Resting electrical membrane potential and Nernst potential considering multiple ions · Vm = (RT/F)·ln(ΣP·[ion]out / ΣP·[ion]in)
| Ion | [in] mM | [out] mM | P relative |
|---|---|---|---|
| K⁺ | |||
| Na⁺ | |||
| Cl⁻ |
-72.52
mV-89.06
mV66.6
mV-88.58
mVCalculates the equilibrium potential for a single ion. is the gas constant, the absolute temperature (in Kelvin, K), the ion's valence, and Faraday's constant.
The Membrane Potential () considers multiple ions simultaneously. Note that concentrations are inverted due to its negative valence.
Determines the net direction and magnitude of an ion's flow across the membrane. For cations, a negative sign indicates the force pushes it into the cell (influx), while a positive sign indicates efflux.
Living cells maintain a voltage difference across their cell membrane. This is due to the unequal distribution of ions (mainly , , and ) and selective permeability. At rest, the membrane is much more permeable to Potassium, so the approaches .
While Nernst assumes permeability to a single type of ion, the GHK equation calculates the potential taking into account the relative permeabilities () of all ions simultaneously. If one ion's permeability overwhelmingly dominates, GHK mathematically collapses into Nernst for that ion.
Ion concentrations change slowly. However, permeability () changes in milliseconds by opening channels. In an action potential, increases drastically, causing the to shoot up towards the Sodium equilibrium potential.
The GHK equation assumes monovalent ions (). Calcium is divalent () and cannot be directly summed. For different valences, the complex Extended GHK Equation is required. Therefore, Calcium's contribution is analyzed by calculating its Nernst independently to understand its Driving Force.