What happens during the phototransduction cascade in light?

• A. Light increases cyclic GMP and opens Na+ channels
• B. Light activates rhodopsin, decreasing cyclic GMP, closing Na+ channels, and stopping neurotransmitter release
• C. Light causes hyperpolarization of photoreceptors without ion channel changes
• D. Light has no effect on phototransduction

Answer: (B)

The key idea is that light triggers a cascade that lowers the chemical that keeps the photoreceptor’s channels open, leading to hyperpolarization and reduced neurotransmitter release. Light activates rhodopsin, which then activates a G protein (transducin). This powers phosphodiesterase to break down cGMP. With less cGMP, the cyclic nucleotide–gated Na+/Ca2+ channels close. Less positive influx means the cell becomes more negative (hyperpolarizes), so it releases less neurotransmitter (glutamate) onto bipolar cells. In the dark, cGMP remains high, channels stay open, the cell depolarizes and releases more neurotransmitter. This sequence is why the correct description is that light activates rhodopsin, lowers cGMP, closes Na+ channels, and reduces neurotransmitter release.