Electrical Excitability and Action Potentials

[mimosa2411]

Hello,
I find it very hard to read and understand the current panel from my physiology textbook(I have attached a the picture).

What I have learned in physiology is when positive ions flow out of a cell it is called "a positive current"(also when negative ions enter the cell) or when positive ions enter the cell it is called " a negative current"(also when negative ions leave the cell). It is a definition we have learned.


In the attached picture:
(i) When there is a hyperpolarization from -80mV to -140 mV, the current is negative which is right because the potassium ions will flow out of the cell. What I don't understand is the terms. Why is the negative current called an "inward current"? The potassium ions is flowing out therefore it will be more logic to me that the current have to be called ""outward current" instead?

And the opposite situation will be for the figure of the depolarization.

Hope that you will help a physiology dummy 🙂

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[Whats up Doc!]

Hello,
I find it very hard to read and understand the current panel from my physiology textbook(I have attached a the picture).

What I have learned in physiology is when positive ions flow out of a cell it is called "a positive current"(also when negative ions enter the cell) or when positive ions enter the cell it is called " a negative current"(also when negative ions leave the cell). It is a definition we have learned.


In the attached picture:
(i) When there is a hyperpolarization from -80mV to -140 mV, the current is negative which is right because the potassium ions will flow out of the cell. What I don't understand is the terms. Why is the negative current called an "inward current"? The potassium ions is flowing out therefore it will be more logic to me that the current have to be called ""outward current" instead?

And the opposite situation will be for the figure of the depolarization.

Hope that you will help a physiology dummy 🙂

So K has a resting membrane potential of -85 mV, what that means is if K had its own way the membrane would be -85 instead of ~-70 (resting membrane potential). So first a membrane potential of -140 mV could not possibly be set up by K rushing out of the cell. Secondly the definition of Inward current is the flow of positive charge into the cell (BRS Physiology 5th Edition), furthermore inward current depolarizes the membrane potential. Judging by the diagrams this looks like Boron & Boulpaep (B&B) is that right?

 

[mimosa2411]

Exactly this is Boron & Boulpaep (B&B).

 

[mimosa2411]

Thank you for the answer! 🙂

 

[Reckoner]

(i) When there is a hyperpolarization from -80mV to -140 mV, the current is negative which is right because the potassium ions will flow out of the cell. What I don't understand is the terms. Why is the negative current called an "inward current"? The potassium ions is flowing out therefore it will be more logic to me that the current have to be called ""outward current" instead?

The short answer is it's done by convention, based on how these recordings work. Inward current means positive charge moving into a cell. I like to remember it like this: you have an electrode outside the neuron to monitor electrical changes going on. When positive charge flows into the cell, it moves away from that electrode, so it gets recorded as negative current. The opposite is true when cations flow out of the cell and toward the electrode. That's a big oversimplification, but it works as a memory device.

What they're showing in A is that artificially hyperpolarizing the cell from -80 to -140 causes no ionic current. Ionic current meaning the flux of ions across the membrane. The little spike you see is a capacitative current, which basically means ions are redistributing on to or off of the surface of the membrane without actually crossing it. You can imagine it as more positive charges rushing to the outside surface of the membrane because all of the sudden the inside of the cell is a lot more negative and those extracellular cations want in. The electrode outside the cell "sees" that as a very brief movement of positive ions toward the cell (i.e. away from the electrode), so it records a small inward spike.

In B, they're artificially depolarizing the cell. Once again you see a very quick capacitative current as ions move on/off the membrane, but this time you also get ion channels opening and ions flowing across the membrane, which leads to the inward (sodium) followed by outward (potassium) current. Hope that helps.

 

[Whats up Doc!]

No problem and yeah B&B can be really difficult at times overall its a great book but I would recommend buying BRS Physiology 5th edition, its like spark notes for physiology and its amazing how much better some of their explanations are and its pretty cheap.

 

[mimosa2411]

Thank you I will immediately try to find this book! 😀

 

[mimosa2411]

The short answer is it's done by convention, based on how these recordings work. Inward current means positive charge moving into a cell. I like to remember it like this: you have an electrode outside the neuron to monitor electrical changes going on. When positive charge flows into the cell, it moves away from that electrode, so it gets recorded as negative current. The opposite is true when cations flow out of the cell and toward the electrode. That's a big oversimplification, but it works as a memory device.

What they're showing in A is that artificially hyperpolarizing the cell from -80 to -140 causes no ionic current. Ionic current meaning the flux of ions across the membrane. The little spike you see is a capacitative current, which basically means ions are redistributing on to or off of the surface of the membrane without actually crossing it. You can imagine it as more positive charges rushing to the outside surface of the membrane because all of the sudden the inside of the cell is a lot more negative and those extracellular cations want in. The electrode outside the cell "sees" that as a very brief movement of positive ions toward the cell (i.e. away from the electrode), so it records a small inward spike.

In B, they're artificially depolarizing the cell. Once again you see a very quick capacitative current as ions move on/off the membrane, but this time you also get ion channels opening and ions flowing across the membrane, which leads to the inward (sodium) followed by outward (potassium) current. Hope that helps.

You are amazing! Thank you very much!