The cell membrane is a lipid bilayer with an intrinsically low permeability to charged ions. However, a variety of structures span the membrane through which ions can enter or leave the cell. These include ion channels through which ions passively diffuse and ion pumps which actively transport ions across the membrane. Pumps regulate ionic gradients, and channels determine membrane potential and underlie action potentials.

Resting Membrane Potential (Figure 10a)

The resting membrane is more permeable to K⁺ and Cl⁻ than other ions, and is therefore semipermeable. The cell contains negatively charged molecules (e.g. proteins) which cannot cross the membrane. This fixed negative charge attracts K⁺ but repels Cl⁻, leading to accumulation of K⁺ within the cell and loss of Cl⁻. However, the consequent increase in K⁺ concentration gradient drives K⁺ back out of the cell. An equilibrium is reached when the electrical forces exactly balance those due to concentration differences (Gibbs–Donnan equilibrium); the net force or electrochemical gradient for K⁺ is then zero. The opposing effect of the concentration gradient means fewer K⁺ ions move into the cell than are required by the fixed negative charges. The inside of the cell is therefore negatively charged compared to the outside (charge separation), and a potential develops across the membrane. Only a small charge separation (e.g. 1 in ∼100 000 K⁺ ions) is required to cause a potential of ∼−100 mV. If the membrane was only permeable to K⁺ and no other cations, the potential at equilibrium (K⁺ equilibrium potential, E_K) would be defined by the K⁺ concentration gradient, and calculated from the Nernst equation. As cardiac muscle intracellular [K⁺] is ∼120 mmol/L and extracellular [K⁺] ∼4 mmol/L E_K = ∼−90 mV (Figure 10a).

In real membranes K⁺ permeability (P_K) at rest is indeed greater than for other ions, so the resting membrane potential (RMP) is close to E_K (∼−85 mV). RMP does not equal E_K because there is some permeability to other ions; most notably Na⁺ permea­bility (P_Na

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