Magnitude of the Action Potential Is Close to ENa

Earlier in this chapter we saw how operation of the Na+/K+ pump generates a high concentration of K+ and a low concentration of Na+ in the cytosol, relative to those in the extracellular medium. The subsequent outward movement of K+ ions through nongated K+ channels is driven by the K+ concentration gradient (cytosol > medium), generating the resting membrane potential. The entry of Na+ ions into the cytosol from the medium also is thermodynamically favored, driven by the Na+ concentration gradient (medium > cytosol) and the inside-negative membrane potential (see Figure 7-20). However, most Na+ channels in the plasma membrane are closed in resting cells, so little inward movement of Na+ ions can occur (Figure 7-32a).

If enough Na+ channels open, the resulting influx of Na+ ions will overwhelm the efflux of K+ ions though open resting K+ channels. The result would be a net inward movement of cations, generating an excess of positive charges on the cytosolic face and a corresponding excess of negative charges (due to the CP ions "left behind" in the extracellular medium after influx of Na+ ions) on the extracellular face (Figure 7-32b). In other words, the plasma membrane is depolarized to such an extent that the inside face becomes positive.

The magnitude of the membrane potential at the peak of depolarization in an action potential is very close to the Na+

(a) Resting state (cytosolic face negative)

Exterior

150 mM 4 mM

12 mM 140 mM Cytosol

12 mM 140 mM Cytosol

K+ channel (open)

Na+ channels (closed)

(b) Depolarized state (cytosolic face positive)

Exterior

150 mM 4 mM

12 mM 140 mM Cytosol

12 mM 140 mM Cytosol

K+ channel (open)

▲ FIGURE 7-32 Depolarization of the plasma membrane due to opening of gated Na+ channels. (a) In resting neurons, nongated K+ channels are open, 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. See text for details.

equilibrium potential ENa given by the Nernst equation (Equation 7-2), as would be expected if opening of voltage-gated Na+ channels is responsible for generating action potentials. For example, the measured peak value of the action potential for the squid giant axon is 35 mV, which is close to the calculated value of ENa (55 mV) based on Na+ concentrations of 440 mM outside and 50 mM inside. The relationship between the magnitude of the action potential and the concentration of Na+ ions inside and outside the cell has been confirmed experimentally. For instance, if the concentration of Na+ ions in the solution bathing the squid axon is reduced to one-third of normal, the magnitude of the depolarization is reduced by 40 mV, nearly as predicted.

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