Figure 4.

Ionic model of genetically engineered light sensitive cardiac cell. The electrochemical state of the cell is characterized in terms of n_ion = 8 ion concentrations, c_ion = [c^e_Na, c^e_K, c^e_Ca, cⁱ_Na, cⁱ_K, cⁱ_Ca, c^up_Ca, c^rel_Ca], the extracellular and intracellular sodium, potassium, and calcium concentrations, and the sarcoplasmic reticulum calcium uptake and release. Ion concentrations are controlled through n_crt = 12 ionic currents, I_crt = [I_Na, I_bNa, I_f, I_NaCa, I_NaK, I_ChR2, I_K, I_bK, I_CaL, I_CaT, I_up, I_rel], where the baseline autorhythmic cell model (26) has been enhanced with the channelrhodopsin current I_ChR2 (shown in blue). The channels are governed by n_gate = 10 gating variables g_gate = [g_m, g_h, g_ChR2, g_y, g_x, g_dL, g_fL, g_fCa, g_dT, g_fT], which may be functions of the current membrane potential ϕ.