Fig. 5.

Theoretical reaction/diffusion model of Ca²⁺ at the IHC AZ. a Theoretical model of Ca²⁺ influx at a 430 nm × 67 nm cluster of 120 Ca²⁺ channels (blue symbols, width of 10 nm) shows that the local increase in free [Ca²⁺] near the channel mouth, before the Ca²⁺ ions bind to the Ca²⁺-indicator or the non-fluorescent buffers, can reach values as high as 150 µM. Blue symbols indicate the positions of simulated Ca²⁺ channels. Inset shows a magnification of the area marked by the green square. Scale bar: 100 nm. b [Ca²⁺], as calculated from the simulated distribution of OGB-5N at the synapse (“reported [Ca²⁺]”), reaches peak values of 45 µM. The lateral diffusion of Ca²⁺ ions and buffers makes it impossible to acquire the [Ca²⁺] at the channel mouth and results in an elongated Ca²⁺ domain. Numbers indicate the [Ca²⁺] at the contour lines. c Increasing the simulated [OGB-5N] (here to 400 µM) results in a lower reported peak [Ca²⁺] of ~40 µM. d Increasing the simulated [EGTA] (here to 2 mM) results in a reported peak [Ca²⁺] of ~44 µM. e Same data as in b, but additionally convolved with a Gaussian PSF with a FWHM of 64 × 64 × 542 nm, mimicking 2D-STED imaging. The reported [Ca²⁺] after convolution reaches peak values of up to 10 µM, still considerably lower than the actual concentration near the channels