Understanding Neuron Polarization

[davgonz90]

From what I understand, neurons at rest are in a state of polarization, with Na+ ions abundant on the outside of the cell and K+ ions abundant on the inside of the cell. During depolarization, sodium ions rush in, creating a highly positive charge on the inside of the cell relatively to the outside.

HERE'S MY QUESTION: How does the rush of K+ ions outside of the cell after depolarization restore a negative charge inside the cell - aren't there still an abundance of Na+ ions inside the cell when this happens?

 

[DiracPool]

How does the rush of K+ ions outside of the cell after depolarization restore a negative charge inside the cell -

They don't. It's the active transport process of the sodium-potassium pump that restores the hyperpolarized state.

aren't there still an abundance of Na+ ions inside the cell when this happens?

There is until the the pump pumps them out and restores the hyperpolarized state.

http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nakpump.html
https://faculty.washington.edu/chudler/ap.html

 

[Pythagorean]

From what I understand, neurons at rest are in a state of polarization, with Na+ ions abundant on the outside of the cell and K+ ions abundant on the inside of the cell. During depolarization, sodium ions rush in, creating a highly positive charge on the inside of the cell relatively to the outside.

HERE'S MY QUESTION: How does the rush of K+ ions outside of the cell after depolarization restore a negative charge inside the cell - aren't there still an abundance of Na+ ions inside the cell when this happens?

The amount of ions that actually flow (for both Na and K) is actually quite small compared to their respective reservoirs. So yes, there are (slightly) more Na ions inside, but now there's (slightly) more K ions leaving the cell.

And in the long term, you have the pumps DiracPool mentioned pushing Na back out and pulling K back in. Note though, that the ion pumps are not responsible for repolarization or hyperpolarization; that's all potassium.

 

[DiracPool]

Note though, that the ion pumps are not responsible for repolarization or hyperpolarization; that's all potassium.

Well, that's true for the initial re-polarization after an action potential as the efflux of Potassium abates well after the the sodium ion channels close. However, the -70mV hyperpolarized resting potential is maintained by the sodium-potassium pump.

 

[Pythagorean]

I guess I've never heard the resting potential referred to as the "hyperpolarized resting potential". I associated hyperpolarization with the potassium over shoot and then once it gets back to -70mV it's just "polarized".

 

[DiracPool]

I guess I've never heard the resting potential referred to as the "hyperpolarized resting potential". I associated hyperpolarization with the potassium over shoot and then once it gets back to -70mV it's just "polarized".

Well, the polarization of at least the dendritic arbors of in vivo populations of neurons oscillate with local gamma and global alpha rhythms. So the membrane potential oscillates between a more hyperpolarized and a more depolarized state, with the pulse probability of spike trains increasing during the more depolarized phases. So, I guess I think of membrane polarization more in terms of hyperpolarized and depolarized than just "polarized" per se.