A muscle cell has an intracellular $\left[\mathrm{Na}^{+}\right]$of $14 \mathrm{mM}$ and an extracellular $\left[\mathrm{Na}^{+}\right]$of $140 \mathrm{mM}$. Assuming that $2.3 \mathrm{RT} / \mathrm{F}=60 \mathrm{mV}$, what would the membrane potential be if the muscle cell membrane were permeable only to $\mathrm{Na}^{+}$?

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Quizplus · Accepted Answer

[IV B $3 \mathrm{a}, \mathrm{b}$ ]. The Nernst equation is used to calculate the equilibrium potential for a single ion. In applying the Nernst equation, we assume that the membrane is freely permeable to that ion alone. $\mathrm{E}_{\mathrm{Na}^{+}}=2.3 \mathrm{RT} / \mathrm{zF} \log \mathrm{C}_{\mathrm{e}} / \mathrm{C}_{\mathrm{i}}=60 \mathrm{mV} \log 140 / 14=60$ $\mathrm{mV} \log 10=60 \mathrm{mV}$. Notice that the signs were ignored and that the higher concentration was simply placed in the numerator to simplify the log calculation. To determine whether $\mathrm{E}_{\mathrm{Na}^{+}}$is $+60 \mathrm{mV}$ or $-60 \mathrm{mV}$, use the intuitive approach $-\mathrm{Na}^{+}$will diffuse from extracellular to intracellular fluid down its concentration gradient, making the cell interior positive.

A Muscle Cell Has an Intracellular [Na+]\left[\mathrm{Na}^{+}\right][Na+] Of 14mM14 \mathrm{mM}14mM And an Extracellular

A Muscle Cell Has an Intracellular $\left[\mathrm{Na}^{+}\right]$ Of $14 \mathrm{mM}$ And an Extracellular