In phototransduction, does the closing of sodium channels lead to depolarization or hyperpolarization of the rod?

• A. Depolarization
• B. Hyperpolarization
• C. No change
• D. Repolarization

Answer: (B)

In darkness, the rod’s cGMP levels keep the cyclic nucleotide–gated Na+ channels open, allowing a steady inward Na+ (and Ca2+) current that keeps the cell relatively depolarized and releasing neurotransmitter onto bipolar cells. When light activates the phototransduction cascade, cGMP is broken down, the channels close, and the inward Na+ current stops. Without that positive inward current, the membrane potential becomes more negative—a hyperpolarization. This hyperpolarization reduces calcium influx at the synaptic terminal, leading to less neurotransmitter release. So closing the sodium channels makes the rod more negative inside, i.e., hyperpolarizes it.