Update 2026-06-04-modulation-of-future-behavior.md
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# Intro
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The synapse uses three interlocking signal systems to translate present activity into future behavioral bias. Ca²⁺ is the universal event recorder — each compartment reads its concentration dynamics differently (amplitude and speed of rise in the postsynapse, residual accumulation in the presynapse, IP3-triggered waves in the astrocyte), so the same ion encodes distinct instructions depending on where and how it appears. cAMP/PKA is the contextual gate: driven by neuromodulatory broadcast (dopamine, norepinephrine), it doesn't write changes itself but determines whether the Ca²⁺ signal gets committed to permanent structure — by priming AMPA receptor insertion, silencing the LTD phosphatase machinery via DARPP-32, and activating CREB-driven gene expression for structural proteins. mGluRs provide the overflow sensing layer: when glutamate spills beyond the cleft, group II/III mGluRs on the presynapse activate a Gi-mediated autoinhibitory brake, while group I mGluRs on the astrocyte trigger the IP3→Ca²⁺→D-serine cascade that amplifies NMDA coincidence detection — a push-pull architecture that simultaneously throttles excessive release and widens the postsynaptic learning window.
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Here is the full descriptive synthesis, mirroring the pseudocode structure exactly.
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Together these three systems form a hierarchical filter: Ca²⁺ asks did something happen?, mGluRs ask was it excessive?, and cAMP/PKA asks was it worth saving? — and only when all three align does the synapse commit to rewriting its future response.
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---
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## Global state: what each compartment tracks
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The three compartments maintain distinct but coupled state variables across three time horizons. At the fast scale, the presynapse tracks residual calcium and its readily-releasable vesicle pool — both of which encode the very recent history of firing. The postsynapse tracks membrane voltage and the amplitude and speed of its calcium rise, which together encode the instruction for future change. The astrocyte tracks glutamate concentration both inside and outside the cleft, its own local and global calcium state, and its fuel output. At the intermediate scale, the shared signal layer tracks whether the mGluR overflow sensors have fired, and whether the neuromodulatory context gate has been set — specifically whether PKA has primed the AMPA insertion machinery, silenced the forgetting phosphatase, and enabled gene expression in the nucleus. At the slow scale, all three compartments track their own physical architecture: the postsynapse its receptor count and spine size, the presynapse its docking slot count and vesicle channel clustering, and the astrocyte its wall distance from the synapse, its matrix density, and its baseline co-agonist supply.
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---
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## Fast scale: what happens spike by spike
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Every action potential sets off a precise sequence across all three compartments simultaneously. The presynapse converts the electrical event into a chemical wavefront: calcium floods in, drives probabilistic vesicle release from the readily-releasable pool, and leaves a residual trace that biases the next release upward if spikes keep arriving and downward if they stop. The amount of glutamate released fills the cleft and begins diffusing outward.
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The astrocyte responds in two parallel arms the moment glutamate spills beyond the cleft boundary. The first arm activates astrocytic mGluR5 receptors via a Gq cascade, triggering an internal calcium rise that is directly proportional to how much glutamate has escaped — this calcium rise drives D-serine release, widening the postsynaptic NMDA detection window. The second arm simultaneously activates presynaptic mGluR2/3 receptors via Gi, suppressing adenylyl cyclase and reducing vesicle release probability — a direct autoinhibitory brake on the very source of the overflow. These two arms run in opposite directions from the same trigger: the astrocyte brakes the presynapse while amplifying the postsynaptic learning window at the same time.
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The postsynapse responds to the glutamate wavefront through its AMPA receptors, depolarizing the membrane. But full calcium entry through NMDA receptors only occurs if two conditions are met simultaneously: the membrane must be sufficiently depolarized to eject the magnesium block, and D-serine released by the astrocyte must be present as a co-agonist. This is the coincidence detection step — both conditions are required, and the astrocyte's D-serine supply is what makes it a three-party coincidence rather than a two-party one.
