In light, are ON-center bipolar cells depolarized or hyperpolarized?

ON-center bipolar cells are depolarized in the presence of light due to the mechanism of the phototransduction pathway. When light hits the photoreceptors, specifically the rods and cones, it leads to the hyperpolarization of these cells. Since ON-center bipolar cells are connected to photoreceptors that release glutamate, the change in the release of this neurotransmitter is crucial.

In darkness, photoreceptors release high levels of glutamate, which inhibits ON-center bipolar cells through metabotropic glutamate receptors, causing them to be hyperpolarized. However, when light is present, the photoreceptors hyperpolarize and reduce their glutamate release. This decrease in inhibition allows the ON-center bipolar cells to depolarize and, in turn, signal to the ganglion cells that there is an increase in light intensity.

Thus, the response of ON-center bipolar cells is a direct consequence of the reduced glutamate release, leading to depolarization in light, facilitating the overall signal processing of visual information. This mechanism is essential for the accurate perception of brightness and contrast in the visual field.