Fig. 1.

Biophysical properties of channelrhodopsin2 (ChR2). A: ChR2 is a light- and voltage-dependent ion channel; all-trans-retinal, intracellularly available and covalently bound to ChR2, acts like a chromophore (sensing photons) to facilitate ChR2 opening and the transport of cations with differential preference from H⁺ to Ca²⁺. B: ChR2 produces voltage- and light-dependent current. The resultant current is predominantly inward/excitatory with a fast peak and sustained component; ChR2 exhibits strong inward rectification with a reversal potential around 0 mV [current-voltage relationship for the sustained component shown from Jia et al. (64)]; shown is also a current-irradiance relationship and selected traces for ChR2 current under different voltage clamps (top) and irradiance levels (bottom). V_m, membrane potential.