Extracellular ion concentrations and neural membrane potential

In summary, the book is saying that if the volume of a cell doesn't change, the concentrations of potassium and chloride inside the cell will be the same no matter what.
  • #1
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I'm learning about a very basic model neuron, in which only potassium and chloride is permeable.

Why is it that when the extracellular concentration of potassium is increased, the neuron become depolarized, whereas when extracellular concentration of chloride is reduced, the neural membrane potential remains the same? Intuitively it would seem that whatever applies to potassium also applies to chloride.

Even when sodium permeability is added to the simple neuron model, it seems chloride can be ignored as it more or less takes care of itself no matter where the membrane potential goes.
 
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  • #2
I believe this is your issue-

basic model

Chloride does contribute to resting membrane potential and any changes in intra- or extracellular conc. of it would definitely alter it.

Since I can't figure out complex fractions inside parentheses in LaTeX, I've just attached a picture. Sorry for the blurriness.
 

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  • #3
aroc91 said:
I believe this is your issue-



Chloride does contribute to resting membrane potential and any changes in intra- or extracellular conc. of it would definitely alter it.

Since I can't figure out complex fractions inside parentheses in LaTeX, I've just attached a picture. Sorry for the blurriness.

Ahh excellent, thanks!

Though I do wonder what my textbook was talking about when it says:

... The model cell membrane is permeable to potassium and chloride, but not to sodium or to the internal anion.

...

In neurons and in many other cells, the resting membrane potential is sensitive to changes in extracellular potassium concentration but is relatively unaffected by changes in extracellular chloride. To understand how this comes about it is useful to consider the consequences of such changes in the model cell.

...

The increase in extracellular potassium reduces the concentration gradient for outward potassium movement, while initially leaving the electrical gradient unchanged. As a result there will be a net inward movement of potassium ions. As positive charges accumulate on its inner surface, the membrane is depolarized. This, in turn, means that chloride ions are no longer in equilibrium, and they move into the cell as well. Potassium and chloride entry continues until a new equilibrium is established, with both ions at a new concentration ratio consistent with the new membrane potential, in this example -68 mV.

...

Similar considerations apply to changes in extracellular chloride concentration, but with a marked difference: When the new steady state is finally reached, the membrane potential is essentially unchanged. ... Chloride leaves the cell, depolarizing the membrane toward the new chloride equilibrium potential (-68 mV). Potassium, being no longer in equilibrium, leaves as well. ... Because the intracellular concentration of potassium is high, the fractional change in concentration produced by the efflux is relatively small. However, the efflux of chloride causes a sizable fractional change in the intracellular chloride concentration, and hence in the chloride equilibrium potential. As chloride continues to leave the cell, the equilibrium potential returns toward its original value. The process continues until the chloride and potassium equilibrium potentials are again equal and the membrane potential is restored.

But if efflux of chloride changes chloride's equilibrium potential a lot because there's not that much of it, then the same must be true for an influx of chloride when potassium is introduced outside the cell. I don't understand the book's explanation for this phenomenon.

Later, when the book explains the constant field equation using potassium and sodium, it says

How do these considerations apply to chloride? As for all other ions, there must be no net chloride current across the resting membrane. As already discussed, chloride is able to reach equilibrium simply by an appropriate adjustment in internal concentration, without affecting the steady-state membrane potential.

Would you happen to know what the book is trying to say here? Thanks.
 
  • #4
I googled the passage you quoted and got http://homepage.univie.ac.at/andreas.franz.reichelt/intro2cogsci2/protected/nichols_2001_neuron_ch5.pdf .

Looking at Fig. 5.2 and the accompanying text, in both cases, potassium and chloride leave or enter the cell together leave the cell together, so the change of potassium and chloride concentration in the cell must be the same, if the cell volume is unchanged. However, in one case the cell volume increases, while in the other case the cell volume decreases, since water moves to maintain osmotic balance. After taking into account the different final cell volumes, you should be able to get the final concentrations given in the book.
 
  • #5


This phenomenon is due to the selective permeability of the neural membrane. In a basic model neuron, only potassium and chloride are permeable, meaning they can pass through the membrane and affect the overall membrane potential. When the extracellular concentration of potassium is increased, it can easily pass through the membrane and cause depolarization, as there are more positive ions outside the cell than inside. However, chloride is less permeable and therefore has a smaller impact on the membrane potential. Additionally, the equilibrium potential for chloride is close to the resting membrane potential, meaning that even if the extracellular concentration is reduced, the movement of chloride ions will not significantly affect the overall potential. This is why when sodium permeability is added to the model, chloride can be ignored as it does not play a major role in regulating the membrane potential. Overall, the selective permeability of the neural membrane and the different equilibrium potentials of different ions contribute to the observed effects on the membrane potential.
 

1. What is the role of extracellular ion concentrations in determining the neural membrane potential?

The extracellular ion concentrations play a critical role in determining the neural membrane potential, which is the difference in electrical charge between the inside and outside of a neuron. This potential is essential for the proper functioning of nerve cells and the transmission of signals throughout the nervous system. The concentration of ions, such as sodium, potassium, and chloride, in the extracellular fluid directly affects the movement of these ions across the neural membrane, thus influencing the membrane potential.

2. How do changes in extracellular ion concentrations affect the neural membrane potential?

Any changes in extracellular ion concentrations can significantly impact the neural membrane potential. For example, an increase in extracellular potassium concentration can lead to a depolarization of the membrane, making it more likely for the neuron to fire an action potential. On the other hand, a decrease in extracellular sodium concentration can result in hyperpolarization, making it less likely for the neuron to fire. Therefore, maintaining the proper balance of extracellular ion concentrations is crucial for the normal functioning of neurons.

3. What mechanisms control extracellular ion concentrations in the body?

The body has various mechanisms in place to regulate extracellular ion concentrations. These include active transport processes, such as sodium-potassium pumps, which actively move ions across the neural membrane. Additionally, the body can also regulate ion concentrations through diffusion, where ions move from areas of high concentration to areas of low concentration. The kidneys also play a vital role in maintaining extracellular ion balance by filtering and reabsorbing ions in the blood.

4. How do neural membrane potentials contribute to nerve signaling?

Neural membrane potentials are crucial for nerve signaling as they allow neurons to communicate with each other and transmit signals throughout the nervous system. When a neuron receives a stimulus, it triggers a change in the membrane potential, leading to an action potential. This electrical signal travels along the neuron and can trigger the release of neurotransmitters, which then transmit the signal to the next neuron. In this way, neural membrane potentials play a vital role in the rapid communication between neurons.

5. What factors can disrupt extracellular ion concentrations and the neural membrane potential?

Several factors can disrupt extracellular ion concentrations and the neural membrane potential. These include certain diseases, such as kidney disease or electrolyte imbalances, which can affect the body's ability to regulate ion levels. Additionally, toxins, drugs, or injuries can also disrupt the normal functioning of neurons and alter extracellular ion concentrations. It is important to maintain a healthy diet and lifestyle to ensure proper regulation of extracellular ion levels and maintain a stable neural membrane potential.

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