FIGURE 22-9 Depolarization of the plasma membrane due to opening of gated Na⁺ channels. (a) In resting neurons, a type of nongated K⁺ channel is open part of the time, but the more numerous gated Na⁺ channels are closed. The movement of K⁺ ions outward establishes the inside-negative membrane potential characteristic of most cells. (b) Opening of gated Na⁺ channels permits an influx of sufficient Na⁺ ions to cause a reversal of the membrane potential. In the depolarized state, voltage-gated K⁺ channels open and subsequently repolarize the membrane. Note that the flows of ions are too small to have much effect on the overall concentration of either Na⁺ or K⁺ in the cytosol or exterior fluid.