Channelrhodopsin Photocycle: Four-State Gating Kinetics

Channelrhodopsin-2 is the light-gated ion channel at the heart of optogenetics: a single blue photon isomerizes its retinal chromophore and drives the protein through a multi-state photocycle, briefly opening a cation pore before it desensitizes and relaxes back to the dark-adapted state. This simulator renders a population of 400 independent channel molecules on a patch of membrane, each stochastically hopping between the dark-closed (C1), open (O1), desensitized-open (O2) and closed-desensitized (C2) states of the standard four-state ChR2 kinetic model, driven by a controllable light pulse. Live readouts track the whole-population open fraction and the resulting Ohmic photocurrent — including its sign flip as membrane voltage crosses the channel's reversal potential — while a scrolling trace shows the characteristic fast-rise, desensitizing-sag, biexponential-decay current shape recorded in real ChR2 electrophysiology.