When a cell membrane of an excitable cell is exposed to a depolarizing current, which of the fol-lowing occurs?

a. Increased influx of calcium into the cell
b. Rapid influx of sodium ions into the cell
c. Increased efflux of potassium out of the cell
d. Rapid efflux of protein molecules out of the cell

ANS: B
An action potential begins when the cell membrane of the excitable cell is exposed to a depolar-izing current and eventually reaches its excitation threshold potential. The initial phase of this type of action potential consists of a rapid upstroke or depolarization. This change in membrane potential to a more positive value occurs because of a rapid influx of sodium ions into the cell.