What occurs in the light state of phototransduction?

What occurs in the light state of phototransduction?

• A. cGMP is synthesized, Na+ channels open, and glutamate is released.
• B. cGMP is hydrolyzed, Na+ channels close, leading to hyperpolarization and decreased glutamate release.
• C. cGMP increases, channels remain open, depolarization continues.
• D. Photopigment is destroyed, no signal is transduced.

Answer: (B)

In the light state, phototransduction shifts from a depolarized, transmitter-releasing mode to a hyperpolarized, reduced-release mode. Light-activated photopigment (rhodopsin) triggers a G protein cascade that activates a cGMP phosphodiesterase. This enzyme hydrolyzes cGMP, lowering its intracellular concentration. With less cGMP, the cGMP-gated Na+/Ca2+ channels close, cutting the inward current and hyperpolarizing the photoreceptor. The membrane becomes more negative, reducing Ca2+ entry through voltage-gated channels and thereby decreasing glutamate release onto downstream neurons. In darkness, cGMP is high and channels stay open, keeping the cell depolarized and continuously releasing glutamate. The photopigment isn’t destroyed in this process; it’s activated and then regenerated. So the light state is driven by cGMP hydrolysis, channel closure, hyperpolarization, and decreased glutamate release.

In the light state, phototransduction shifts from a depolarized, transmitter-releasing mode to a hyperpolarized, reduced-release mode. Light-activated photopigment (rhodopsin) triggers a G protein cascade that activates a cGMP phosphodiesterase. This enzyme hydrolyzes cGMP, lowering its intracellular concentration. With less cGMP, the cGMP-gated Na+/Ca2+ channels close, cutting the inward current and hyperpolarizing the photoreceptor. The membrane becomes more negative, reducing Ca2+ entry through voltage-gated channels and thereby decreasing glutamate release onto downstream neurons. In darkness, cGMP is high and channels stay open, keeping the cell depolarized and continuously releasing glutamate. The photopigment isn’t destroyed in this process; it’s activated and then regenerated. So the light state is driven by cGMP hydrolysis, channel closure, hyperpolarization, and decreased glutamate release.