# Tripartite Synapse — Biological Reference (companion to v14 pseudocode) > Companion to `tripartite_synapse_v14_pseudocode.md` · principle: `logic_principles_v3` (The > Timescale Ladder). Explains the biology each variable and behavior conflates. v14 adds > per-component plain-language intros, regroups budget refill under RECOVER, adds presynaptic > short-term potentiation (possible_tag → VGCC coupling occupancy, the presynaptic parallel to > postsynaptic AMPA-surface occupancy), and surfaces four timescale tiers — FAST (ms–s, traces), > MEDIUM (s–min, occupancy + evidence), SLOW (hr, tag), PERSISTENT (NIGHT-written capacity). --- ## The three synaptic components and their support structures A SYNAPSE is composed of three first-class components: - **PRE** — presynaptic bouton (the axon's terminal at this synapse) - **POST** — postsynaptic spine (the dendrite's terminal at this synapse) - **ASTRO** — astrosynapse, the perisynaptic astrocytic process (the astrocyte's terminal) Each has an upstream support structure that supplies it: - **AXON** supplies PRE (transmission + transport from soma) - **DEND** supplies POST (integration + transport from soma) - the **astrocyte cell body** supplies ASTRO (energy + ECM material) - **SOMA** is the integrating center and the root of neuronal material The compartment analogy: AXON:PRE = DEND:POST = astrocyte-body:ASTRO = supply line : terminal. --- ## Resource variables ### DAY budget (one per component) Aggregates fast energy AND fast consumables — everything needed to run moment-to-moment. - **pre_budget** — ATP for VGCC gating, vesicle fusion (SNARE), VATPase vesicle refill, plus fast consumables: vesicle membrane lipids, synaptotagmin recycling. - **post_budget** — ATP for the NaK pump (membrane reset after current), NMDA current handling, plus fast actin monomers for transient spine changes and receptor-recycling lipids. - **dend_budget** — ATP for bAP propagation (NaK reset along branch), local translation (ribosome running cost), SERCA Ca²⁺ resequestration, plus fast mRNA consumed by translation. - **soma_budget** — ATP for AP generation (Na⁺/K⁺ currents + NaK reset), CREB phosphorylation, nuclear Ca²⁺ handling, plus shipping running costs. - **axon_budget** — ATP for AP propagation at nodes of Ranvier, kinesin/dynein motor running cost, fast myelin maintenance. - **astro_central_budget** — ATP from glycolysis at the astrocyte cell body; funds EAAT clearance, serine→D-serine synthesis, lactate export, fast process motility. ### astro_lactate[i] Lactate exported from the astrocyte cell body to synapse i. Biologically: glucose → (glycolysis) → lactate, released into extracellular space, absorbed by neuronal MCT2 transporters, converted to pyruvate → TCA → ATP in the neuron's mitochondria. The astrocyte is the primary fast-energy supplier to pre, post, and dend. ### NIGHT energy (one per component) — NOT recoverable ATP for structural assembly. Distinct from DAY budget because it is spent on building, and the work of assembly is thermodynamically gone once done (cannot be recovered by disassembly). - pre_energy: RIM/Munc13 incorporation, VGCC clustering. - post_energy: CaMKII anchoring, actin polymerization, PSD scaffold remodeling. - dend_energy: mitochondria incorporation, cytoskeletal reinforcement. - soma_energy: ribosome biogenesis, ion-channel incorporation. - axon_energy: myelination, microtubule stabilization. - astro_energy: process retraction, ECM secretion, racemase upregulation. ### NIGHT material (one per component) — RECOVERABLE Slow structural proteins. Recoverable because disassembly (LTD) returns the proteins to a reusable pool (ubiquitin-proteasome → amino acids; internalized receptors → endosomal reserve). - **soma_material** (root) — all neuronal structural proteins from CREB-driven synthesis: AMPA subunits, PSD scaffold, AZ scaffold, mRNA transcripts (Arc, BDNF), organelles. - **dend_material** — from soma: Arc/plasticity mRNA, mitochondria, cytoskeletal proteins, AMPA subunits in transit to spines. - **post_material** — from dend: AMPA receptor subunits (GluA1/2), PSD scaffold (PSD-95, SHANK, Homer), structural actin, CaMKII. - **axon_material** — from soma: kinesin/dynein motors, microtubule components, myelin proteins. - **pre_material** — from axon: RIM, Munc13, VGCC subunits, structural vesicle proteins. - **astro_material** (root: astrocyte cell body) — EAAT proteins, serine racemase, ECM proteins (Glypicans, Thrombospondins), process cytoskeleton. **Why energy and material are separate in