Some questions about action potentials

[CausativeAgent]

My question arises from study during my Human A+P class, but it goes beyond what I'd be expected to know for the class so I decided to post it here.

When potassium exits the cell body during repolarization, is this due to its concentration gradient? If so, why does the chemical gradient get "preference" in deciding the direction of K+ movement, as opposed to the electrical gradient, which I would think would favor depolorization, ie. moving the absolute value of the charge closer to zero. I have a similar confusion about the movement of chloride ions into the cell during hyperpolarization.


Thanks for reading.

 

[Cincinnatus]

For these kinds of questions, you typically talk about a combined "electrochemical gradient" rather than thinking about the concentration and voltage gradients separately. The Nernst equation (or similar results) can be used to calculate the corresponding voltage across the membrane for a given concentration gradient.

Ions enter and leave the cell as their driving forces with respect to this electrochemical gradient dictate. The particular shape of the action potential is due to the properties of the different ion channels involved...

 

[denverdoc]

just to add to Cincinnatus response, you can look at the instantaneous driving force on any ion as the difference between the Nernst eqn and the transmembrane voltage. But that in itself may not tell you much about current flow of any species.

Its becaause conductances (the reciprocal of resistance)come in two types, there is a passive component which is more or less constant, and as Cinci offered, conductances that vary as both a function of time and voltage thru various voltage gated channels. generally these will account for the shapes of an AP, but which will also certainly change depending on concentration gradients. If you look at the actual number of ions that change sides during an AP it is a relatively small number and unlikely to affect the overall concentrations.

 

[CausativeAgent]

That equation is a bit past my level of comprehension at this point, but it's nice to at least know the factors involved.

 

[denverdoc]

That equation is a bit past my level of comprehension at this point, but it's nice to at least know the factors involved.

Hey you're thinking about it, and the nernst eqn is fairly easy to derive at least for one ion, basically by saying the electrical driving force exactly offsets a concentration gradient at a particular voltage, ie when diffusion/entropy considerations are exactly offset by voltage potential gain moving against that same concentration gradient.