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

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En provenance du cours de Duke University

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.

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Electricity in Solutions

This week's theme focuses on the foundations of bioelectricity including electricity in solutions. The learning objectives for this week are: (1) Explain the conflict between Galvani and Volta; (2) Interpret the polarity of Vm in terms of voltages inside as compared to outside cells; (3) Interpret the polarity of Im in terms of current flow into or out of a cell.; (4) Determine the energy in Joules of an ordinary battery, given its specifications; (5) State the “big 5” electrical field variables (potentials, field, force, current, sources) and be able to compute potentials from sources (the basis of extracellular bioelectric measurements such as the electrocardiogram) or find sources from potentials.

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

Biomedical Engineering, Pediatrics

Hello again.

This is Roger for the Bioelectricity course.

We're in section 1, subsection 8.

In this subsection, we just had some arbitrary, but

still realistic potentials in, to our nerve fiber in the core conductor model,

and then gets some voltages associated with it.

Let's suppose we first measure, that is to say,

in concept measure, potentials at all the points A through F.

We would do that by first attaching the negative lead to have a volt meter to

a reference point.

The reference point can be any convenient point.

It doesn't have to be a particular one.

It is most commonly taken to be a point outside the fiber.

And at some convenient distance away from the fiber, but is the,

often the major consideration is that it, they mechanically stable and

easy to hold it in place.

So the negative lead is attached to that place and

kept attached to that place, and then the positive lead, I've marked it here in red,

is moved around from A to B to C to D to E to F,

taking a measurement at each site in turn.

Now, since this is simply an example, I have made up some examples of

potentials that might be found at each of these different locations.

So, these should be potentials phi, at all these locations.

If I were to move my red point over to the reference point,

I would measure a value of zero.

When I move the red arrow to point A, let's say I get minus 20 millivolts.

You notice that it's a minus value, we can have either minus or plus values,

negative or positive values.

Similarly if I now move my red, my arrow to point D,

I get a value of minus 1 millivolts.

So in general the magnitudes outside are lower than the magnitudes inside, but

that's not always the case.

Going down through each of the points, we get this table of values,

I've imagined, and we'll use them for illustration.

Potential fields or

fields that one might measure experimentally, often people do that.

They also are conceptual entities.

People often refer to what's there as if they had measured it,

even if they have not.

Knowing the potential field, we're in a position to find the transmembrane and

axial voltages.

First let's do the transmembrane voltage, if we ask,

what is the transmembrane voltage between point A and point D?

We have that in the equation on the right.

The voltage between those two points, V m,

is phi A minus phi D.

V m is always measured with the inside positive, minus the corresponding outside.

Putting in the number values from the previous table, we have Vm is minus 20,

minus, minus 1.

So Vm is, minus 19.

There's something to notice here.

In this example, as in many examples, Vm is approximately the same

as as is, is the value of phi at A.

That is to a first approximation.

You can say, well, phi D.

We'll ignore that because phi D is about equal to zero.

So that means Vm and phi A are equal.

It's true in this example.

It's true in many other examples, but it's not true in every case.

So you have to be careful.

If we now look at the axial voltage, we say what is the voltage at point A,

relative to point A, point B.

So Vab, that voltage, phi at A minus phi at B.

Notice the polarity.

Minus 20, minus, minus 60, or positive 40 moll volts.

That's a useful value because it helps us understand what is happening,

moving down the fiber in an electrical sense.

With these voltages present,

electrical energy would be given to charges when they move to the right.

So that concludes this section.

We've found some voltages from the potentials.

We'll make use of this in the next section.

Thank you for watching and listening.

I'll see you again soon.

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