NIGHT but combined in DAY:** during DAY both are fast consumables replenished on the same timescale, so one `budget` variable suffices. During NIGHT they diverge — material is recoverable after LTD, energy is not — so they must be two variables. This asymmetry (material returns to the pool, energy is gone) is what makes one synapse's depression genuinely fund another's potentiation. --- ## Structural variables (strength ceilings — written in NIGHT) Each aggregates several correlated structural properties into one capacity. - **pre_structure** — active zone capacity: slot_ceiling (number of vesicle docking slots) + VGCC_coupling (Ca²⁺-channel proximity to slots, sets release efficiency) + refill_ceiling (max RRP replenishment rate). - **post_structure** — spine sensitivity capacity: slot_ceiling (number of PSD anchoring slots for AMPA) + spine_volume (local reserve and actin machinery) + reserve_ceiling (endosomal AMPA pool size). - **dend_structure** — branch capacity: bAP_fidelity(position) (mitochondrial density sets propagation strength, attenuates with distance) + translation_ceiling (local mRNA capacity) + transport_speed (cytoskeletal integrity). - **soma_structure** — somatic output capacity: baseline_threshold (inverse: ion-channel density at axon initial segment) + AP_reliability (Na⁺ channel density) + synthesis_ceiling (ribosome density + CREB machinery). - **axon_structure** — axonal capacity: propagation reliability (myelination density) + transport_ceiling (motor density + microtubule integrity) + mitochondrial density. - **astro_structure** — astrosynaptic environmental capacity: perisynaptic_distance⁻¹ (wall proximity — closer = more glutamate contained) + EAAT_density (clearance ceiling) + Dserine_tonic (baseline co-agonist) + ECM_integrity. **Self-reinforcing both directions:** tighter wrap + more tonic D-serine make future potentiation easier; looser wrap + zero tonic D-serine make future depression easier. --- ## Budget ceilings (endurance ceilings — written in NIGHT) - **{component}_budget_ceiling** — the maximum fuel the component can hold / the maximum duration of sustained behavior. Biologically: mitochondrial density and local fuel-storage capacity. Built by activity-driven mitochondrial biogenesis; lost by mitophagy when idle. Parallel to structure: structure is strength capacity, budget_ceiling is endurance capacity. --- ## Trace variables ### fast_trace (one per component) — DAY only, decays automatically The local record of recent activity that biases the next behavior. - **pre_fast_trace** — residual presynaptic Ca²⁺ after spikes (τ≈100ms). Biases NT release (facilitation) and provides tagging eligibility. - **post_fast_trace** — spine Ca²⁺ amplitude × rise-speed (τ≈tens ms). Encodes the LTP-vs-LTD instruction (fast rise → CaMKII → potentiation; slow rise → phosphatase → depression). - **dend_fast_trace** — branch Ca²⁺ from bAP + spine spillover (τ≈300ms). Integrates branch co-activity. - **soma_fast_trace** — nuclear Ca²⁺ from each AP (τ≈seconds). Drives toward CREB activation. - **axon_fast_trace** — propagation load (τ≈seconds). High load → Na⁺ inactivation at branch points → propagation failure (this is axonal short-term depression). - **astro_fast_trace** — perisynaptic Ca²⁺ from mGluR5 activation by glutamate spillover (τ≈seconds). Drives D-serine release. ### soma timing traces (emergent refractory + adaptation + alignment) - **soma_Na_inactivation** (τ≈ms) — sodium-channel inactivation after an AP. Its recovery IS the refractory period (emergent, not a hardcoded timer). High → absolute refractory; decaying → relative refractory; recovered → normal. - **soma_adaptation** (τ≈100s of ms) — slow K⁺ channel (SK/M-type) activation accumulating over a spike train, raising threshold. This is spike-frequency adaptation. - **soma_refractory_alignment** — deposited when a suprathreshold input arrives during refractoriness (a missed coincidence). Speeds future recovery so the soma aligns to its input rhythm. Bottom-up: no rhythm is represented; alignment emerges from accumulated local mismatches and decays when mismatches stop (self-limiting). ### possible_tag (one per component) — intermediate, τ≈s–min Graded accumulation of tagging eligibility. For POST, this is the CANDIDATE tag lifetime. ### endurance_need (one per component) — intermediate, τ≈s–min Deposited when budget depletion interrupts a behavior that was on a LOCALLY successful trajectory. Records that fuel — not structure, not significance — was the binding constraint on a