Photon absorption triggers phototransduction, leading to the closing of which channels?

• A. cGMP-gated Na+ channels
• B. voltage-gated calcium channels
• C. potassium channels
• D. chloride channels

Answer: (A)

Phototransduction changes the chemical state inside the outer segment so that channels gated by a messenger molecule are closed. Specifically, light activates a cascade that lowers cGMP levels. Those cGMP-gated cation channels rely on cGMP to stay open, so when cGMP drops, they close. With fewer cations entering, the cell becomes more negative (hyperpolarizes), and neurotransmitter release diminishes. This direct sequence—light reducing cGMP and closing cGMP-gated Na+ (and Ca2+) channels—is what drives the photoresponse.

Voltage-gated calcium channels aren’t directly gated by light; they respond to membrane potential, so they’re affected downstream as the cell hyperpolarizes. Potassium and chloride channels aren’t the primary targets of the light-activated cascade in this context.