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Why do sodium channels close in vertebrate photoreceptors exposed to light?
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Photoreception
The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.
In vertebrate photoreceptors, sodium channels close in response to light due to a decrease in the concentration of the second messenger cGMP (3β²5β²-cyclic guanosine monophosphate).1
The process functions as follows:
- Dark Conditions: In the absence of light, cGMP binds to sodium channels in the cell membrane, keeping them open and allowing a continuous influx of sodium ions that maintains the cell in a depolarized state.1
- Light Exposure: When light is absorbed, rhodopsin molecules become excited and activate a G-protein called transducin. Transducin then activates an enzyme known as phosphodiesterase, which cleaves cGMP into 5β²-GMP.1
- Channel Closure: As cGMP levels fall, the molecule no longer binds to the sodium channels, causing them to close. This reduction in the influx of positive sodium ions leads to the hyperpolarization of the cell.1
This hyperpolarization inhibits the release of the neurotransmitter glutamate, effectively signaling the presence of light to the rest of the retina.1