From what I understand, the greater permeability of the neuronal plasma membrane to K+ ions (which diffuse out) than Na+ ions (which diffuse in) helps to maintain the -60 mV resting membrane potential. However, I am not sure why. After all, K+ ions are larger so should the membrane not be less permeable to them? Could it be to do with the greater charge density of Na+ ions or is there another reason altogether? Does it have something to do with the 'leakiness' of ion channels?
Also, why is it that the resting potential is maintained and does not become increasingly more negative? Should it not be that because 3 Na+ ions are pumped out and 2 K+ are pumped in continuously, and more K+ ions are diffusing out than Na+ is diffusing in, that the potential is becoming increasingly negative? Is movement of other ions also involved in maintaining this potential?