The NMDA receptor is a ligand gated non selective cation channel. It allows Na+ and to a lesser extent Ca2+ and K+ to pass through. I read that Mg2+ clogs up the pore. Why would Mg2+ block the channel but not Ca2+? Ca2+ is larger so I'd expect that to be more likely to block an ion channel. Is it because Mg2+ is more electronegative than Ca2+?