A sodium potassium pumps is used in the active transport of sodium and potassium ions across a cell membrane. The pumps moves 3 sodium ions out of the cell for every 2 potassium ions that it brings into the cell. The movement of these ions is against their concentration gradients, which means the pump needs energy in order to transport these ions. In a cell, most of the Na+ exists outside of the cell and the K+ is inside the cell. Charge is measured at the membrane and in a neutral cell, the inner layer is negative where the outside is positive. Cells move torward the membrane potential of the ion it's most permeable to, so if a cell has more leakage channels for K+ then the membrane potential for K+ is what is desired.
Extracellular potassium will result in a less negative membrane potential because an addition of extracellular potassium results in a decrease in the concentration gradient. There is more potassium inside of the cell to begin with so an increase in extracellular potassium decreses the chemical gradient, creating a less negative membrance potential.
Decreasing extracellular sodium will not have a similar impact on the membrane potential. This actually has little effect on the membrane potential since the membrane is not very permeable to sodium. Leakage potassium channels exist so the membrane is more permeable to potassium than to sodium. So potassium has more movement available. An increase in extracellular potassium would then decrease the amount of potassium leaving the cell through these leakage channels since the gradient is less steep. This results in a less negative (more positive) membrane potential. Becasuse sodium is not as permeable, reduction of extracellular sodium would not have a similar effect because active transport is required regardless.
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