Fig. 7. Schematic of light- and dark-adapted photoreceptor cell. Levels of cGMP are high in the dark and the sodium channels are open, depolarizing the cell. In the light, the cGMP levels decrease rapidly, causing the cell to hyperpolarize. (Courtesy of Richard A. Alvarez). (Basic and Clinical Science Course, Section 1: Fundamentals and Principles of Ophthalmology. San Francisco: American Academy of Ophthalmology, 1990:188)