in the dark, are the sodium channels opened or closed in rods? What is the molecular basis behind this? What is this movement of positive charge across the membrane called?
they are opened. sodium channels are stimulated to open (they are gated) by an intracellular second messenger called cGMP. Evidently, cGMP is continually produced in the photoreceptor by the enzyme guanylyl cyclase, keeping the Na channels open. The movement of positive charge across the membrane is called the dark current, since it occurs when the eye isn't exposed to light.