![]() The relative contribution of each ion is governed not only by its concentration gradient and valence, but also by its relative permeability. Instead, the value of V m will be determined by the relative contribution of each ion to the membrane potential. In this case, V m will not be at the equilibrium potential for either ion and, thus, no ion will be at equilibrium. If channels for two or more ions are present (and are open), both ions contribute to the membrane potential. Of course, it must be assumed that the ion channels are open in order to allow transmembrane movement of the ionic species for which the channel is specific. If only one ionic species is present in the system and/or only one type of ion channel is present, the Nernst potential also determines the resulting membrane potential ( V m). The Nernst equation can be derived based on simple thermodynamic principles (see lecture notes on the Resting Membrane Potential and Derivation of the Nernst Equation).Īs mentioned above, the Nernst equation calculates the equilibrium potential ( V Eq.) for an ion based on the charge on the ion (i.e., its valence) and its concentration gradient across the membrane.Typically, but not always, the concentrations are noted in mM. Note that the concentration unit must match that of out. ![]() in is the concentration of the ionic species X in the intracellular fluid.Note that the concentration unit must match that of in. out is the concentration of the ionic species X in the extracellular fluid.F is the Faraday's constant and is equal to 96485 C.mol -1 (Coulombs per mole).For example, z is +1 for Na +, +1 for K +, +2 for Ca 2+, -1 for Cl -, etc. ![]()
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