  In this pedagogical paper a coherent explanation of the resting potential of
nerve cells is given in terms of its determining factors. These are the
currents of active transport of the ions to which the membrane is permeable,
their membrane permeabilities and their concentrations in the extracellular
fluid. They play the role of the independent variables in the problem and
simultaneously also determine the concentrations of the permeating ions inside
the cell. The resting state is assumed to be a stationary state, and the
resting potential is understood to be the membrane potential in this state. The
explanation presented here differs from the conventional one found in textbooks
with respect to the handling of the intracellular concentrations of the
permeant ions. In the conventional description the causal relations between
these and the independent variables are not taken into account, and the
intracellular concentrations are treated like additional independent variables.
The "causal" theory presented here leads to a simple formula for the resting
potential in terms of the currents of active transport (pumping by the
Na,K-pump and carrier transport) and the membrane permeabilities for sodium and
potassium ions. In addition it leads to two results not obtained in the
conventional treatment: An upper bound exists for the magnitude of the resting
potential, and the difference between the resting potential and the diffusion
potential, which can be measured after poisoning of the ion pumps, is small.
