Experimental strategy. The purpose of our study was to elicit light-induced APs not by coexpression of wild type ChR2 with voltage-gated Na⁺ channels, but by coupling ChR2 either directly to Na_v1.5 (ChR2-Na_v1.5, Na_v1.5-ChR2) or to the accessory β1 subunit (β1-ChR2, β1-ChR2-ChR2). The seven transmembrane regions of ChR2 (blue), the four large domains of voltage-gated Na⁺ channels (red), the β1 subunit (orange), and the membrane-spanning region of the human H⁺/K⁺ATPase (gray) are illustrated. In Na_v1.5-ChR2, β1-ChR2, and β1-ChR2-ChR2, peptide [GGGS]₃ was incorporated into all linker regions (symbolized by a black line). For detailed amino acid composition of the constructs, see Materials and methods.