The effects of changes in ion concentrations for a neuron

[rico11]

Hey everyone,

 

My question is, what would happen if you raised the extracellular potassium concentration for a neuron to be very large (like 220 mMol)?

 

I believe it would cause an influx of K+ ions through the membrane and you'd end up getting a very positive charge inside the cell. What effect would that have on action potentials? Would they just stop then? I guess one AP would fire, then the cell would stay depolarized. What would this do to Na/K ATPase? Would it just stop as there is already a lot of K+ or would it keep going?

 

Also, what would happen if you raised the extracellular Na+ concentration to very large (like 220 mMol)? I assume there would be less of a change because Na+ already has a large concentration outside the cell, but it may result in more K+ efflux to balance out the resting Vm, since there would be more positively charged ions outside the membrane.

 

What would happen if you removed all Na+ from the extracellular fluid? Specifically, what would happen to Vm? I suppose action potentials would not occur because for an action potential to fully happen the cell needs Na+ to depolarize the cell to around +30 (I believe?) And I would think K+ would leak out, going down the electrochemical gradient, which is stronger due to the absence of Na+. I'm confused with what that would do to the Vm though. Wouldn't it eventually flow to an equilibrium, or no?

 

Please help, I've got ideas, but I need some clarification and help to understand this.

[Ringer]

Is this a homework question? Think about what the membrane is there for. Although there is some leaking with small ions, the membrane is meant to keep a stable environment by not letting things in and out, that's why channels and transports are needed. If that's so what would that change by putting high levels of positive charge outside with low levels of positive charge inside. With that change how would that affect the neurons ability to reach threshold potential?

[rico11]

It's a prep question for a test I have on Tuesday, so I want to make sure I have everything correct. My general thought is that everything will flow down the electrochemical gradient, but what effect will that have on ATPases? Would it be better for me to just disregard them and assume they'll be overwhelmed? For the higher EC [Na+], can I assume that too much Na+ will come in through ion channels and the ATPases/K+ leak channels won't be able to establish the correct -70 mV Vm? Or does the neuron have gated Na+ channels that won't let Na+ in? But I would assume since the outside would be even more positive, the channels would have to open to let more positive ions in the intracellular fluid. I feel like I know a lot about the neuron, but not enough to completely say what would happen in this situation. Any clarification?

[Ringer]

It really depends on what your professor likes to focus on and what they tend to ignore. I know that may sound dumb, but that has been my experience most of the time. If the professor wants you to think about the entire interaction between the effects on ATPases, voltage gated proteins, etc you could go with something like the ATPases would not be able to regulate voltage properly to keep up with the influx of K+ to maintain resting potential. This influx through the leaky channels would cause the membrane potential to most likely reach threshold and cause a single action potential, if looking at a single neuron. After that action potential the K+ gradient would not allow the membrane potential to reach the point where the K+ channels close and the Na+ channels would not be able to return to their resting conformation.

 

If you have the Nernst equation (I think that's the right one) you can figure out what the potentials would become and what the effect would be. The permeability of Na+ is about 2.5% that of K+ so if you think about the leaky channels the influx should be more negligible, but its effect of bringing the membrane potential so far below threshold would cause a large problem in actually getting an action potential. but if the threshold is reached it would have a similar effect of not allowing RMP to be restored.