快樂小藥師 Im pharmacist nichts glücklich

這個圖在解說威而剛為何會引起視力障礙時，應該會很有幫助的

Mechanism of phototransduction. The membrane potential of photoreceptors is depolarized in the dark due to sustained opening of cyclic guanosine monophosphate (cGMP)-dependent cation channels. The absorption of light by the visual pigment rhodopsin stimulates the G protein transducin to increase cGMP phosphodiesterase activity. cGMP is broken down to guanosine monophosphate (GMP), which causes cation channels to close and results in a hyperpolarizing receptor potential.