This is a statistically significant difference (P?=?0.001). Such an area lacking SR would be a barrier to propagation. Figure 5A shows that the side-to-side junction between cells is narrower than the intercalated disc observed in end-to-end junctions. Staining is quantified in Fig.?5B from the regions indicated by rectangles in Fig.?5A. Again, the membrane dye shows a narrower staining than the E