internal:** The bouton **grows or retracts**, changing the active zone size and vesicle pool. This is a **presynaptic, internal** structural change.
* **Postsynaptic Internal:** The spine **changes its volume and number of AMPA receptor slots**. This is a **postsynaptic, internal** structural change.
* **Interaction (Global Scaling):** The soma sends **homeostatic scaling signals** to all synapses (including this one) to maintain network stability, providing a **top-down, interactive modulation** that can override local weights.

### **Conclusion**

* **STP/STD** are predominantly **presynaptic, internal** phenomena operating on **fast to medium timescales** (ms to s), governed by vesicle dynamics and residual calcium.
* **LTP/LTD** are initiated by a **postsynaptic, internal** calcium-based decision on a **slow timescale** (s to min). Their expression involves a **critical bidirectional interaction**: retrograde signals (NO/BDNF for LTP, eCBs for LTD) from the postsynapse induce **presynaptic, internal** changes in baseline $P_r$.
* These changes are then stabilized on **metabolic and structural timescales** through astrocytic support and physical remodeling of both synaptic components, embedding the memory trace in the physical architecture of the connection.

---

---

---

**In biology, presynaptic release probability (Pr) emerges from a dynamic molecular machine, not a simple number.** Changing Pr involves coordinated modifications to multiple components of the vesicle release apparatus. Here's the biological process breakdown:

## **The Pr Machinery: Three Core Components**

Pr = f(**Calcium Influx** × **Vesicle Readiness** × **Fusion Machinery Sensitivity**)

### **1. Modulating Calcium Influx**

**Target:** Voltage-Gated Calcium Channels (VGCCs, mainly P/Q and N-type).

- **Increasing Pr:** Phosphorylation of VGCCs by kinases (PKA, PKC) enhances their open probability or prolongs open time.
- **Decreasing Pr:**
  - Direct inhibition by G-proteins (e.g., via activated CB1 receptors during LTD).
  - Physical relocation of channels away from release sites.
  - Dephosphorylation by phosphatases (calcineurin).

**Biological Process:** A retrograde messenger (e.g., NO) activates a kinase cascade in the presynaptic terminal, leading to VGCC phosphorylation → more Ca²⁺ enters per action potential → higher Pr.

### **2. Modulating Vesicle Readiness (Docking/Priming)**

**Targets:** Docking proteins (Syntaxin, SNAP-25), priming proteins (Munc13, Munc18), and the vesicle pool itself.

- **Increasing Pr:**
  - **Munc13 activation:** Residual Ca²⁺ binds to calmodulin, which binds to Munc13, increasing its priming activity. This is the main mechanism for **short-term facilitation**.
  - **Phosphorylation of priming proteins** by PKC/CaMKII makes them more active.
  - Increased expression or recruitment of vesicles to the "readily releasable pool" (RRP).
- **Decreasing Pr:**
  - Dephosphorylation of priming proteins.
  - Physical depletion of RRP during high-frequency firing (STD).
  - Ubiquitination and degradation of priming proteins.

### **3. Modulating Fusion Machinery Sensitivity (Ca²⁺ Sensor)**

**Target:** The primary Ca²⁺ sensor Synaptotagmin and the SNARE complex (Syntaxin, Synaptobrevin, SNAP-25).

- **Increasing Pr:**
  - Phosphorylation of Synaptotagmin increases its Ca²⁺ affinity.
  - Phosphorylation of SNARE proteins (e.g., SNAP-25 by PKC) enhances fusion kinetics.
  - Assembly of more SNARE complexes.
- **Decreasing Pr:**
  - Cleavage of SNARE proteins by toxins (e.g., botulinum).
  - Increased binding of inhibitory proteins like Complexins.

## **Specific Biological Pathways for Pr Changes**

### **Fast Pr Increase (Facilitation, <100ms)**

**Process:** Action potential → Ca²⁺ influx → residual Ca²⁺ binds to calmodulin → Ca²⁺-calmodulin binds to Munc13 → Munc13 increases vesicle priming rate → more vesicles become release-ready for the next spike.

**Biological signature:** Transient, activity-dependent, decays with Ca²⁺ clearance.

### **Slow Pr Increase (LTP Expression, Minutes+)**

**Process:** Retrograde NO diffuses into presynaptic terminal → activates soluble guanylyl cyclase → produces cGMP → activates Protein Kinase G (PKG) → PKG phosphorylates multiple targets:

1. **VGCCs** → increased Ca²⁺ influx
2. **Munc18** → enhanced vesicle priming
3. **Synaptotagmin** → increased Ca²⁺ sensitivity **Plus:** Local protein synthesis of new vesicle proteins.

