••• bio a nerve cell in its resting state has a membrane potential of −70 mv, meaning that the potential inside the cell is 70 mv less than the potential outside due to a layer of negative charge on the inner surface of the cell wall and a layer of positive charge on the outer surface. this effectively makes the cell wall a charged capacitor. when the nerve cell fires, sodium ions, na , flood through the cell wall to briefly switch the membrane potential to +40 mv. model the central body of a nerve cell—the soma—as a 50-μm-diameter sphere with a 7.0-nm-thick cell wall whose dielectric constant is 9.0. because a cell’s diameter is much larger than the wall thickness, it is reasonable to ignore the curvature of the cell and think of it as a parallel-plate capacitor. how many sodium ions enter the cell as it fires?