<prompt> A muscle cell has an intracellular of 14 mM and an extracellular of 140 mM. Assuming that 2.3 F 60 mV, what would the membrane potential be if the muscle cell membrane were permeable only to Na?80 mV-60 mV-80 mV60 mV  <response>: Nernst equation 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. ENa 2.3 zF log CeCi ENa 60 mV log 14014 60 mV log 10 ENa 60 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 ENa is 60 mV or -60 mV,- Use the intuitive approach Na will diffuse from extracellular to intracellular fluid down its concentration gradient, making the cell interior positive. 