**Biological signature:** Persistent, requires gene expression for maintenance, structurally embedded.

### **Slow Pr Decrease (LTD Expression, Minutes+)**

**Process:** Retrograde eCBs bind presynaptic CB1 receptors → activates Gi/o proteins → βγ subunits directly inhibit VGCCs → reduced Ca²⁺ influx → decreased Pr.
**Additionally:** Gi/o inhibits adenylyl cyclase → reduced cAMP → reduced PKA activity → decreased phosphorylation of release machinery.

**Biological signature:** Reversible initially, can become structural with prolonged activation.

## **The Complete Biological Cascade for LTP-Induced Pr Increase**

```
Retrograde NO/BDNF
      ↓
Presynaptic receptor activation
      ↓
Second messenger cascade (cGMP/cAMP)
      ↓
Kinase activation (PKG/PKA/PKC)
      ↓
Phosphorylation of multiple targets:
1. VGCCs → ↑ Ca²⁺ influx per AP
2. Munc13/18 → ↑ vesicle priming rate
3. Synaptotagmin → ↑ Ca²⁺ sensitivity
4. SNARE proteins → ↑ fusion efficiency
      ↓
Increased Pr from baseline 0.2 → 0.4
      ↓
More reliable vesicle release per AP
      ↓
Structural reinforcement (Days+):
- More active zone material
- More docked vesicles
- Closer VGCC-vesicle coupling
```

## **Simulation vs. Biological Reality**

| In Simulation                   | In Biology                                                                                      |
|---------------------------------|-------------------------------------------------------------------------------------------------|
| `Pr = 0.3`                      | Pr emerges from: 5 docked vesicles × 40% channel open probability × 60% fusion probability      |
| Update rule: `Pr += 0.1` if LTP | LTP: NO → cGMP → PKG → phosphorylates 3 proteins → increases Ca²⁺ influx 30% & priming rate 40% |
| Single variable                 | Distributed property of ~50 protein species                                                     |
| Instant change                  | Change occurs over: 100ms (fast), 1min (slow), 24hr (structural)                                |

## **Key Biological Insights**

1. **Pr is multi-parametric:** Not one number but the product of multiple probabilities.
2. **Redundancy:** Multiple pathways converge on Pr (kinases, protein synthesis, structural changes).
3. **Timescale hierarchy:**
   - **Fast (ms):** Modify existing proteins (phosphorylation)
   - **Slow (min):** Synthesize new proteins
   - **Structural (days):** Change physical architecture
4. **Metabolic dependence:** All phosphorylation requires ATP. During energy crisis, kinases fail → Pr decreases even with "LTP signals."
5. **Cross-talk:** The same kinase (e.g., PKC) can phosphorylate both VGCCs and SNARE proteins, creating coordinated enhancement.

In essence, **Pr is the presynaptic terminal's "readiness state"**—a complex, dynamic equilibrium of dozens of interacting proteins that the neuron tunes through phosphorylation, gene expression, and structural remodeling to implement learning and adaptation.

---

---

---

**The presynapse does NOT release neurotransmitter every time a presynaptic spike arrives.** This is a fundamental principle of synaptic transmission: **release is probabilistic, not deterministic.**

## **What Pr Actually Influences**

**Pr (Release Probability)** determines the **likelihood** that a given action potential will trigger **vesicle fusion and neurotransmitter release**. It's essentially the synapse's **"coin toss" probability** for transmission.

### **Biological Reality:**

For a typical central synapse:

- **Pr ≈ 0.2-0.5** (meaning only 20-50% of spikes cause release)
- Some specialized synapses (like the neuromuscular junction) have Pr ≈ 0.9
- Some cortical synapses have Pr as low as 0.1

### **What Happens at a Presynaptic Terminal When a Spike Arrives:**

```
Action Potential arrives at terminal
    ↓
VGCCs open → Ca²⁺ enters
    ↓
**PROBABILISTIC DECISION POINT**
    ↓
├── **With probability Pr (e.g., 0.3):**
│       ↓
│   Ca²⁺ binds to Synaptotagmin on docked vesicle
│       ↓
│   SNARE complex zippers completely
│       ↓
│   Vesicle membrane fuses with presynaptic membrane
│       ↓
│   **GLUTAMATE RELEASED** → Postsynaptic response
│
└── **With probability 1-Pr (e.g., 0.7):**
        ↓
    Ca²⁺ enters but insufficient to trigger fusion
        ↓
    Vesicle remains docked but unfused
        ↓
    **NO RELEASE** → No postsynaptic response
```

