Thus, when o and i are known, E X can be calculated, and o or i can be calculated if E X and the other compartmental ion concentration are known.
This chart can be used as a template to calculate any of the three parameters in the Nernst equation, providing the two other parameters are known. Reflecting on that publication while teaching the topic to current second-year undergraduate neuroscience students prompted me to develop a flowchart to help students understand the logical sequence of steps that are required to satisfactorily implement calculations involving the Nernst equation. Although the Nernst equation contains only three parameters, it is the cause of much student anguish, an issue I attempted to address in a recent publication regarding the logarithmic transformation that lies at the heart of the Nernst equation ( 13). The equation and its utility in determining membrane potential ( E m) are considered sufficiently important to justify dedicated chapters in textbooks ( 2, 8, 11, 12) and has been the subject of numerous articles in this ( 3– 6, 9, 10, 13, 14, 16) and other journals ( 1, 15). E X = R T z F ln o i = 61.5 z log 10 o i where E X is the reversal potential o and i are the extracellular and intracellular ion concentrations, respectively z is the valence of the ion R is the gas constant T is the absolute temperature and F is Faraday’s constant ( 7), initially reducing to the factor 26.7 mV at 37☌, then to 61.5 mV, if log 10 rather than ln is used, since log 10 x = ln x/ln 10.
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