Figure 2. All-optical electrophysiology. A, Comparison of fluorescence signals recorded simultaneously from a GECI, GCaMP6f, and a GEVI, QuasAr2, expressed as a fusion construct in a rat hippocampal neuron. Subthreshold depolarizations, such as indicated by the arrow, do not have a correlate in the Ca2+ signal. B, Spatially resolved all-optical electrophysiology in a cultured rat hippocampal neuron. The blue region indicates the optically stimulated patch of dendrite. The action potential initiated in an unstimulated process and propagated back into the soma and into the dendritic arbor. Movie frames were calculated by sub-Nyquist interpolation of data acquired at a 1 s exposure time. Scale bar, 50 μm. Bottom right, Immunostaining of the same cell with anti-EGFP (EGFP; green) and anti-AnkyrinG (AnkG; magenta). Scale bar, 25 μm. Magenta arrows, Site of action potential initiation; distal end of the axon initial segment. C, Single-trial optical recordings of APs initiated by pulses of blue illumination (10 ms, 7.5 mW/cm2). Signal represents whole-soma fluorescence without photobleaching correction or background subtraction (modified from Hochbaum et al., 2014).