Fig. 5

Limit threshold ER radius as function of RyR density. The ER radius r above which stable calcium waves can be elicited for arbitrarily large dendrites (for a given RyR channel density ρ, cf. Fig. 4) can be fitted by a function of the form \(r(\rho ) = \frac{ac+b\rho }{\rho - c}\). The parameter values \(a = 0.377 \ \upmu \mathrm{m}\), \(b = 0.0115 \ \upmu \mathrm{m}\) and \(c = 0.637\ \upmu \mathrm{m}^{-2}\) have been determined using a least squares fitting. The value of c indicates that such a limit ER radius exists only for RyR densities larger than approximately 0.64 \(\upmu \mathrm{m}^{-2}\). This is due to calcium buffering in the cytosol: Simulations with the cytosolic buffer concentration reduced by 90% result in a fitting curve with \(c = 0.007\ \upmu \mathrm{m}^{-2}\). Neurons could therefore modify RyR densities according to intracellular buffering properties to then maintain a minimal ER “footprint” for stable calcium waves