Measurement of the dyad length. The lengths of the dyads, regions of close apposition between jSR membranes with either a t-tubule or surface membrane, were measured using Fiji (ImageJ). (A i) jSR adjacent to a t-tubule. Extended SR is also visible in this image (SR) and displays neither CSQ in its interior nor RyR2 on its surface. Scale bar = 50 nm. (B i) Yellow line displays the length of the jSR. (A ii and B ii) Sevenfold magnification of the regions in A i and B i.