Action potentials actively propagating along dendrites are responsible for calcium concentration transients detected by OGB-1 dye.

<p><b><i>A</i></b>. All detected Ca_i²⁺ increases coincided with action potentials. In this trace three action potentials correspond to three steps of OGB-1 signal increases. <b><i>B</i></b>. During current injection all 7 action potentials caused OGB-1 fluorescence intensity increases. <b><i>C</i></b>. The first action potential from B is shown on faster time scale with a corresponding OGB-1 fluorescence signal. It is clear that all increase in OGB-1 fluorescence occurred within few milliseconds of the action potential occurrence. The fluorescence sampling rate was 440 Hz. The vertical bar serves as a scale for both the voltage (20 mV) relative fluorescence change, dF/F, 5%. <b><i>D E</i></b>. Calcium influx was similar during action potentials evoked by a voltage step a visual stimulus. In <b><i>D</i></b> current traces evoked by a visual stimulus (a ~20° wide black circle in the center of the receptive field) during voltage steps (a 40 mV, 20 ms long voltage step, bottom black traces) are shown in slow in fast time scales (left right panels correspondingly). Leak currents capacitance charging transients were subtracted for clarity. In <b><i>E</i></b> fluorescence traces for both conditions are aligned at the onset of the responses. Grey filled circles correspond to visually evoked stimulus while black filled circles correspond to voltage step comm. In addition, a fluorescence trace corresponding to OFF visual stimulus is shown as a dark grey line. Although no membrane currents are shown for this stimulation, two action potentials were also evoked during this OFF stimulus albeit with larger interval that explains slightly slower rate of rise of the fluorescence signal. For all three conditions the fluorescence measurements were taken in the same dendrite section at about 100 μm from the soma. <b><i>F G</i></b>. OGB-1 fluorescence increase could be detected up to >300 μm from the soma in the area of fine dendritic branches near the edge of the optic tectum. In all three locations the onset of the OGB-1 signal increase was nearly simultaneous, probably limited by the fluorescence signal sampling rate of 100 Hz. A vertical thick grey broken line in <b><i>G</i></b> denotes the onset of the inward synaptic currents while the horizontal thin black broken line denotes the 0 pA baseline for the current trace.