How don't neurons get enough K+ to balance out their inner negative charge?

sodium.dioxid

If resting potential is negative, then K+ ions shouldn't be passively exiting the cell. It doesn't make sense for a male that is attracted to females to leave a club full of females and go outside where there are lots of males. There are females to be had!

tal444

The neuron's firing depends on the K+ not being inside. The sudden rush of K+ is what causes the firing. Neurons wouldn't work otherwise.

sodium.dioxid

There are two levels to learning anything in biology. One is by saying that "things wouldn't work otherwise". The other level is trying to understand the seeming contradictions. THAT is the one I'm interested in.

aytell

There are two forces at work here:
1. Electric force--because the inside of the cell is negative, it pulls K+ in.
2. Diffusion force--because the concentration of K+ is much, much higher inside the cell, K+ tends to move out. This is because things move from regions of high concentration to regions of low concentration, like a drop of food coloring spreading out in a glass of water.

In the resting state, the diffusion force (pushing K+ out) is much stronger than the electromotive force (pulling K+ in), so K+ moves out of the cell.

In terms of gradients: the concentration gradient pushing K+ out is stronger than the electrical gradient pulling K+ in.

(And by the way, K+ doesn't create the action potential; Na+ does.)

somasimple

But if the external side is positive by the presence of positive ions across the membrane, why Na⁺ ions are not stopped/repulsed by them since the electrostatic force that exists in the extra cellular compartment near the membrane is stronger that the ones that may exist on the other side of the membrane?

Neurofreak114

Na+ IS repelled by the excess of Na+ and other positive charges. Only you have to factor
in permeability.

Neurons are designed to maintain a negative resting membrane potential. Meaning they have
channels that open only at specific threshold voltages. Extracellular signals at the synapse
stimulate the opening of ion channels making the synapse more "permeable" to these
specific ions. If the voltage within the cell reaches a specific threshold, voltage gated Na+
channels open allowing an all or none (non-decrementing) action potential.

So there can't simply be an influx of ions into or out of the cell. You need channels to
open to allow permeability. There are diverse types of channels. One's that respond to
mechanical pressure, light, vibration, voltage, chemical signals etc.

Neurofreak114

There are no contradictions, there is only a balance of polar opposites.
Understand equilibrium and you will understand biology.

yin and yang

somasimple

If the membrane is permeable, at rest, to K⁺ there is an excess of K⁺ outside the cell. The Na⁺ ions will encounter them firstly when the membrane will change its state?

Neurofreak114

The membrane has negligible/leaky permeability to K+ at rest. K+ permeability increases
AFTER the influx of Na+

Also if the membrane is fully permeable to K+ then it won't be at the resting membrane potential. K+ will move out as it is less concentrated outside the cell, this efflux causes hyperpolarization of the cell (decreasing voltage below resting potential V_r).

as "aytell" stated already there are 2 forces at work.
One caused by the the concentration gradient, the other by the voltage (charge difference).
There are a sequence of events happening.

1. Some stimulus (light/acoustic/mechanic/ligand/etc) mediated voltage increase causes threshold level voltage.
2. At this threshold Na+ channels are frequently open and allow influx from outside (positive
and high in Na+ concentration) to the inside (negative and low in Na+ concentration). This
known as the rising portion of the action potential (also called depolarization)
3. Increasing voltage due to Na+ influx triggers the opening voltage gated K+ channels, resulting in a net efflux of K+ from the cell. Against its electrical gradient (- to +) and towards its concentration gradient (high to low concentration). The concentration gradient dominates over the electrical gradient in this circumstance, resulting in the falling phase of
the action potential and slight hyperpolarization.

The movement of ions is determined by the Goldman equation. It is the net sum
that determines the fate of all ion movements.

somasimple

From Resting Potential

somasimple

And, if Na⁺ channels are closed at rest and permeability to K⁺ at rest is near to 1 then the Na⁺/ K⁺ exchanging pump may not function at all?

If a theoretical model is clear/simple you must/may be able to draw every step between each phase. In that case, I can't draw it.

Neurofreak114

Na+/K+ pump is an ATPase
Phosphate bond hydrolysis ΔG=−30.5 kJ/mol

it pumps 3 Na+ and 2 K+ against their respective electrochemical gradients.
this is a form of active transport and does not depend on the gradients unless the
energy stored in a phosphate bond is comparable to the gradient differential.

somasimple

From Na/K pump
At rest Na⁺ channels are closed so no Na⁺ is entering the cell.
The pump can not function.

Pythagorean

The pump is not the same protein as the receptor that operates the Na flood channels. They're two completely different kinds of gateways in/out of the cell.

somasimple

Yes, did I said something different? Does it change the problem?

Pythagorean

Why do you think the pump cannot function?

somasimple

At rest:
Na⁺ channels are closed. Sodium intake may be quite low.
K⁺ channels are open so potassium ions goes out.

Na/K pump is theorized as functioning all the time.

If sodium intake is meant low and potassium outtake is meant high, how a pump that may put outside 3 Na+ (taken from inside) and may put inside 2 K+ (taken from the outside) find sodium ions, inside, when they remain outside because the sodium permeability is low?