forming success. Requires NO dopamine (homeostatic, not associative). **Local success proxy per component** (each uses only its own state + arrived signals): - PRE: own fast_trace high (was releasing strongly), optionally amplified by retrograde messenger (endocannabinoid / NO / BDNF) that has arrived. - POST: own Ca²⁺ climbing toward tagging threshold (naturally local). - DEND: own branch strongly active (high branch voltage/Ca²⁺) when propagation fell short. - SOMA: own nuclear Ca²⁺ climbing toward CREB. - AXON: own propagation load high (was carrying a strong train). - ASTRO: own local glutamate/Ca²⁺ high (was under heavy clearance/D-serine demand). ### tag (one per component) — DAY→NIGHT bridge, τ≈hours The validated record of significance that survives to NIGHT and gates strength commits. Formed by coincidence of local eligibility + non-local validation (dopamine). **POST is special — two-phase, three coincidences:** - CANDIDATE: local Ca²⁺ above threshold + astrosynapse D-serine present (coincidence 1). - amplified when bAP confirms soma fired (coincidence 2). - STABLE: CANDIDATE + dopamine within stabilization window (coincidence 3). Biologically: early CaMKII creates a labile tag (early-LTP); PKA driven by dopamine via D1R stabilizes it (late-LTP). Without dopamine, the candidate degrades — early-LTP reverses. --- ## Behaviors — biological meaning ### PRE | AP — neurotransmitter release `NT_flux = RRP × sat(pre_fast_trace, K_release)` models continuous NT release proportional to the readily-releasable pool and a saturating Ca²⁺ drive (synaptotagmin's cooperative Ca²⁺ sensitivity, simplified to a saturating curve). RRP depletes as released (short-term depression as a consequence) and refills via VATPase (energy-throttled, so low budget deepens depression). The mGluR2/3 brake is presynaptic autoinhibition by spillover (Gi → reduced VGCC opening). ### POST | NOT_bAP — three calcium sources, two plasticity cases - **Source 1 (AMPA):** glutamate opens AMPA → depolarizing current + small Ca²⁺; the depolarization begins ejecting the NMDA Mg²⁺ block. - **Source 2 (NMDA):** if depolarized enough (Mg²⁺ ejected) AND D-serine present (astrocyte co-agonist) AND glutamate bound → large Ca²⁺ influx. This is the coincidence detector. - **Source 3 (bAP, separate context):** back-propagating AP adds depolarization + Ca²⁺, amplifying an existing signal supralinearly. - **Case 1 (STP):** high Ca²⁺ drives AMPA receptors from the local reserve to the surface, bounded by the anchoring-slot ceiling. Fast, reversible, NO dopamine. When Ca²⁺ falls, receptors drift back — short-term depression as a passive consequence, never signaled. - **Case 2 (LTP tag):** high Ca²⁺ + (later) dopamine sets the tag that NIGHT uses to raise the slot ceiling. NIGHT builds slots; DAY fills them. ### DEND | bAP — bidirectional signaling Propagates the bAP from soma toward spines (fidelity attenuates with distance — distal spines get weaker confirmation, are harder to potentiate) and integrates spine signals toward the soma. ### SOMA | AP — integration, firing, emergent timing Fires when integrated branch input exceeds a threshold that is the baseline (from structure) raised by adaptation and modulated by neuromodulators, gated by the emergent refractory state. Each AP deposits three traces (inactivation → refractory, adaptation → threshold rise, nuclear Ca²⁺ → plasticity). The soma is the coincidence detector at the cellular scale (nuclear Ca²⁺ + dopamine → CREB), and the production bottleneck: its tag gates how much material all downstream components get in NIGHT. ### AXON | AP — reliable propagation with frequency-dependent failure Propagation reliability is set by myelination and degraded by high-frequency load (Na⁺ inactivation at branch points = axonal STD). The axon also transports material to boutons and sets the timescale of presynaptic structural commits. ### ASTRO | CONTINUOUS — gatekeeper and energy hub Clears glutamate (EAAT), supplies D-serine (the NMDA co-agonist that gates postsynaptic LTP), and distributes lactate to the territory by demand-weighting (active synapses generating more clearance load pull more fuel; slow synapses get less). The same spillover that excites the astrocyte (mGluR5 → Ca²⁺ → D-serine) also brakes the presynapse (mGluR2/3 → Gi) — one signal, opposite effects via different receptors. The astrocyte is the energy root and the gain control of the whole synapse. --- ## NIGHT operations — biological meaning - **Step 1 (replenish/distribute):** overnight protein synthesis