FIGURE 4.

Primary human sinoatrial node (SAN) and atrial cardiomyocyte and sinoatrial node models. Simulated action potentials [transmembrane voltage (V_M)] and Ca²⁺ transients ([Ca²⁺]_i), model structure, simulated ion currents and fluxes, and special features are highlighted. Traces were simulated using the human SAN cell models from Fabbri 2017 (120), as well as models from Bai 2018 (121), Grandi 2011 (90), Koivumäki 2011 (121,122) Courtemanche 1998 (45), and Nygren 1998 (44, 118). Models were obtained from https://www.cellml.org/ or implemented based on the model equations. AP, action potential; APD, action potential duration; CYT, cytosol; I_Cab, background Ca²⁺ current; I_Ca,L, L-type Ca²⁺ current; I_Ca,T, T-type Ca²⁺ current; I_Clb, background Cl⁻ current; I_ClCa, Ca²⁺-dependent Cl⁻ current; I_f, hyperpolarization-activated cyclic nucleotide-gated “funny” current; I_K1, basal inward-rectifier K⁺ current; I_K,ACh, acetylcholine-activated inward-rectifier K⁺ current; I_Kp, plateau K⁺ current; I_Kr, rapid delayed-rectifier K⁺ current; I_Ks, slow delayed-rectifier K⁺ current; I_Kur, ultrarapid delayed-rectifier K⁺ current; I_Na, Na⁺ current; I_Nab, background Na⁺ current; I_NaK, Na⁺-K⁺-ATPase current; I_NCX, Na⁺/Ca²⁺ exchange current; I_pCa, plasmalemmal Ca²⁺-ATPase current; I_sus, sustained K⁺ current; I_to, transient outward K⁺ current; J_leak, Ca²⁺ leak from sarcoplasmic reticulum; J_rel, Ca²⁺-release flux from the sarcoplasmic reticulum; JSR, junctional sarcoplasmic reticulum; J_up, Ca²⁺ uptake flux into the sarcoplasmic reticulum; NSR, network sarcoplasmic reticulum; SR, sarcoplasmic reticulum; SS, subspace Ca²⁺ domain.