Fig. 2

a Chemical structure of the voltage-sensitive merocyanine–rhodanine dye that we have been using (NK2761). b Optical recordings of spontaneous electrical activities from the pre-fused right and left cardiac primordia of a 6-somite chick embryo using 10 × 10-element photodiode array. The preparation was stained with NK2761, and each trace shows changes in transmission detected by one element at 37.1 °C. A 700 ± 11 nm interference filter was used. Nine traces obtained from the left primordium exhibit spike-signals which we attribute to spontaneous action potentials (indicated by arrowheads). Three traces showed very tiny signals (asterisks). Note that the signals appeared rhythmically and were nearly synchronized among the different positions where the signals were detected. No signals were detected from the right primordium. c Second representation of spontaneous electrical activity recorded simultaneously from many adjacent separate regions of primordial area in 6-somite embryo shown in a. Traces are arranged so that relative positions correspond to relative positions of regions of the heart imaged onto the detector. Here, very small but distinct optical signals were detected by 13 elements of the photodiode array (filled triangles). In some positions, output from amplifiers overflowed because of large fluctuation of baselines (open triangles). From examination of other portions of the recording, we think that no signals appeared in these areas. d Sketch of the 6-somite chick embryo. The square overlaid on the sketch is schematically indicating the size of the photodiode array relative to the size of the image of the cardiac primordial area, and shows the relative position of the photodiode array. The arrows with “Cephalic” and “Left” indicate the directions in the sketch. Modified from Ref. [11]