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Once the spike is complete, the astrocyte vacuums up residual glutamate via its transporter proteins, and the harder it works at this clearance the faster it runs its glycolysis engine, converting blood glucose into lactate and pumping it into the extracellular space. Both the presynapse and postsynapse absorb this lactate to power their reset pumps — refilling the vesicle pool and restoring the resting membrane potential respectively. The energy supply is therefore coupled to activity: busier synapses generate more demand on the astrocyte, which in turn fuels faster recovery.
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---
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## Intermediate scale: temporary tuning between spikes
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If firing is sustained rather than isolated, the system begins temporary adjustments that do not yet commit to structural change. In the presynapse, sustained high-frequency firing keeps residual calcium elevated, progressively increasing release probability and mobilizing vesicles from deep reserve storage into the readily-releasable pool — a priming of the launchpad without permanently expanding it. If frequency is low and sparse, the reverse happens: the pool depletes faster than it refills and release probability falls.
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In the postsynapse, sustained high-frequency input keeps the membrane depolarized long enough to hold the magnesium block off continuously, allowing calcium to accumulate gradually rather than in isolated pulses. This accumulation is the early signal for plasticity, but it is not yet sufficient on its own to commit a structural change.
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In the astrocyte, sustained activity keeps mGluR5 activated and D-serine release elevated above its baseline pulse level, maintaining a wider NMDA detection window for as long as the high-frequency drive continues.
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The neuromodulatory broadcast then sets the critical context gate. If dopamine or norepinephrine levels cross their respective thresholds — signaling that the current activity pattern is behaviorally significant — PKA activity rises and phosphorylates three targets in sequence. It lowers the threshold for AMPA receptor insertion by priming the GluA1 subunit at Ser845. It silences the LTD phosphatase PP1 by phosphorylating DARPP-32, effectively blocking the forgetting machinery from running while the save signal is present. And it translocates to the nucleus to phosphorylate CREB, enabling the gene expression needed to build new structural proteins. Acetylcholine from the basal forebrain acts in parallel, lowering the global LTP threshold — making the entire system more sensitive to incoming patterns during periods of high attention.
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---
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## Slow scale: the commit decision and structural rewriting
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Once the fast and intermediate dynamics have run, the system evaluates a three-layer filter to decide whether to permanently rewrite its architecture. The first layer asks whether a genuine event occurred — specifically whether postsynaptic calcium rose above the high threshold at sufficient speed to activate the LTP kinase pathway. The second layer asks whether that event was excessive enough to saturate the cleft and trigger mGluR5 on the astrocyte. The third layer asks whether the neuromodulatory context validated the event as worth saving — whether PKA has primed the insertion machinery and silenced the forgetting machinery.
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Only when all three conditions align does structural rewriting proceed, and when it does, all three compartments are rewritten simultaneously and in the same direction.
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In the potentiation branch, the postsynapse anchors new AMPA receptors into the membrane via CaMKII — a process made easier because PKA has already lowered the insertion threshold — and the spine itself physically enlarges. The presynapse expands its active zone, clusters more calcium channels directly beneath the docking area to tighten the coupling between electrical events and vesicle release, and increases its baseline release probability. The astrocyte retracts its process walls inward toward the synapse, secretes structural matrix proteins to seal and stabilize the channel, upregulates its baseline D-serine synthesis for chronic NMDA priming, and reduces its glutamate clearance rate — meaning future signals will linger longer in the cleft rather than being vacuumed away immediately.
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In the depression branch, the phosphatase PP1 wins instead of CaMKII, and all three changes run in reverse simultaneously. The postsynapse internalizes receptors and the spine shrinks. The presynapse removes docking slots, scatters its calcium channels away from the active zone, and pulls vesicles back into deep reserve storage. The astrocyte secretes matrix metalloproteinases to dissolve the structural scaffold, cuts its D-serine supply to starve the NMDA gate chronically, and extends its process walls outward away from the synapse, loosening the diffusion barrier so that future glutamate bleeds away faster rather than concentrating at the cleft.
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If the calcium event occurred but the neuromodulatory save signal did not arrive, only transient changes happen — early receptor insertion and brief facilitation — both of which reverse within minutes without trace. If no threshold was crossed at all, nothing changes and the current structural state is simply held.
