True or false: The amount of sodium channels closed in the initiation of phototransduction is directly proportional with how much the membrane potential changes.

True or false: The amount of sodium channels closed in the initiation of phototransduction is directly proportional with how much the membrane potential changes.

• A. True
• B. False
• C. Not enough data
• D. It varies with light intensity

Answer: (D)

Photoreceptors respond to light by closing cGMP-gated sodium channels, which stops the dark inward current and causes the cell to hyperpolarize. The key point is that how much the membrane potential shifts depends on how much light is present, because light drives the phototransduction cascade that lowers cGMP and closes channels. But the relationship between the number of channels that close and the resulting voltage change is not a simple one-to-one, linear proportion. The conductance change, the driving force for ions, and the cell’s baseline currents create a nonlinear response, and as light intensity increases the system approaches saturation where additional channel closures yield smaller incremental voltage changes. So the magnitude of the membrane potential change varies with light intensity, making that option the best choice.

Photoreceptors respond to light by closing cGMP-gated sodium channels, which stops the dark inward current and causes the cell to hyperpolarize. The key point is that how much the membrane potential shifts depends on how much light is present, because light drives the phototransduction cascade that lowers cGMP and closes channels. But the relationship between the number of channels that close and the resulting voltage change is not a simple one-to-one, linear proportion. The conductance change, the driving force for ions, and the cell’s baseline currents create a nonlinear response, and as light intensity increases the system approaches saturation where additional channel closures yield smaller incremental voltage changes. So the magnitude of the membrane potential change varies with light intensity, making that option the best choice.