Factors Affecting Ion Current Across a Cell Membrane

What factors govern the amount of current carried across the membrane by a particular ion? We would expect that one important factor would be the difference between the equilibrium potential for the ion and the actual membrane potential. As an example, consider the movement of potassium ions across the membrane. We know that if R = Eā€ž, there is a balance between the elec-

m k trical and concentrational forces for potassium and there is no net movement of potassium across the membrane. In this situation, then, iK = 0. As shown in Figure 5-2, if Em does not equal EK, the resulting imbalance in electrical and concentrational forces will drive a net movement of potassium across the membrane. The larger the difference between Em and EK, the larger the imbalance between the electrical and concentration gradients and the larger the net movement of potassium. Thus, iK depends on Em - EK. This difference is called the driving force for membrane current carried by an ion.

We would also expect that the permeability ofthe membrane to an ion would be an important determinant of the amount of membrane current carried by that ion. If the permeability is high, the ion current at a particular value of driving force will be higher than if the permeability were low. Thus, because pK is much greater than pNa, the potassium current resulting from a 10 mV difference between Em and EK will be much larger than the sodium current resulting from a 10 mV difference between Em and ENa. This is, in electrical terms, the reason that the steady-state membrane potential of a cell lies close to EK rather than ENa: in order for Equation (5-4) to be obeyed, the driving force for sodium entry (Em - ENa) must be much greater than the driving force for potassium exit (Em ā€” ek).

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