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---
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## The critical asymmetry
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The astrocyte's perisynaptic wall distance is the variable that makes both outcomes self-reinforcing rather than merely additive. When it moves inward during potentiation, it concentrates glutamate at the cleft, maintains D-serine near the postsynapse, and tightens the presynaptic feedback loop — making future high-frequency events even more likely to cross the threshold. When it moves outward during depression, it dilutes the signal, starves the NMDA gate, and loosens the presynaptic feedback — making future events even less likely to reach threshold. The astrocyte therefore does not simply mirror what the neurons decide: it actively deepens the valley the synapse has already rolled into, in whichever direction that happens to be.
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## Pseudocode
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[pseudocode](tripartite_synapse_full_pseudocode.html)
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## signal state variables
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```Gen
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// ── Ca²⁺ : event recorder ──────────────────────────────────────
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pre_Ca_residual // leftover Ca²⁺ between spikes — encodes recent history
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post_Ca_amplitude // peak rise magnitude in spine
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post_Ca_rise_speed // rate of rise — fast=LTP, slow=LTD
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### global state variables
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#### ── Fast (ms–s): wave propagation ─────────────────────────────
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##### Presynapse
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pre_Ca_residual // leftover Ca²⁺ between spikes — short-term trace
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vesicle_release_prob // P(0.1–1.0) per docking slot
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RRP_pool // readily-releasable vesicle pool
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reserve_pool // chained vesicles in deep storage
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##### Postsynapse
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membrane_potential // Vm — depolarization state
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NMDA_Mg_block // bool — mechanical clamp on/off
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post_Ca_amplitude // peak [Ca²⁺] rise in spine
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post_Ca_rise_speed // d(Ca)/dt — fast=LTP signal, slow=LTD signal
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##### Astrocyte
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glutamate_cleft // [glu] in synaptic cleft
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glutamate_spillover // extrasynaptic [glu] — saturates mGluRs
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astro_Ca_local // IP3-triggered local rise near synapse
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astro_Ca_global // soma-wide wave — network overload flag
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D_serine_release // gliotransmitter — NMDA co-agonist pulse
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lactate_output // fuel export rate to pre and post
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// ── cAMP/PKA : context gate ────────────────────────────────────
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#### ── Intermediate (s–min): temporary tuning ────────────────────
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mGluR2_3_activation // presynaptic Gi — autoinhibitory brake
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mGluR5_activation // astrocytic Gq — IP3→Ca²⁺→D-serine cascade
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cAMP_level // set by dopamine/NE via Gs → adenylyl cyclase
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PKA_activity // downstream of cAMP
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GluA1_Ser845_primed // bool — AMPA insertion threshold lowered
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DARPP32_phospho // bool — PP1 (LTD phosphatase) silenced
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GluA1_Ser845_primed // bool — AMPA insertion threshold lowered by PKA
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DARPP32_phospho // bool — PP1 (LTD phosphatase) silenced by PKA
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CREB_active // bool — structural gene expression enabled
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// ── mGluRs : overflow sensor ───────────────────────────────────
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glutamate_spillover // extrasynaptic [glu] — only high when cleft saturated
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mGluR2_3_activation // presynaptic Gi — autoinhibitory brake
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mGluR5_activation // astrocytic Gq — IP3 → Ca²⁺ → D-serine cascade
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```
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#### ── Slow (h–weeks): structural architecture ───────────────────
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AMPA_count // surface receptors — postsynaptic sensitivity
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spine_volume // physical size of dendritic spine
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active_zone_size // docking slot count
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RRP_pool_capacity // max readily-releasable pool
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VGCC_clustering // Ca²⁺ channels beneath active zone
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perisynaptic_distance // how close astrocyte walls are to synapse
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ECM_integrity // extracellular matrix density
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D_serine_tonic_level // baseline co-agonist supply (sustained)
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glutamate_clearance_rate // EAAT transporter density
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## layer 1 — Ca²⁺: did something happen?
