Figure 2.

An example (diffusion computation) illustrating the effect of Ca²⁺ indicators on Ca²⁺ signals inside a small cellular compartment and the relationship between fluorescence recordings and the underlying kinetics of free Ca²⁺. (A) Diagram depicting the modeled Ca²⁺ store (yellow) with Ca²⁺ channels (nine 50-nm channel clusters depicted by dots) facing the cell cytoplasm (~1-μm wide compartment). (B) Simulated Ca²⁺ entry time course; the total entry corresponds to ~10⁴ Ca²⁺ ions (which is approximately twice the spike-evoked presynaptic Ca²⁺ influx in small central synapses [Koester and Sakmann 2000], consistent with the estimate for presynaptic Ca²⁺ store release reported earlier [Scott, Lalic, and others 2008]). (C) Concentration profiles at five time points during Ca²⁺ entry. Top row: free Ca²⁺ landscapes, with no Ca²⁺ buffers present, shown within the 10-nm layer adjacent to the Ca²⁺ store membrane. Middle row: same as top row but in the presence of 0.2 mM Fluo-4 (typical imaging protocol). Bottom row: concentration profile of Ca²⁺-bound Fluo-4 averaged over a 100-nm depth adjacent to the Ca²⁺ store interface, [CaF]_Σ, to reflect recorded fluorescence. False color scale bars as shown. Models adapted and modified from Scott, Lalic, and others (2008) and Henneberger and others (2010) were constructed and run online using Virtual Cell (VCell) version 4.7 at the National Resource for Analysis and Modeling, National Institutes of Health, Connecticut.