From 91851fd2d47b6092e6686148ed9f6cc3417c86a0 Mon Sep 17 00:00:00 2001 From: ocrampal Date: Sat, 6 Jun 2026 09:58:24 +0200 Subject: [PATCH] Update elements/astrocyte/appunti/2026-06-04-modulation-of-future-behavior.md Calcium --- ...026-06-04-modulation-of-future-behavior.md | 20 +++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/elements/astrocyte/appunti/2026-06-04-modulation-of-future-behavior.md b/elements/astrocyte/appunti/2026-06-04-modulation-of-future-behavior.md index 052d3fd..736fedb 100644 --- a/elements/astrocyte/appunti/2026-06-04-modulation-of-future-behavior.md +++ b/elements/astrocyte/appunti/2026-06-04-modulation-of-future-behavior.md @@ -358,3 +358,23 @@ The second is a **context validation signal** that asks whether the event was be The third is a **protein synthesis requirement** that enforces a time cost on permanent storage — in all organisms where long-term behavioral change has been studied, blocking protein synthesis blocks consolidation, suggesting that the metabolic cost of building new structural proteins is a universal gate that prevents trivial events from consuming permanent resources. Sleep and hippocampal replay are therefore best understood not as the mechanism of consolidation but as a **vertebrate solution to the scaling problem** — how to consolidate thousands of experiences per day across billions of synapses without running the protein synthesis machinery continuously at full cost during waking. Simpler organisms consolidate fewer experiences, face less interference, and can afford to let repetition and spacing do the work that sleep does in more complex nervous systems. + +# Calcium in pre, post, astro + +**In the postsynapse**, the calcium amplitude and rise-speed filter works because CaMKII and the phosphatases PP1/PP2B have different sensitivities to calcium-bound calmodulin, and calmodulin itself has different binding kinetics depending on how fast calcium rises. + +Calmodulin has four calcium binding sites and its activation is highly cooperative — it does not activate linearly with calcium concentration but switches sharply above a threshold. When calcium rises fast and high, as during a strong high-frequency burst, calmodulin saturates quickly and activates CaMKII. CaMKII then autophosphorylates at Thr286, which is the critical step — once autophosphorylated it remains active even after calcium falls back to baseline, effectively converting a transient calcium event into a sustained kinase signal that outlasts the trigger. This persistence is what gives CaMKII its memory-like property and is what drives AMPA receptor insertion. + +When calcium rises slowly and to a lower amplitude, as during weak low-frequency input, calmodulin activates preferentially the phosphatases PP2B (calcineurin) and downstream PP1 instead, because these enzymes have higher affinity for calcium-calmodulin complexes at lower occupancy. PP1 then dephosphorylates AMPA receptors, triggering their internalization and driving LTD. + +So the filter is not a simple threshold — it is a **kinetic competition** between two enzyme systems with different calcium-calmodulin affinities. Fast large rise activates the low-affinity high-gain system (CaMKII). Slow small rise activates the high-affinity low-gain system (PP2B/PP1). The same calcium messenger routes to opposite outcomes depending purely on its dynamics. + +**In the presynapse**, the calcium filter is structurally simpler but operates on a different principle — **proximity and timing** rather than kinetic competition. Calcium enters through VGCCs clustered directly beneath the active zone, and the vesicles docked at that zone sit within nanometers of the channel mouth. The local calcium concentration at the release site reaches extremely high values — estimated at hundreds of micromolar — for a very brief window of microseconds before diffusing away. Synaptotagmin, the calcium sensor on the vesicle membrane, has a low affinity but fast on-rate, meaning it only fires in response to this extremely high local transient, not to the diffuse residual calcium that lingers afterward. + +The residual calcium that accumulates with repeated spikes — the pre_Ca_residual in the pseudocode — acts on a completely different target: Munc13 and RIM proteins at the active zone, which have higher affinity for calcium but slower kinetics. These proteins respond to the sustained low-level residual and increase the size of the readily-releasable pool and the probability of release — this is facilitation. So the presynaptic filter distinguishes between the sharp local transient (triggers release via synaptotagmin) and the slow diffuse residual (modulates future release probability via Munc13/RIM). Two calcium signals, two sensors, two time scales, within the same compartment. + +**In the astrocyte**, the calcium filter is the least understood of the three but operates through IP3 receptor gating. IP3 receptors on the endoplasmic reticulum have a bell-shaped calcium dependence — they open in response to rising calcium but are inhibited at very high calcium concentrations. This means the astrocyte's internal calcium release is self-limiting: a moderate IP3 signal produces a local calcium rise that drives D-serine release, but an excessive signal triggers the global soma wave that activates the circuit-breaker response instead. + +The key filter here is therefore the **spatial containment of the IP3 signal**. Under normal high-frequency activity, IP3 production is local to the perisynaptic process and the calcium rise stays local — driving D-serine release proportionally. Only when multiple neighboring synapses fire simultaneously does IP3 accumulate enough to propagate as a regenerative wave across the entire astrocyte via gap junctions to adjacent astrocytes, triggering the global alarm. The astrocyte is therefore filtering not just amplitude but **spatial coherence** — a single strong synapse produces a local response, but coordinated overactivity across a territory produces a qualitatively different global response. + +The common thread across all three compartments is that none of them use a simple concentration threshold. Each uses a molecular mechanism that is sensitive to the **dynamics** of the calcium signal — its speed, its spatial spread, its duration, or its subcellular location — rather than just its peak value. This is what allows the same ion to encode fundamentally different instructions depending on the context in which it arrives.