Volume 145, No. 3, March 2, 2015. Pages 173–184.In the original online early release version of this article, the text contained the following three errors. In the last sentence of the section Protein expression silencing in Materials and methods, Fig. S2 should have been cited instead of Fig. S1. In the legend to Fig. 4, the second to last sentence should have said “bin widths of 0.1 sarcomere−1 s−1” instead of “0.05 sarcomere−1 s−1.” And in the last sentence of that legend, it should have said “0.05 sarcomere−1 s−1” instead of “0.025 sarcomere−1 s−1.” The HTML, PDF, and print versions now appear correctly. For reference, the corrected sentences follow:The second control was designed to block the Tbx5 protein, which is important in vertebrate limb development (Tamura et al., 1999), allowing us to easily test the efficiency and specificity of the injection by verifying the absence of pectoral fins in morphant larvae (see Fig. S2) and the unaffected presence of PJF.For frequencies above 0, the symbols are plotted at the center frequency of bin widths of 0.1 sarcomere−1 s−1. In the case of the morphant cells, sparks were sufficiently rare that the mean value of the frequencies in the first bin is 0.0065 ± 0.0028 sarcomere−1 s−1 (n = 8; ±SEM), considerably less than the position of the central frequency (0.05 sarcomere−1 s−1).