i_K i_Ca i_S tau 1 V_m V theta_m -1 g_Ca m_infinity V V_Ca g_Ca partialdiff m_infinity partialdiff unknown element V V_Ca g_Ca m_infinity V_Ca V_K V V_K delta n_infinity n tau delta partialdiff n_infinity partialdiff unknown element delta 1 V_n V theta_n -1 g_K n V V_K g_K V V_K S_infinity S tau_S partialdiff S_infinity partialdiff unknown element 1 V_S V theta_S -1 g_S S V V_K g_S k_S V V_K k_S tau tau_S unknown element