Equilibrium Potentials II Answer 4

• We found that the values of sodium, potassium and chloride ion concentrations inside the cell were 12 mM, 125 mM, and 5 mM (respectively), and the concentrations of these ions outside the cell were 120 mM, 5 mM, and 125 mM (respectively).
• Then the estimated resting potential across the membrane is $$\begin{align} V_m &= 58\ \text{mV} \times \log_{10} \frac{1 \cdot 120\ \text{mM} + 1 \cdot 5\ \text{mM} + 1 \cdot 5\ \text{mM}} {1 \cdot 12\ \text{mM} + 1 \cdot 125\ \text{mM} + 1 \cdot 125\ \text{mM}} \\ &= 58\ \text{mV} \times \log_{10} \frac{130}{262} \\ &= -17.7\ \text{mV}, \end{align}$$ which is midway between the Nernst or equilibrium potentials of $-81\ \text{mV}$ that we calculated for the chloride and potassium ions, and $+58\ \text{mV}$ that we calculated for the sodium ions.