Correct option is C
Explanation-
In darkness (i.e., absence of light), photoreceptors (rods and cones in the retina) are depolarized due to a continuous inward flow of Na⁺ ions. This is known as the dark current.
In the outer segment of photoreceptors, Na⁺ channels remain open due to the presence of cGMP (cyclic guanosine monophosphate). This allows a steady flow of Na⁺ (sodium ions) into the cell, which keeps it depolarized. This depolarization leads to the release of neurotransmitters even in the absence of light.
When light hits photoreceptors:
1. cGMP levels drop.
2. Na⁺ channels close.
3. The cell hyperpolarizes.
4. Neurotransmitter release decreases.
Incorrect Options:
Option a - Closing of Na⁺ channels in the outer segment of photoreceptors – This happens in light, not in dark.
Option b - Opening of K⁺ channels in the inner segment of photoreceptors – This happens normally, but does not cause the dark current.
Option d - Closing of K⁺ channels in the outer segment of photoreceptors – Irrelevant; K⁺ efflux continues and doesn’t create the inward dark current.
So, the correct answer is option c - Opening of Na⁺ channels in the outer segment of photoreceptors
