[somasimple]

Hi All,

It is a fact that Na+ ions cross the membrane and enter the cell during the rising phase of the action potential. The process happens because Na ions channels are open.
Then the ions channels becomes inactivated/closed for a while.

What happens to the Na+ ions that entered the cell?

[modest]

Hello somasimple. I don't know much about biology (thank you Sam Cooke), but I think the answer you're looking for is really well set out here:

The Action Potential

Steps 1 - 4 are a bit too much copyrighted material to copy over, but, it looks like what you're looking for. If I interpret correctly, the Na is pumped out of the cell as quickly as possible after the electrical potential is passed on down the line.

~modest

[somasimple]

Hmm,

The fast Na channels are closed.
The Na/K pumps are a slow process and not involved in the active phase of AP.
It doesn't work.

[Monomer]

Originally Posted by somasimple

Hi All,

It is a fact that Na+ ions cross the membrane and enter the cell during the rising phase of the action potential. The process happens because Na ions channels are open.
Then the ions channels becomes inactivated/closed for a while.

What happens to the Na+ ions that entered the cell?

Basically the Na+ ions get pumped back out of the cell by sodium-potassium pumps so that another action potential can occur:

So, when an axon is at rest, the anions give it a negative charge, the sodium pumps keep sodium out and potassium in, and the sodium gates and potassium gates are all closed. Because of the positive-negative difference between the inside and outside, this resting state is called a resting potential. The word potential refers to the fact that there is a potential for change here. We use the same term to refer to a battery that is just sitting there, not connected to anything: It, too, has a resting potential.

When changes occurring in the membranes of the dendrites and the body of the cell reach the axon, the sodium gates respond: some of them open and let sodium ions in, so that the inside starts to become less negative. If this reaches a certain level, called a threshold, more sodium gates respond and let more ions in...

Then we have what is called the action potential -- a moving exchange of ions that runs along the length of the axon. So many sodium ions get in that, for a very short time, the difference between the outside and inside of the cell is actually reversed: The inside is positive and the outside negative.

Then the situation changes: The sodium gates close and the potassium gates open up. Potassium rushes out of the cell, which brings the charge inside the cell back down to where it was -- negative on the inside, positive on the outside.

Notice, though, that the sodium is now inside the cell and the potassium is outside, that is, they are in the wrong places. So, the sodium-potassium pumps get back to work and pump the sodium back out and the potassium back in, and things are back to where we started.

Steps in an Action Potential

1. At rest the outside of the membrane is more positive than the inside.
2. Sodium moves inside the cell causing an action potential, the influx of positive sodium ions makes the inside of the membrane more positive than the outside.
3. Potassium ions flow out of the cell, restoring the resting potential net charges.
4. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions.

The Action Potential
the nervous system
Na�/K�-ATPase - Wikipedia, the free encyclopedia
Action potential - Wikipedia, the free encyclopedia

[somasimple]

Reconstruction of the Action Potential

At the same time, depolarization slowly activates the voltage-dependent K+ conductance, causing K+ to leave the cell and repolarizing the membrane potential toward EK.

Hodgkin and Huxley's reconstruction of the action potential and all its features shows that the properties of the voltage-sensitive Na+ and K+ conductances, together with the electrochemical driving forces created by ion transporters, are sufficient to explain action potentials.

Na K pumps are not included in the description.

[somasimple]

The Refractory Period

The depolarization that produces Na+ channel opening also causes delayed activation of K+ channels and Na+ channel inactivation, leading to repolarization of the membrane potential as the action potential sweeps along the length of an axon

[HydrogenBond]

Another important detail is, Na+ cations are kosmotropic and K+ cations are chaotropic. What that means is Na+ ions will create structure in water. The K+ ions create disorder in water. Same charge but different affects on the water. Kosmotropic water helps push proteins into order which is why sodium can't leak in very easily. The water it induces sort of puckers the pump's exit mouth. K+ ions by making chaotropic water loosens the inside mouth of the pump. (as a visualization).

The firing of the pump happens when the Na+ is on inside the membrane. The ATP only works when the Na+ is setting up water structure on the inside of the pump. This adds the extra energetics. The chaotic water of K+ does not add enough kick. It is a smart affect, just water energetics. The neurotransmitters are often chaotropic and counter the external Na+ kosmostropic affect, allowing the pump's exit mouth to relax so Na+ can leak inside, easier. It is not quite that simple, but it is the water's order or disorder that gives the extra energy and protein push or pull to complete the energy balance.

In a resting neuron, because Na+ is all over the outside, the water on the surface is very ordered. This makes it easier to conduct potential, both Na+ and H+. On the inside, is just the opposite due to the K+ ions. The cell likes the K+, because it creates disorder inside the cell water and this helps loosen up the innards.

When Na+ leaks into the dendrites, a local change occurs in the inside water near the pumps. Now there is order in chaos. There is both an energy and density difference between this area and the bulk inside cell water. The Na+ migration from inside dendrite to inside axon is following the density and energy difference in the water creating sort of a stream as it attempts to equilibrate the inside water. If the Na+ sticks anywhere onto the inside membrane it form that ordered water to help fire the pump.

[somasimple]

I'm already aware of such a context but thanks.
Kosmotropes and Chaotropes

Your explanation is more congruent with energy consumption but dissagree with a constant activation of the Na/K pump.

BTW, it doesn't explain how a slow process is able to restore initial conditions so fast.