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```Gen
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function Ca_event_recorder(spike_history, input_freq):
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#### fast time scale — wave propagation (ms → s)
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function fire_action_potential(input_freq):
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// Presynapse: residual Ca²⁺ = trace of recent firing
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##### Presynapse: launch wavefront
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pre_Ca_residual += spike_influx(input_freq)
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pre_Ca_residual *= decay(τ ≈ 100ms) // fades unless spikes keep arriving
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vesicle_release_prob *= facilitation(pre_Ca_residual)
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released_vesicles = binomial(RRP_pool, vesicle_release_prob)
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glutamate_cleft = released_vesicles × quantal_content
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RRP_pool -= released_vesicles
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// Postsynapse: amplitude + speed encode the instruction
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post_Ca_amplitude = NMDA_influx(glutamate_cleft, membrane_potential)
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post_Ca_rise_speed = d(post_Ca_amplitude) / dt
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if post_Ca_amplitude > Ca_HIGH and post_Ca_rise_speed > fast_threshold:
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activate(CaMKII) // → LTP kinase pathway
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elif post_Ca_amplitude > Ca_LOW and post_Ca_rise_speed < slow_threshold:
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activate(PP1, PP2B) // → LTD phosphatase pathway
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else:
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pass // sub-threshold — no instruction encoded
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// Astrocyte: local vs global Ca²⁺ = two different alarms
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astro_Ca_local = IP3_release(mGluR5_activation) // activity-proportional
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astro_Ca_global = soma_wave(astro_Ca_local > OVERLOAD_threshold)
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if astro_Ca_local > local_threshold:
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D_serine_release += gliotransmitter_pulse() // widens NMDA window
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if astro_Ca_global:
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trigger(shockwave_lockdown) // circuit-breaker
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```
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## layer 2 — mGluRs: was it excessive?
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```Gen
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function mGluR_overflow_sensor():
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// Only fires when cleft is genuinely saturated (low-affinity receptors)
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##### Astrocyte: overflow sensing and co-agonist release
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glutamate_spillover = extrasynaptic_diffusion(glutamate_cleft)
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if glutamate_spillover > spillover_threshold:
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// Presynapse arm: Gi → brake
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mGluR2_3_activation = True
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cAMP_level -= Gi_inhibition(adenylyl_cyclase) // suppress PKA locally
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mGluR5_activation = True // Gq arm → IP3 → Ca²⁺ → D-serine
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astro_Ca_local += IP3_cascade(PLC)
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D_serine_release += proportional_to(astro_Ca_local)
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mGluR2_3_activation = True // Gi arm → brake presynapse
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cAMP_level -= Gi_inhibition(adenylyl_cyclase)
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vesicle_release_prob -= VGCC_suppression() // autoinhibitory brake
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// Astrocyte arm: Gq → amplify (push-pull)
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mGluR5_activation = True
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astro_Ca_local += IP3_cascade(PLC_activation) // feeds back into layer 1
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D_serine_release += proportional_to(astro_Ca_local)
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##### Astrocyte: check for network overload
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astro_Ca_global = soma_wave(astro_Ca_local > OVERLOAD_threshold)
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if astro_Ca_global: trigger(shockwave_lockdown)
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// Net: same overflow signal brakes pre, amplifies post-learning window
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return (mGluR2_3_activation, mGluR5_activation)
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```
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##### Postsynapse: wavefront strikes resonator
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AMPA_current = glutamate_cleft × AMPA_count
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membrane_potential += AMPA_current
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## layer 3 — cAMP/PKA: was it worth saving?
