Explanation

In resting nerve fibre (a nerve fibre that is not conducting an impulse), sodium ions ($Na^{+}$) predominate in the extra cellular fluid, whereas potassium ions ($K^{+}$) predominate in the intracellular fluid (within the fibre). This result in the fact that, the resting membrane has only a poor permeability for $Na^{+}$ although it has a higher permeability for $K^{+}$.