Which of the following is true of photoreceptors in the light (relative to the dark)?
A) The photoreceptors are depolarized.
B) Sodium channels are open.
C) The synaptic terminal is releasing more transmitter.
D) Phosphodiesterase is more active.
E) cGMP levels are elevated.

Question

Quizplus · Accepted Answer

In the light, photoreceptors (rods and cones) hyperpolarize due to the closure of sodium channels, which is opposite to depolarization. This process is mediated by a decrease in cGMP levels, caused by the activation of phosphodiesterase (PDE). PDE breaks down cGMP, leading to the closure of sodium channels, thus reducing the release of neurotransmitter from the synaptic terminal. Therefore, in the light, phosphodiesterase activity is indeed more active, making D the correct choice.

Which of the Following Is True of Photoreceptors in the Light