Upper panel: Sketch illustrating the distribution of cardiac connexins Cx43, Cx40, and Cx45. Cx43 is the dominating connexin in the working myocardium of the ventricles, which in addition host small amounts of Cx45. Both left and right atria co-express Cx40, Cx43, and Cx45. The cells of the Purkinje system express Cx40. Cx45 dominates in both the sino-atrial and the atrio-ventricular nodes. Lower panel: Schematic presentation of the cardiac cell junction (ICD, intercalated disk). It includes four main components: gap junction channels, ion channels, desmosomes, and adherens junctions. Adherens junctions and desmosomes are situated at the sites perpendicular to the long axis of the cell, whereas gap junctions are preferentially located at sites parallel to the long axis. In mammalian hearts, adherens junctions and desmosomes can form combined junctions, termed area composita. N-cad: N-cadherin; PKG: plakoglobin (γ-catenin); PKP2: plakophilin-2; αE: αE-catenin; αT: αT-catenin; β-cat: β-catenin; DSC2: desmocollin-2; DSG2: desmoglein-2; DSP: desmoplakin; DSM: desmin (intermediate filament).