peaks (CREB-driven, gated by soma_tag — corresponds to slow-wave-sleep replay). Soma material flows to branches/axon then spines/boutons; astrocyte material flows to astrosynapses, tag-weighted. - **Step 2 (strength commits):** tagged components build structure — more slots, tighter coupling, tighter astrosynaptic wrap. Coherence bonus when pre+post+astro all tagged (the whole synapse agrees). astro_structure self-reinforces. - **Step 2b (endurance commits):** components with high endurance_need build budget_ceiling — mitochondrial biogenesis. Competes with step 2 for the same material/energy. - **Step 3 (passive decay):** both ceilings decay; maintenance from the remaining pool resists decay only where sufficient. Depotentiation and endurance-loss are both by neglect — no signal weakens anything; unmaintained capacity simply drifts down. Recovered material (not energy) returns to pools. - **Step 4 (homeostatic scaling):** if the soma fired too much overall, all synapses scale down proportionally (sleep-associated global downscaling), preserving relative differences. - **Step 5 (clear traces):** fast traces, possible tags, endurance needs, and soma timing traces reset; tags below expiry clear, above-expiry tags carry forward (multi-night consolidation); structure and budget_ceiling persist. ### Shockwave lockdown Emergency global astrocytic Ca²⁺ wave → GABA + ATP release → mass AMPA internalization and hyperpolarization. Bypasses budget gates. A circuit breaker against runaway excitation. --- ## Pool-filling: private reserve vs contested supply The pseudocode uses two filling primitives, distinguished by where the resource comes from. **`fill` (private reserve).** The pool is replenished from a source the component owns outright, uncontested by siblings, bounded by the component's own ceiling and a rate cap. - RRP refill — vesicles mobilized from the bouton's own reserve pool toward the docking-slot ceiling, rate-limited by VATPase. The reserve is private to the bouton. - SOMA self-replenish — the soma fuels itself from its own mitochondria toward its budget ceiling. No other component draws on it. **`refill` (contested supply).** The pool is replenished from a supply that multiple components compete for, rationed by demand (gap to ceiling). - pre/post/dend/axon budgets — drawn from astrocytic lactate (shared across all synapses the astrocyte wraps) plus shipment from soma/axon/dendrite (shared across downstream targets). **Neither primitive (their own forms).** Some inflows are not fills toward a ceiling: - AMPA surface insertion — Ca²⁺-driven rate from the spine's private endosomal reserve, with an explicit passive drift-back (short-term depression) when Ca²⁺ is low. Not a steady fill. - D-serine release — demand-driven (saturating in astro Ca²⁺) and budget-limited, like NT release; a release process, not a pool top-up. - Root productions — `glycolysis(glucose)` at the astrocyte and `CREB_synth(soma_tag)` at the soma are the system's energy and material roots: raw inflows capped only by the external vascular supply, not fills toward an internal ceiling. The distinction matters biologically: a private reserve guarantees a component some autonomy (the bouton can refill its RRP from its own vesicles even when lactate is scarce), while a contested supply couples a component's fate to its neighbours' demands (operational budget fails first where many active synapses compete for the same lactate). --- ## PRE ↔ POST interaction: local computation, message-only coupling The presynapse and postsynapse never read each other's internal state. They interact only by writing to and reading from shared cleft channels. Each side computes entirely locally on what it has: its own variables plus whatever signals have arrived in the cleft. This is the message-passing realization of the locality principle. **Forward channel — glutamate (PRE → POST and ASTRO).** The presynapse writes glutamate via NT_flux. The postsynapse reads it (AMPA, NMDA) and the astrosynapse reads it (clearance, mGluR5). The astrosynapse clears it. PRE never knows whether POST responded — it only emits. **Gate channel — astro_Dserine (ASTRO → POST).** The astrosynapse writes D-serine; the postsynapse reads it as the obligatory NMDA co-agonist. POST cannot open NMDA without this arrived signal, but it does not read the astrocyte's state — only the delivered D-serine. **Backward channel + — retro_NO (POST → PRE).