Figure 4. Ultra-fast Ca²⁺ spark recordings.

A, x-t line scan image recorded at 40,000 lines/s. B, Top: one pixel-wide (0.3 µm) ΔF/F₀ profile recorded from the center of the spark. For noise reduction data were averaged to 10,000 lines/s. The red trace represents a 5-point moving average. Bottom: d(ΔF/F₀)/dt (s⁻¹), first derivative of the ΔF/F₀ signal of the rising phase of the spark. Vertical dashed lines mark maxima of the d(ΔF/F₀)/dt signal identifying maxima of Ca²⁺ release flux. The discrete peaks of the d(ΔF/F₀)/dt signal were used to identify step-like increases of the ΔF/F₀ signal (marked by horizontal solid black lines). Grey bars indicate discrete d(ΔF/F₀)/dt levels. A discrete d(ΔF/F₀)/dt level was defined when at least two d(ΔF/F₀)/dt peaks of the same amplitude were observed.