Figure 1.

ChR2's amplitude response function. (A) Illustration of the three-state Markov channel model described by Equations 1–3. The transition rates between open, O, desensitized, D, and closed, C, states are ϵϕ(t), Γ_r, and Γ_d, respectively. (B) Amplitude response functions |F_ChR2(2πf)| for the model are shown for three ChR2 variants using different mean illumination intensities (0.15–0.6 mW·mm⁻²) and parameters in Table 1. (C) Voltage dependence of ChR2's amplitude response function. ChR2 and ChR2_R both have a voltage-dependent desensitization rate, Γ_d(v), which results in decreased bandwidth as the membrane potential increases. ChR2_A does not have a voltage dependent desensitization rate and therefore has a stable bandwidth across membrane potentials. (D) Predicted amplitude response of each ChR2 variant compared to the experimentally measured response for a mean illumination intensity of 0.35 mW·mm⁻². Error bars are ±1 STD.