Figure 2. Modelling ChR2 properties and delivery modes at the cell level.

(a) Four-state Markov model for I_ChR2 with dark- and light-adapted photocycle branches associated with different peak conductances; each branch comprises closed- (red) and open-channel (green) states. Channel opening rates (blue arrows) are directly proportional to the irradiance (E_e) of light absorbed by ChR2 and open channels approach an E_e-dependent equilibrium between open states O₁ and O₂; all other transitions are purely time-dependent. (b) I_ChR2 in response to illumination (pale blue background; top to bottom: E_e = 0.4, 1, 2, 4, and 10mW mm⁻²) with V_m clamped to —85.6mV. Photoevoked ChR2 current has well-defined transient and steady-state phases resulting from the transition from full dark adaptation to an equilibrium between the two operating modes. The I_ChR2 model qualitatively reproduces experimental records from whole-cell patch-clamp recordings in a stable HEK-ChR2 cell line²¹, as shown in Supplementary Fig. S1. (c&d) Schematics for modelling gene delivery (GD) and cell delivery (CD) of ChR2; generic ionic currents are shown in myocytes. (e) Optically- (blue) and electrically-evoked (red) action potentials (APs) and underlying I_ChR2 in a myocyte with gene-delivered ChR2. Illumination at 2× threshold (E_e = 0.468mW mm⁻² over 10ms) elicited depolarising I_ChR2 (bottom), which triggered an optically-evoked AP. (f) Response to illumination in an inexcitable ChR2-rich donor cell (CD; dashed blue) coupled to a normal myocyte by a 500 MΩ resistance (solid blue); a photoevoked AP in the latter cell is compared to an electrically-evoked AP (red). Light delivered to the donor cell was at 2× threshold for optically eliciting an AP in the myocyte (E_e = 0.652mW mm^_2 over 10ms). During the AP plateau phase, myocyte V_m (≈ 2.77mV) was between the plateau V_m of the electrically-induced AP (≈ 15.3mV) and the donor cell resting level (−40mV).