## **The Biological Basis of This Stochasticity**

### **1. Calcium Nanodomain Stochasticity**

- Ca²⁺ channels are ~20-30 nm from vesicle release sites
- When a channel opens, only ~100-300 Ca²⁺ ions enter
- These ions form a brief, localized "nanodomain"
- **Random diffusion and buffering** mean the Ca²⁺ concentration at the sensor varies randomly
- If <5-10 µM at the sensor → no fusion; if >10 µM → fusion

### **2. Molecular Stochasticity**

- **Synaptotagmin** (Ca²⁺ sensor) requires 3-5 Ca²⁺ ions to bind
- **SNARE complex** zippering is thermally driven and probabilistic
- These molecular processes have intrinsic randomness at nanoscale

### **3. Vesicle State Stochasticity**

- Vesicles exist in different "readiness states":
  - **Docked & primed** (immediately releasable)
  - **Docked but unprimed**
  - **Reserve pool** (undocked)
- Transition between states is probabilistic

## **Why Evolution Selected Probabilistic Release**

### **Advantages of Low/Moderate Pr:**

1. **Dynamic Range:** Allows both **facilitation** (Pr increases with repeated spikes) and **depression** (Pr decreases with vesicle depletion)
2. **Noise Filtering:** Random failures filter out isolated, potentially noisy spikes while allowing bursts to get through (due to facilitation)
3. **Energy Efficiency:** Release machinery (vesicle fusion/recycling) is energetically expensive. Why waste energy on every spike?
4. **Information Capacity:** Probabilistic release allows synapses to encode information in **release probability** as well as spike rate
5. **Plasticity Substrate:** The stochastic nature provides a substrate for learning through Hebbian mechanisms

## **What Actually Gets Released When Release Occurs**

Even when release occurs, it's not always "all or nothing":

### **Quantal Release:**

- **Miniature EPSCs (mEPSCs):** Single vesicle release (~1000-5000 glutamate molecules)
- **Univesicular Release:** Most CNS synapses release **0 or 1 vesicle** per active zone per spike
- **Multivesicular Release:** Some synapses can release 2-3 vesicles simultaneously

### **Key Measurement:**

```
EPSC amplitude = Q × n × Pr
```

Where:

- **Q** = quantal size (response to one vesicle)
- **n** = number of release sites (active zones)
- **Pr** = release probability per site

## **How This Plays Out in Neural Computation**

### **Example: Cortical Pyramidal Neuron Synapse**

```
Spike train arrives: [1, 1, 1, 1, 1, 1] (6 spikes at 50 Hz)

With Pr = 0.3:
Actual releases might be: [0, 1, 0, 0, 1, 1]

Postsynaptic effect:
- Spike 1: No response
- Spike 2: EPSP
- Spike 3: No response
- Spike 4: No response
- Spike 5: EPSP
- Spike 6: EPSP

But due to facilitation:
Pr increases to 0.5 by spike 6
```

## **Simulation Implications**

In simulations, you would implement this as:

```python
import numpy as np

class Synapse:
    def __init__(self, Pr=0.3):
        self.Pr = Pr  # Release probability
        self.vesicles_available = True
        
    def process_spike(self):
        if not self.vesicles_available:
            return 0  # No release due to depletion
            
        # Probabilistic release decision
        if np.random.random() < self.Pr:
            # Release occurs
            self.vesicles_available = False  # Temporary depletion
            self.trigger_recycle_timer()  # Schedule vesicle recycling
            return 1  # 1 vesicle released
        else:
            # Release fails probabilistically
            return 0
```

## **Biological Determinants of Pr**

**High Pr synapses** tend to have:

- Denser VGCC clustering near release sites
- Higher Ca²⁺ sensitivity of Synaptotagmin
- More efficient priming machinery (Munc13)
- Larger readily releasable pool

**Low Pr synapses** tend to have:

- Sparse VGCC distribution
- Lower Ca²⁺ sensitivity
- Less efficient priming
- Smaller readily releasable pool

## **The Big Picture**

**Pr is the synapse's "transmission reliability setting"** that:

1. **Filters** spike trains probabilistically
2. **Encodes** short-term plasticity through dynamic changes
3. **Embeds** long-term memory through stable modifications
4. **Conserves** energy by not releasing on every spike

This stochasticity isn't a bug—it's a feature that allows synapses to be **temporal filters, memory elements, and computational units** all at once. The fact that transmission isn't guaranteed is precisely what gives synapses their rich computational properties, enabling phenomena like paired-pulse facilitation, synaptic filtering of high-frequency bursts, and providing a substrate for Hebbian learning through coincident detection failures.