In the dark state, which description applies to photoreceptors?

In the dark state, which description applies to photoreceptors?

• A. cGMP decreases; Na+ channels close.
• B. cGMP is hydrolyzed; Na+ channels close.
• C. The photopigment remains in an inactive state.
• D. cGMP is synthesized continuously, Na+ channels are open, and glutamate is released.

Answer: (D)

In darkness, photoreceptors stay depolarized because the level of cGMP remains high, which keeps the cGMP-gated Na+ channels open. This ongoing Na+ (and Ca2+) influx creates the dark current, so the cell is tonically depolarized and continuously releases glutamate onto bipolar cells. The photopigment is in its inactive state in the dark, and there’s no activation of the phototransduction cascade to lower cGMP. So the best description is: continuous synthesis of cGMP, open Na+ channels, and ongoing glutamate release.

In darkness, photoreceptors stay depolarized because the level of cGMP remains high, which keeps the cGMP-gated Na+ channels open. This ongoing Na+ (and Ca2+) influx creates the dark current, so the cell is tonically depolarized and continuously releases glutamate onto bipolar cells. The photopigment is in its inactive state in the dark, and there’s no activation of the phototransduction cascade to lower cGMP. So the best description is: continuous synthesis of cGMP, open Na+ channels, and ongoing glutamate release.