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##### NMDA gate: coincidence check
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if membrane_potential > -40mV and D_serine_release > threshold:
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NMDA_Mg_block = False // Mg²⁺ ejected
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post_Ca_amplitude += NMDA_influx(glutamate_cleft)
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post_Ca_rise_speed = d(post_Ca_amplitude) / dt
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```Gen
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function PKA_context_gate():
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##### Astrocyte: vacuum trailing echoes + fuel pipeline
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glutamate_cleft -= glutamate_clearance_rate × Δt
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lactate_output += glycolysis_rate(glutamate_clearance_rate)
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membrane_potential restored by NaK_ATPase(lactate_output)
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RRP_pool refilled by VATPase(lactate_output)
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// Neuromodulators set the gate via Gs protein
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if dopamine_level > D1_threshold or norepinephrine_level > β_threshold:
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cAMP_level += Gs_activation(adenylyl_cyclase)
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PKA_activity = proportional_to(cAMP_level)
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#### intermediate time scale — temporary tuning (s → min)
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function short_term_plasticity(input_freq, duration):
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// Target 1: prime AMPA insertion
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##### Presynapse: facilitate or depress based on Ca²⁺ history
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if input_freq > 20Hz:
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vesicle_release_prob *= 1.3 // residual Ca²⁺ primes launchpad
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mobilize(reserve_pool → RRP_pool) // break storage chains
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elif input_freq < 5Hz:
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vesicle_release_prob *= 0.7 // RRP depleted faster than refill
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##### Postsynapse: NMDA gate primed if frequency sustained
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if input_freq >= 50Hz and duration > 1s:
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NMDA_Mg_block = False // sustained depolarization
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post_Ca_amplitude accumulates // early-LTP signal rises
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##### Astrocyte: sustained volume → escalate co-agonist
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if astro_Ca_local > local_threshold:
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D_serine_release += gliotransmitter_pulse() // widens NMDA window
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##### Neuromodulators: set context gate via Gs protein
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if dopamine_level > D1_threshold or NE_level > β_threshold:
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cAMP_level += Gs_activation(adenylyl_cyclase)
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PKA_activity = proportional_to(cAMP_level)
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phosphorylate(GluA1, site=Ser845)
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GluA1_Ser845_primed = True // lowers threshold for CaMKII to anchor receptors
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// Target 2: silence the forgetting machinery
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GluA1_Ser845_primed = True // lowers CaMKII threshold
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phosphorylate(DARPP32)
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DARPP32_phospho = True // inhibits PP1 → LTD pathway blocked
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DARPP32_phospho = True // silences PP1 — blocks LTD
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translocate(PKA → nucleus) → phosphorylate(CREB)
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CREB_active = True // enables structural gene expression
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// Target 3: enable structural gene expression
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translocate(PKA → nucleus)
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phosphorylate(CREB)
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CREB_active = True // new receptors, cytoskeleton, scaffolding
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```
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##### Acetylcholine: lower LTP threshold globally
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LTP_threshold *= (1 / (1 + ACh_level × mAChR_gain))
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## hierarchical filter — commit decision
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```Gen
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#### slow time scale — structural commit (h → weeks)
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function commit_to_structural_change():
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// All three layers must align
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##### Hierarchical filter: three conditions must align
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event_detected = post_Ca_amplitude > Ca_HIGH // layer 1: did something happen?
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overflow_sensed = mGluR5_activation == True // layer 2: was it excessive?
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context_validated = DARPP32_phospho and GluA1_Ser845_primed // layer 3: worth saving?
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##### ── Branch 1: LTP — potentiation ──────────────────────────────
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if event_detected and overflow_sensed and context_validated:
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activate(CaMKII) // Ca²⁺ signal now gets converted
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AMPA_count += receptor_insertion(CaMKII, GluA1_Ser845_primed)
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active_zone_size += structural_expansion(CREB_active)
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ECM_integrity += astrocyte_sealing(astro_Ca_local)
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return "potentiated"
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###### Postsynapse: anchor receptors, enlarge spine
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activate(CaMKII)
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AMPA_count += receptor_insertion(CaMKII, GluA1_Ser845_primed)
