It is important to note that the depolarization of the membrane characteristic of 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 gradient. A typical nerve cell has about 10 voltage-gated Na+ channels per square micrometer (^m2) of plasma membrane. Since each channel passes «5000-10,000 ions during the millisecond it is open (see Figure 7-18), a maximum of 105 ions per ^m2 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 1 micrometer (^m) long and 10 ^m in diameter. The volume of this segment is 78 ^m3 or 7.8 X 10-14 liters, and it contains 4.7 X 108 Na+ ions: (10—2 mol/L) (7.8 X 10-14 L) (6 X 1023 Na+/mol). The surface area of this segment of the axon is 31 ^m2, and during passage of one action potential, 105 Na+ ions will enter per ^m2 of membrane. Thus this Na+ influx increases the number of Na+ ions in this segment by only one part in about 150: (4.7 X 108) ^ (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.
Because so few Na+ and K+ ions move across the plasma membrane during each action potential, the Na+/K+ pump that maintains the usual ion gradients plays no direct role in impulse conduction. When this pump is experimentally inhibited by dinitrophenol or another inhibitor of ATP production, the concentrations of Na+ and K+ gradually become the same inside and outside the cell, and the membrane potential falls to zero. This elimination of the usual Na+ and K+ electrochemical gradients occurs extremely slowly in large squid neurons but takes about 5 minutes in smaller mammalian neurons. In either case, the electrochemical gradients are essentially independent of the supply of ATP over the short time spans required for nerve cells to generate and conduct action potentials. Since the ion movements during each action potential involve only a minute fraction of the cell's K+ and Na+ ions, a nerve cell can fire hundreds or even thousands of times in the absence of ATP.
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