explain light adaptation
light adaptation reverses the changes in the retina that result from dark adaptation.
When you step out into bright light from a movie theater, initially the cones are hyperpolarized as much as possible (toward the equilibrium potential for K). If the cones stayed in this state, we would be unable to see changes in light level. as we discussed above, the constriction of the pupil helps a bit in reducing the light entering the eye. However, the most important change is a gradual depolarization of the membrane back to about -35 mV.
The reason this happens stems from the fact that the cGMP-gated sodium channels we discussed previously also admit calcium, In the dark, Ca enters the cones and has an inhibitory effect on guanylyl cyclase. When the cGMP-gated channels close, the flow of Ca into the photoreceptor is curtailed; as a result, more cGMP is synthesized (because the enzyme is less inhibited), thereby allowing the cGMP-gated channels to open agian. Stated more simply, when the channels close, a process is initiated that gradually reopens them even if the light level doesn't change. Calcium also appears to affect photopigments and phosphodiesterase in ways that decrease their response to light.