Why is the neuronal plasma membrane more permeable to potassium ions than sodium ions?

Question

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?

Answer 1

The cell membrane is inherently quite impermeable to any ion, because it is made of hydrophobic fatty acids. Ions are hydrophilic because of their charge and small size. Metal ions are relatively small and, indeed, have therefore a high charge density. They gather a lot of water around them and hence, they are hydrophilic.

Therefore ions, including Na⁺ and K⁺ are transported via dedicated ion channels, which form specialized pores in the membrane. They consist of vast families with many members and a host of different characteristics. Hence, the size of the ion does not matter, because they are transported through channels.

There are indeed leaky K⁺ channels, but not leaky Na⁺ channels. And exactly this feature is the reason cells are negative on the inside, because positively charged K⁺ leaks from the cell.

There is just so much the Na⁺,K⁺-ATPase can do. In the end it burns ATP to fuel each cycle to pump out Na⁺ and pump in K⁺. At a certain point it cannot transport anymore Na⁺ out (K⁺ diffuses relatively freely) because the concentration gradient of Na⁺ and the electrical gradient (inside negative) gets too steep.