From 8a577b4fada5764200410477950b06316cf62e64 Mon Sep 17 00:00:00 2001 From: ocrampal Date: Mon, 29 Jun 2026 21:32:16 +0200 Subject: [PATCH] Create 2026-06-29-biological-reference_v15.md --- .../2026-06-29-biological-reference_v15.md | 407 ++++++++++++++++++ 1 file changed, 407 insertions(+) create mode 100644 elements/neuron/appunti/2026-06-29-biological-reference_v15.md diff --git a/elements/neuron/appunti/2026-06-29-biological-reference_v15.md b/elements/neuron/appunti/2026-06-29-biological-reference_v15.md new file mode 100644 index 0000000..aeba0d4 --- /dev/null +++ b/elements/neuron/appunti/2026-06-29-biological-reference_v15.md @@ -0,0 +1,407 @@ +# Tripartite Synapse — Biological Reference (companion to v15 pseudocode) + +> Companion to `tripartite_synapse_v15_pseudocode.md` · principle: `logic_principles_v3`. +> v15 distributes NIGHT into each component as an iterated cycle. Biologically this reflects +> that consolidation is not a single event but proceeds across the repeated slow-wave cycles of +> NREM sleep: each cycle the producers (soma protein synthesis, astrocyte glycolysis/ECM +> synthesis) make a bounded batch of material and energy, it is transported one hop down the +> dendrite/axon/astrocytic processes, and tagged synapses incorporate what arrives. Distal sites +> consolidate later because their material arrives over more cycles. The night ends when there is +> nothing left worth consolidating (tags spent) or the night's metabolic supply is exhausted — +> a heavier learning day therefore demands a longer or fuller night, and unfinished consolidation +> carries to the following night. Energy spent on construction is irreversible (the system's one +> one-way flow); material released by pruned structure is recycled. + +--- + +## 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. + + +--- + +## NIGHT as iterated NREM cycles — the biology + +The distributed, cyclic NIGHT models sleep-dependent consolidation more faithfully than a single +commit step. + +**Why cycles, not one event.** NREM sleep proceeds in repeated cycles (the ultradian ~90-minute +rhythm, and within it the <1 Hz slow oscillation with its up- and down-states). Protein synthesis, +hippocampal–cortical replay, and synaptic renormalization all advance incrementally across these +cycles rather than in a single consolidation moment. Modeling NIGHT as a loop of cycles captures +this: each cycle is a small, local round of produce → transport → incorporate. + +**Production each cycle (the roots).** The soma's CREB-driven transcription/translation produces a +batch of structural material per cycle, gated by the soma's own tag (replay-driven activity). +The astrocyte cell body produces a batch of energy (glycolysis) and ECM material per cycle, capped +by glucose. These are the two roots; everything downstream lives on what they ship. + +**Transport over cycles (the descent).** Material and energy move one hop down the supply chains +per cycle — soma → dendrite/axon → spine/bouton; astrocyte body → astrosynapses — by the same +motor transport that carries cargo by day, now at the consolidation timescale. A distal bouton on +a long axon therefore receives its material only after several cycles, so its consolidation lags +a proximal one. This is the NIGHT-scale image of the transit delay. + +**Incorporation and tag consumption (the commit).** A tagged synapse incorporates arrived material +into structure (more receptor slots, tighter active zone, tighter astrocytic wrap) or into budget +capacity (mitochondrial biogenesis), spending energy on the assembly. The tag is consumed in +proportion to what was built — the molecular tag (CaMKII/PKA-maintained eligibility) is discharged +as capture completes. A strong tag is satisfied early; a marginal one waits for later cycles. + +**Two ways the night ends.** Either the standing tags are all spent (consolidation finished — the +rested case) or the night's metabolic budget is exhausted (ran out of night — the overloaded +case). Unspent tags are not discarded: they persist (decaying slowly) into the next day and +compete again the next night, so importance is re-tested across nights and a marginal memory may +consolidate over several nights or, if it decays first, never. + +**Energy is the irreversible cost.** Material released when an unmaintained structure is pruned +returns to the pool and is reused; the energy burned to build or to prune is gone. Across a +lifetime this energy throughput bounds how much the system can ever consolidate — the metabolic +arrow of time underlying the whole model.