Resting potential
Origin of the resting potential - theories. Goldman equation. The effect of electrogenic pump. Resting potential components.
The resting potential is the difference in electrical charge between the inside and outside of a cell at rest. In excitable cells it is the membrane potential in the absence of excitation, with the inside negative.
The resting potential depends on which ions can cross the membrane at rest. Each ion's influence on the membrane potential depends on two things:
- 1. That ion's driving force
- 2. That ion's permeability
The stronger the driving force and the higher the permeability, the more that ion affects the membrane potential.
- Driving force is the difference between the membrane potential and the ion's equilibrium potential: Eₘ − Eᵢₒₙ. For example, at a resting potential of −73 mV, the driving force on potassium (equilibrium potential −80 mV) is 7 mV, and on sodium (+60 mV) it is −133 mV.
- Permeability is how easily an ion crosses the membrane. It is usually measured as electrical conductance, in siemens.
At rest, potassium has a small driving force but a very high permeability, while sodium has a huge driving force but almost no permeability. Potassium therefore carries about 20 times more current and dominates the resting potential.
At the peak of an action potential the situation reverses. Sodium permeability is high, so the membrane moves close to the sodium equilibrium potential and away from the potassium one.
When several ions can cross the membrane, its potential is calculated with the Goldman-Hodgkin-Katz equation. The result is a weighted average of the ions' equilibrium potentials, where each ion's weight is its relative permeability.
Two theories explain the origin of the resting potential:
- 1. Hodgkin-Katz theory
Basic principle - when the membrane is permeable for several ions, the resting potential is not due to an equilibrium but steady state.
- 2. Bernstein “potassium” theory: The resting potential is a Nernst (membrane) equilibrium potential caused by the concentration difference of potassium ions (only) in the intra- and extracellular phases.
Effect of the electrogenic pump: the sodium-potassium pump moves 3 Na+ out for every 2 K+ in. This makes a small direct contribution that leaves the inside more negative (number 4 in the picture below).