Final answer:
When exposed to light, sodium channels in retinal rod and cone cells close, leading to hyperpolarization of the cell membrane, which is essential for visual signal transduction to the brain.
Step-by-step explanation:
When a rod or cone cell in the retina is exposed to light, the closing of sodium channels leads to hyperpolarization. This process is initiated by a shape change in the molecule retinal, which activates rhodopsin, and in turn starts a cascade of events involving a G-protein. This G-protein activates phosphodiesterase, which converts cGMP to GMP and results in the closure of sodium channels. The closure of these sodium channels causes the photoreceptor cell's membrane to become hyperpolarized, meaning it becomes more negatively charged than during the resting state.
In the absence of light, photoreceptor cells are slightly depolarized, releasing glutamate, but upon exposure to light, they become hyperpolarized and release less neurotransmitter, therefore not stimulating the bipolar cells. This change in the activity of photoreceptor cells is essential for transmitting visual signals to the brain.