Schematic of the new LMC pacemaker model, showing passive channels (rectangles) and active pumps (circles) crossing the cell membrane and the SR membrane. Currents are shown by blue arrows and calcium fluxes by black arrows. By convention, currents indicate the direction that positive charges, real or imaginary, are moving; thus, an outward current of negatively charged chloride ions is here shown as an inwardly directed current of fictitious positive charges, producing an increase in (trans)membrane potential (defined as internal relative to external). GPCR = G-protein-coupled receptor; see Eqs. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, and 19, and Table 1, for explanation of other symbols and abbreviations. Adapted from Fig. 2 of Hancock et al. (2022) with permission from Computers in Biology and Medicine.