When extracellular K+ concentration increase by a certain amount, excitability of cells is higher because the resting potential shifts toward a higher equilibrium potential of K+, therefore causing depolarization. However, when there concentration of extracellular Ca++ increases, excitability of cells decreases. If Ca++ and K+ are both positively charged ions, what is the cause behind their opposite physiological effects?
I'm not a biology student, so detailed answers would be kindly appreciated. Thank you!