no ecb messages

elements/neuron/appunti/2026-06-19-biological-reference-v10.md

@@ -274,3 +274,60 @@ The distinction matters biologically: a private reserve guarantees a component s
 (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.