Cations (+) and anions (−) are
distributed unevenly across the neuronal cell membrane because the membrane is
differentially permeable to these ions. The uneven distribution depends on the
forces of charge separation and diffusion.
The permeability of the membrane to
ions changes with depolarization (toward 0) or hyperpolarization (away from 0).
The typical neuronal resting potential is approximately −90 mV with respect to
the extracellular fluid. The extracellular concentrations of Na+ and Cl−
of 145 and 105 mEq/L, respectively, are high compared to the intracellular
concentrations of 15 and 8 mEq/L.
The extracellular concentration of K+ of 3.5 mEq/L is low compared to the intracellular
concentration of 130 mEq/L. The resting potential of neurons is close to the
equilibrium potential for K+ (as if the membrane were permeable only
to K+). Na+ is actively pumped out of the cell in
exchange for inward pumping of K+ by the Na+-K+-ATPase
membrane pump. Equivalent circuit diagrams for Na+, K+,
and Cl−, calculated using the Nernst equation, are illustrated in
the lower diagram.
