The slice was loaded with the fluorescent voltage-sensitive dye JPW3028 (A). Full frame (80 x 80 pixels (B)) recording at 2 KHz frame rate (D) and partial readout (80 x 12 pixels (C)) recording at 10 KHz frame rate (E). The signal is undersampled at 2 KHz because the upstroke of the spike is shorter than 0.5 ms at 35 C.