Ans. A: Lower extracellular sodiumDecreasing the external Na+ concentration decreases the size of the action potential but has little effect on the resting membrane potential. The lack of much effect on the resting membrane potential would be predicted, since the permeability of the membrane to Na+ at rest is relatively low.Conversely, increasing the external K+ concentration decreases the resting membrane potential.A decrease in extracellular Ca2+ concentration increases the excitability of nerve and muscle cells by decreasing the amount of depolarization necessary to initiate the changes in the Na+ and K+ conductance that produce the action potential. Conversely, an increase in extracellular Ca2+ concentration "stabilizes the membrane" by decreasing excitability