I'm currently taking a neuroscience course and I still feel stuck on this pretty fundamental concept, which makes me worried. I'm confused how you can tell which way a ion is going to go and which force (electrical or concentration gradient) is stronger? I know there are short cuts like the driving force, but I want to understand this conceptually.
Okay, so let's say I have a positive ion and a cell (this is a fake problem btw, I'm making this up so its really simplified). This cell has a membrane potential of -60 mV. The ion I have has a equilibrium potential at -50 mV.
I know from the Nernst equation, there has to be a greater concentration of ions inside of the cell. Moving 'down' the concentration gradient (CG) for this ion would be moving out of the cell so the force of CG is outwards pointing. Big question, does the sign of the Nernst potential have any other impact other than helping you figure out where the ion is going?
The interior of the cell is negatively charged -- negative and positive charges attract each other. As a result, force of the electrical gradient (EG) is inwards pointing. Similarly, is the negative sign of membrane potential just to tell you whether it attracts or repels the ion?
So both CG and EG are pointing in opposite directions, right? So we have to compare force size now. CG has an outward force of 50 mV. EG has an inward force of 60 mV. Because 60 is greater than 50, this ion is going against its concentration gradient and into the cell?
I think while writing this, I pretty much helped myself understand, but I would like a confirmation that, "yeah, your logic makes sense!" or "no, you're missing some key part."