** When the postsynapse's NMDA opens (Mg²⁺ ejected, D-serine present, glutamate bound), nNOS — physically tethered to the NMDA receptor through PSD-95 — synthesises nitric oxide (and, on a slower timescale, BDNF is released). These diffuse retrogradely to the presynapse. Biologically this is the classic retrograde messenger of LTP: it tells the bouton that its release landed on a postsynapse that genuinely responded. In the model, POST emits `retro_NO` proportional to its own NMDA-driven calcium — computed purely from POST's local state — and PRE reads it as `retro_NO_local`. `retro_NO_local` is exactly the grounding of the presynaptic endurance signal. The presynapse's local success proxy is "I was releasing strongly" (`pre_fast_trace` high). On its own that only says the bouton was working hard, not that the work mattered. `retro_NO` adds the missing confirmation — that the postsynapse responded — without PRE ever reading POST's calcium. So PRE deposits endurance need as `pre_fast_trace × (1 + retro_NO_local)`: strong release that was confirmed effective makes the strongest claim that fuel, not futility, was what interrupted a forming success. retro_NO is short-lived (NO degrades and diffuses within seconds), so the channel decays fast — confirmation must be recent to count. **Backward channel − — retro_eCB (POST → PRE).** When the postsynapse is strongly depolarised, it synthesises endocannabinoids (2-AG, anandamide) that diffuse retrogradely and bind presynaptic CB1 receptors, suppressing release. This is depolarisation-induced suppression of excitation (DSE) — a homeostatic negative feedback: an over-driven postsynapse tells the presynapse to release less. In the model, POST emits `retro_eCB` from its own membrane potential, and PRE reads it as `retro_eCB_local`, which reduces the release drive `sat(...) × (1 - retro_eCB_local)`. Again POST computes from its own state; PRE adjusts from the arrived signal; neither reads the other's interior. The two backward channels are opposite-signed messages the postsynapse sends about its own condition: retro_NO says "your input was effective — worth sustaining," retro_eCB says "I am saturated — ease off." Together with the forward glutamate and the D-serine gate, they make the synapse a fully message-coupled system of locally-computing components. **Why RRP refill is in NOT_AP only.** During an AP the bouton releases — RRP depletes. Refill (VATPase reloading vesicles from the reserve pool) is a recovery process that proceeds between spikes. Placing `fill(RRP, ...)` only in the NOT_AP context makes the AP context pure depletion and the NOT_AP context pure recovery. A consequence falls out for free: during sustained high-frequency firing there are many AP steps and few NOT_AP steps, so RRP depletes faster than it recovers — short-term depression deepens with frequency, with no explicit depression rule. The release itself is throttled further when budget is low (VATPase refill is energy-limited), coupling metabolic state to the depth of depression. --- ## Presynaptic short-term potentiation — VGCC coupling occupancy `VGCC_active` is the presynaptic parallel to the postsynaptic `AMPA_surface`. Both are MEDIUM-tier occupancy variables: a current operating value filled toward a NIGHT-built ceiling, no dopamine, reversible, drifting back when undriven. Biologically, `VGCC_active` represents the effective coupling between voltage-gated calcium channels and the vesicle docking slots — how reliably each calcium influx is converted into release. Repeated eligible activity (accumulated `pre_possible_tag`) transiently tightens this coupling — through calcium-channel facilitation, active-zone protein phosphorylation, and channel-to-sensor proximity changes — raising release efficiency without changing the number of channels (which is the structural ceiling `pre_structure.VGCC_coupling`, written only at NIGHT). When eligibility falls, the coupling relaxes back to baseline over seconds-to-minutes: presynaptic short-term depression as the passive consequence of undriven coupling, never a signalled act. This gives the presynapse a genuine intermediate-timescale memory it previously lacked — a "this bouton has been reliably active lately" state that outlasts individual spikes and bursts, filling the gap between the fast trace (residual calcium, ~100 ms) and the tag (hours). It also completes the capacity/occupancy symmetry across the synapse: both PRE and POST now fill a MEDIUM occupancy variable toward a PERSISTENT structural ceiling, rather than PRE reading its ceiling directly as if capacity and occupancy were the same thing.