The depolarization of the membrane during an action potential results from movement of just a small number of Na⁺ ions into a neuron and does not significantly affect the intracellular Na⁺ concentration. A typical nerve cell has about 10 voltage-gated Na⁺ channels per square micrometer (µm²) of plasma membrane. Since each channel passes about 5000–10,000 ions during the millisecond it is open (see Figure 11-23), a maximum of 10⁵ ions per µm² of plasma membrane will move inward during each action potential.

To assess the effect of this ion flux on the cytosolic Na⁺ concentration of 10 mM (0.01 mol/L) typical of a resting axon, we focus on a segment of axon 10 micrometers (µm) long and 1 µm in diameter. The volume of this segment is 78 µm³, or 7.8 x 10⁻¹³ liters, and it contains 4.7 x 10⁹ Na⁺ ions: (10⁻² mol/L) (7.8 x 10⁻¹³ L) (6 x 10²³ Na⁺/mol). The surface area of this segment of the axon is 31 µm², and during passage of one action potential, 10⁵ Na⁺ ions will enter per µm² of membrane. Thus this Na⁺ influx increases the number of Na⁺ ions in this segment by only one part in about 1500: (4.7 x 10⁹) / (3.1 x 10⁶). Likewise, the repolarization of the membrane due to the efflux of K⁺ ions through voltage-gated K⁺ channels does not significantly change the intracellular K⁺ concentration.