The reasoning I've been given is that high extracellular [K⁺] increases the Ev of potassium; therefore, membrane potential increases and the threshold for action potentials is more easily reached. Conversely, high extracellular [Ca²⁺] increases the accumulated positive charge outside the cell membrane and therefore increases the membrane potential.
However, like with K⁺, wouldn't high ECF [Ca²⁺] also increase its Ev and therefore membrane potential of the cell? Vice versa, high ECF [K⁺] should also increase membrane potential by accumulating charge.
Why are the effects different? A guess I had is that the relatively higher permeability of K⁺ makes the effect on equiLiBrium potential larger than the effect of accumulating positive charge.