Resting Potential

Cells create potential differences by pumping ions across membranes. The Nernst equation defines the electrical potential arising from differences in ionic concentration created by the various pumps. It relates the membrane resting potential to the charge and concentration of ions on either side of a membrane.

At equilibrium,

,

where Z is the ionic charge, F is the Faraday constant, [M^Z] is the concentration of ion M of charge Z and is the potential expressed in millivolts.

For monovalent cations (Z = +/-1), the Nernst equation reduces to

The resting potential is the potential difference across the membrane of a nerve axon arising from the imbalance in ionic concentrations (Figure 10.30). This is easily measured in the giant axons of the squid by inserting electrodes and measuring the potential across the membrane.