The cell membrane of a retinal photoreceptor contains sodium channels that at rest (in the dark) are normally and have a resting potential that is compared to other neurons.

open; depolarized

Explanation:

The photoreceptors, i.e., rods and cones, are specialized neurons located in the retina. These cells convert sunlight into electrochemical signals which are interpreted by the central nervous system. The cGMP-gated channels that are expressed in photoreceptor plasma membranes have a major role in this process by regulating the flow of sodium (Na+) and calcium (Ca2+) ions in the outer side of the membrane via changes to cyclic guanosine monophosphate (cGMP) levels which are induced by light. In the dark, high cGMP levels maintain channels in an open state, thereby allowing the passage of Na+ and Ca2+ ions to flow into the outer segment of photoreceptors in order to maintain them in a depolarized state.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.