Physiology 5th Ed.

RESTING MEMBRANE POTENTIAL

The resting membrane potential is the potential difference that exists across the membrane of excitable cells such as nerve and muscle in the period between action potentials (i.e., at rest). As stated previously, in expressing the membrane potential, it is conventional to refer the intracellular potential to the extracellular potential.

The resting membrane potential is established by diffusion potentials, which result from the concentration differences for various ions across the cell membrane. (Recall that these concentration differences have been established by primary and secondary active transport mechanisms.) Each permeant ion attempts to drive the membrane potential toward its own equilibrium potential. Ions with the highest permeabilities or conductances at rest will make the greatest contributions to the resting membrane potential, and those with the lowest permeabilities will make little or no contribution.

The resting membrane potential of excitable cells falls in the range of −70 to −80 mV. These values can best be explained by the concept of relative permeabilities of the cell membrane. Thus, the resting membrane potential is close to the equilibrium potentials for K+ and Cl because the permeability to these ions at rest is high. The resting membrane potential is far from the equilibrium potentials for Na+ and Ca2+ because the permeability to these ions at rest is low.

One way of evaluating the contribution each ion makes to the membrane potential is by using the chord conductance equation, which weights the equilibrium potential for each ion (calculated by the Nernst equation) by its relative conductance. Ions with the highest conductance drive the membrane potential toward their equilibrium potentials, whereas those with low conductance have little influence on the membrane potential. (An alternative approach to the same question applies the Goldman equation, which considers the contribution of each ion by its relative permeability rather than by its conductance.) The chord conductance equation is written as follows:

image

where

Em

= Membrane potential (mV)

gK+ etc.

= K+ conductance etc. (mho, reciprocal of resistance)

gT

= Total conductance (mho)

EK+ etc.

= K+ equilibrium potential etc. (mV)

At rest, the membranes of excitable cells are far more permeable to K+ and Cl than to Na+ and Ca2+. These differences in permeability account for the resting membrane potential.

What role, if any, does the Na+-K+ATPase play in creating the resting membrane potential? The answer has two parts. First, there is a small direct electrogenic contribution of the Na+-K+ ATPase, which is based on the stoichiometry of three Na+ ions pumped out of the cell for every two K+ ions pumped into the cell. Second, the more important indirect contribution is in maintaining the concentration gradient for K+ across the cell membrane, which then is responsible for the K+ diffusion potential that drives the membrane potential toward the K+ equilibrium potential. Thus, the Na+-K+ ATPase is necessary to create and maintain the K+ concentration gradient, which establishes the resting membrane potential. (A similar argument can be made for the role of the Na+-K+ ATPase in the upstroke of the action potential, where it maintains the ionic gradient for Na+ across the cell membrane.)