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@@ -71,8 +71,20 @@ This is where your ATP loop and Ca2+ loop intersect. If the firing frequency is
3. Cumulative CDI: As residual $Ca^{2+}$ builds up in the terminal ($Tr\_Ca$ in your model), the CaM sensors on the VGCCs stay partially "primed" or occupied.The
1. Effect: This means when the next action potential arrives, the channels are already in a semi-inactivated state. Fewer channels are "available" to open, and those that do open close faster.
2. Timescale: This operates on the scale of hundreds of milliseconds to seconds, effectively mapping the decay curve of your calcium clearance pumps.
4. Modulation by the "State" of the ChannelIn the minutes and beyond category, the accumulation of $Ca^{2+}$ changes the structural landscape of the VGCCs through two medium-term signals:A. The "Clogged" Channel Signal (Minutes)If $Ca^{2+}$ accumulation is high enough to keep CDI active for a prolonged period (as in your "self-imposed silence" scenario), the channel spends too much time in the inactivated state.Ubiquitination: Inactivated channels are more susceptible to being tagged by E3 ubiquitin ligases (like Nedd4-1).Elimination: Once tagged, they are endocytosed (removed from the membrane). This is a medium-term "down-scaling" to prevent excitotoxicity.B. The Calcineurin Pathway (Minutes to Hours)Accumulated $Ca^{2+}$ activates Calcineurin (PP2B), a phosphatase.Calcineurin dephosphorylates the VGCCs and their anchoring proteins (like RIM).This physically "loosens" the channels from the Active Zone. They drift away from the release sites, meaning even if they do open, they are too far away from the vesicles to trigger release.Modeling Summary for your LoopsIf you are building this into your simulation, the Availability ($A$) of VGCCs can be modeled as a function of both the instantaneous spike and the integrated trace:$$A = (1 - CDI_{fast}) \times (1 - f(Tr\_Ca))$$Short term: $CDI_{fast}$ resets (mostly) between spikes if pumps are healthy.Medium term: $f(Tr\_Ca)$ grows as ATP drops, locking the "Availability" to near zero.Long term: If $f(Tr\_Ca)$ stays high for $>X$ minutes, trigger a decrement in the $Total\_VGCC\_Count$ (structural elimination).
4. Modulation by the "State" of the Channel
In the minutes and beyond category, the accumulation of $Ca^{2+}$ changes the structural landscape of the VGCCs through two medium-term signals:
1. A. The "Clogged" Channel Signal (Minutes)If $Ca^{2+}$ accumulation is high enough to keep CDI active for a prolonged period (as in your "self-imposed silence" scenario), the channel spends too much time in the inactivated state.
1. Ubiquitination: Inactivated channels are more susceptible to being tagged by E3 ubiquitin ligases (like Nedd4-1).
2. Elimination: Once tagged, they are endocytosed (removed from the membrane). This is a medium-term "down-scaling" to prevent excitotoxicity.
2. B. The Calcineurin Pathway (Minutes to Hours)
Accumulated $Ca^{2+}$ activates
1. Calcineurin (PP2B), a phosphatase.Calcineurin dephosphorylates the VGCCs and their anchoring proteins (like RIM).
2. This physically "loosens" the channels from the Active Zone. They drift away from the release sites, meaning even if they do open, they are too far away from the vesicles to trigger release.
1. Modeling Summary for your Loops
If you are building this into your simulation, the Availability ($A$) of VGCCs can be modeled as a function of both the instantaneous spike and the integrated trace:$$A = (1 - CDI_{fast}) \times (1 - f(Tr\_Ca))$$
1. Short term: $CDI_{fast}$ resets (mostly) between spikes if pumps are healthy.
2. Medium term: $f(Tr\_Ca)$ grows as ATP drops, locking the "Availability" to near zero.
3. Long term: If $f(Tr\_Ca)$ stays high for $>X$ minutes, trigger a decrement in the $Total\_VGCC\_Count$ (structural elimination).
**Long time scale**
In the minutes-to-hours range, the presynapse shifts from "gating" (turning existing channels on/off) to remodeling (changing the physical number of channels). This process is governed by a shift from purely electrical signals to biochemical "state" signals.The primary signal that dictates the density of VGCCs at the terminal is the history of the $Ca^{2+}$ trace, specifically mediated through three core molecular pathways: