Nerves, the heart, and the brain are electrical. How do these things work? This course presents fundamental principles, described quantitatively.

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Bioelectricity: A Quantitative Approach

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Nerves, the heart, and the brain are electrical. How do these things work? This course presents fundamental principles, described quantitatively.

À partir de la leçon

Energy into Voltage

This week we will examine energy, by which pumps and channels allow membranes to "charge their batteries" and thereby have a non-zero voltage across their membranes at rest. The learning objectives for this week are: (1) Describe the function of the sodium-potassium pump; (2) State from memory an approximate value for RT/F; (3) Be able to find the equilibrium potential from ionic concentrations and relative permeabilities; (4) Explain the mechanism by which membranes use salt water to create negative or positive trans-membrane voltages.

- Dr. Roger BarrAnderson-Rupp Professor of Biomedical Engineering and Associate Professor of Pediatrics

Biomedical Engineering, Pediatrics

Hello again.

This is Roger Kochbar talking about bioelectricity, and we are at week two,

segment seven.

I thought it might be useful in this segment to ask a few questions,

first for me and then for you.

Let's look at the ionic concentrations for frog muscle.

What is the Nernst potential for potassium?

What is the Nernst potential for sodium?

And what is the difference?

Now I think earlier in the lectures we were computing these values for

the concentrations present in the nerve of the squid.

But now we'd like to do them in the muscle of the frog.

So first of, what will be the equilibrium potential for

potassium using the concentrations that are present in the frog?

That's normally denoted EK, so I'll write it that way.

That will be RT over F times the logarithm.

Concentration, Extracellular.

Concentration Intracellular.

Now I've written LN as the logarithm so as to emphasize that this is the natural log,

not the log base ten or some other log.

So, when we compute it, we'll compute it that way.

RT over F, we've studied many times up until now.

So we know that's 26 millivolts.

So now the logarithm.

Extracellular over Intracellular.

So that will be 2.2 over 124.

Units aren't important here, as long as they are the same units.

So I won't write those in.

If you look, if you think about taking the logarithm of 2.2 over 124,

it's a number less than 1.

So you know right away that this is going to be equal to minus 26 millivolts.

Times the logarithm of 124 divided by 2.2.

I'll leave that up to you to complete the calculation.

If we ask the same question for sodium, then the question is

answered by substitution of now the sodium concentrations for frog.

4 and 109 into the same basic equation.

The equilibrium potential now is denoted ENa.

It's the trans-membrane potential at equilibrium for sodium,

so it's denoted ENa and that'll be RT over F algorithm.

Na Extracellular, NA Intracellular.

I'll leave it up to you to determine the actual number, but

we're expecting a positive number this time.

Because NaE is greater than NaI.

If now we ask, ask part C of the question, what is the change?

Well, that's easily obtained.

That's simply the value that we obtained in the previous two slides, ENa minus EK.

So we found those, and now you'll fill this and you'll fill this in and

you'll determine the shift.

Thank you very much.

We'll move on to the next segment after looking at

this picture of the beautiful Duke University Library.

Thank you.

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