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spine_volume *= 1.5
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###### Presynapse: expand launchpad, increase output reliability
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active_zone_size *= 1.4 // more docking slots
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RRP_pool_capacity += pool_expansion(active_zone_size)
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VGCC_clustering += cluster_beneath_AZ() // tighter Ca²⁺ coupling
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vesicle_release_prob += 0.1 // driven by VGCC clustering
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###### Astrocyte: seal and insulate the channel
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perisynaptic_distance -= process_retraction() // walls move IN → tighter wrap
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ECM_integrity += secrete(Glypicans, Thrombospondins)
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D_serine_tonic_level += upregulate_synthesis() // sustained NMDA priming
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glutamate_clearance_rate *= 0.85 // tighter wrap → slower diffusion away
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return "potentiated"
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##### ── Branch 2: temporary only — Ca²⁺ rose, no save signal ─────
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elif event_detected and not context_validated:
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return "temporary facilitation only" // Ca²⁺ rose but no save signal
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AMPA_count += transient_insertion() // early-LTP only — reverses in minutes
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vesicle_release_prob += transient_facilitation()
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elif not event_detected and overflow_sensed:
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activate(PP1) // phosphatase wins — LTD
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AMPA_count -= receptor_internalization(PP1)
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return "depressed"
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###### No astrocyte structural change
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return "temporary facilitation only"
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##### ── Branch 3: LTD — active forgetting ─────────────────────────
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elif event_detected and not overflow_sensed and not context_validated:
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###### Postsynapse: internalize receptors, shrink spine
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activate(PP1)
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AMPA_count -= receptor_internalization(PP1)
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spine_volume *= 0.7
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###### Presynapse: dismantle launchpad
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active_zone_size -= docking_slot_removal()
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RRP_pool_capacity -= pool_contraction()
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VGCC_clustering -= scatter_VGCCs() // decouple Ca²⁺ from AZ
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vesicle_release_prob *= 0.6
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###### Astrocyte: dissolve matrix, pull away, cut support
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ECM_integrity -= secrete(MMPs) // molecular scissors
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D_serine_tonic_level = 0 // co-agonist supply cut
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perisynaptic_distance += process_extension() // walls move OUT → loose wrap
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glutamate_clearance_rate *= 1.2 // looser wrap → faster spillover
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return "depressed"
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##### ── Branch 4: baseline ────────────────────────────────────────
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else:
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return "baseline — no change"
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```
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## Conclusion
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###### All structural variables unchanged — system holds current state
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return "baseline — no change"
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##### special case — shockwave lockdown (>100Hz uncoordinated)
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function shockwave_lockdown():
|
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astro_Ca_global = GLOBAL_WAVE // soma-level flood
|
||||
release(GABA, ATP) // gel floods postsynapse
|
||||
AMPA_count -= mass_internalization()
|
||||
membrane_potential = HYPERPOLARIZED
|
||||
cluster(VGCC → beneath_active_zone) // ensures signal survives chaos
|
||||
|
||||
##### energy supply chain — metabolic gating (continuous)
|
||||
function metabolic_loop(Δt):
|
||||
###### Astrocyte: glucose → lactate pipeline
|
||||
glucose_uptake = blood_capillary_supply()
|
||||
lactate_output = glycolysis(glucose_uptake, glutamate_clearance_rate)
|
||||
lactate_output *= load_factor(glutamate_clearance_rate)
|
||||
|
||||
###### Pre + post absorb lactate → power their pumps
|
||||
RRP_pool refill rate ∝ VATPase(lactate_output)
|
||||
membrane_potential reset ∝ NaK_ATPase(lactate_output)
|
||||
|
||||
##### key asymmetry — perisynaptic distance is bidirectional
|
||||
// LTP: astrocyte moves IN → tighter diffusion barrier
|
||||
// → glutamate_clearance_rate ↓ (signal contained, not diluted)
|
||||
// → D_serine_tonic_level ↑ (NMDA gate chronically primed)
|
||||
|
||||
// LTD: astrocyte moves OUT → looser diffusion barrier
|
||||
// → glutamate_clearance_rate ↑ (signal bleeds away faster)
|
||||
// → D_serine_tonic_level = 0 (NMDA gate chronically starved)
|
||||
|
||||
// Result: astrocyte amplifies both directions simultaneously
|
||||
// potentiation becomes self-reinforcing; depression becomes self-reinforcing
|
||||
|
||||
The key architectural decision in this pseudocode is the separation into three explicit layers that feed into a single commit_to_structural_change function. Each layer answers one question independently before the final AND-gate runs — Ca²⁺ detects the event, mGluRs assess its magnitude, and cAMP/PKA validates its context. Notice also that mGluR layer has a push-pull side effect that feeds back into the Ca²⁺ layer (astro_Ca_local is updated by mGluR5_activation), making the system not a strict pipeline but a loop — the overflow sensor actively reshapes what the event recorder sees next.
|
||||
|
||||
## Neuromodulators
|
||||
|
||||
|
||||
Reference in New Issue
Block a user