ABSTRACT
Rod and cone photoreceptors have similar structures. The inner segment contains the nucleus, is rich in mitochondria and has an axon-like process connected to a synaptic terminal called a spherule in rods and a pedicle in cones. The resting potential of the photoreceptor plasma membrane in the dark is quite low, about –40 mV. Light produces a hyperpolarizing receptor potential, the amplitude of which is related to the light intensity. When light photons strike the outer segments a cascade of biochemical events is initiated which results in the closure of the cation channels, reducing the dark current, and hyperpolarizing the photoreceptor. Light adaptation, in which photoreceptors become less sensitive in the face of steady state light exposure, allows them to respond to levels of illumination that vary by as much as four orders of magnitude. The transduction process in rod cells is well understood. Rhodopsin is a member of the G protein receptor superfamily with seven transmembrane segments.
