Rectification of Names

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From the course by Duke University

Bioelectricity: A Quantitative Approach

33 ratings

Duke University

Bioelectricity: A Quantitative Approach

33 ratings

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

From the lesson

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.

Introduction to Week 10:30

What is the Question5:35

About Bioelectricity5:14

Major Sections of the Course5:52

Rectification of Names10:11

Ions in Solution6:55

Core-Conductor Model of a Nerve Fiber9:02

Potential and Voltages in the Fiber5:31

Axial Currents in the Fiber5:08

Membrane Resistance3:53

Membrane Current, Failure & Mystery3:34

Week 1 in Review4:29

Meet the Instructors

Dr. Roger Barr
Anderson-Rupp Professor of Biomedical Engineering and Associate Professor of Pediatrics
Biomedical Engineering, Pediatrics

Hello again.

This is Roger Koch Barr, for the Bioelectricity course.

This is section 1, subsection 5,

which I have called in a flowery fashion the Rectification of Names.

You might call it Terminology.

But, that wouldn't sound as good, so I thought, I'll call it

Rectification of Names, and you would enjoy knowing about that phrase too.

Rectification of Names is a phrase, taken from the Confucian doctrine,

that social harmony occurs by using the proper designations for each thing.

In Bioelectricity we have the problem,

that we are dealing with invisible objects.

Accordingly, we are very dependent on having common terminology, so

that we can talk to each other about the same thing.

Even though we can't actually see it.

So, in this section, I'll mention a few things about the terminology,

that are important to know.

And important to use in the same way, that everybody else does.

So, there's both termination in a few basic facts.

First let's talk about Membranes.

Memebranes are very thin material that forms the covering of cells.

Memebranes of made of two layers, of lipid molecules.

There's a picture here.

It's very thin.

The thickness of the membrane overall, is about Angstroms.

So that's not very much.

Of that thickness about half of it.

Is an electrical insulator, so, that's about 40 angstroms approximately.

If you want to imagine, a lipid bilayer do the following experiment,

do it first in your imagination and then you can just try it if you want to.

Get some olive oil from the grocery store, or from your kitchen cabinet.

Take one drop of olive oil, and put it in the top of a glass of water.

Watch the olive oil spread out, and it will.

And if you look at it very carefully, maybe have to look from the side but

if you put the light just a certain way, you can see that it spreads out,

only so far, and then it stops.

That's a lipid bilayer, and

the thickness of that, coating floating on the surface of the water.

This how thick a lipid bilayer is.

Well, you can see it or see it's reflection there on the water, but,

when you look at it, it is very, very thin.

That is not simply like a lipid bilayer, it is a lipid bilayer.

Now, within that very thin structure, are embedded some other mechanisms.

These are called Channels and Pumps.

And there are other things as well.

Channels are tiny tunnels, that go through the Membrane.

They're important,

because charged ions can use them to go from one side to the other.

Pumps are more complicated structures in the Membrane.

They move ions across the Membrane, using energy that's derived from food.

They use that energy to move the ion's to a place where they don't want to go

all by themselves.

That is to say they move them from regions of lower concentration,

to regions of higher concentration.

These Pumps are vital to every one of us, and

each of us has millions of such pumps operation.

Within our Membranes right now.

Channels and Pumps are very specialized, and that means they

are structured in such a way as to be matched up

to a particular kind of ion such as sodium ions or potassium ions.

And it's important for both Channels and Pumps,

to realize that these are actually within the Membrane, not inside the Membrane.

And in that sense, our Railroad analogy, is not a good one.

Because when we look at the Railroad engine.

We'd say the engine is inside, of the covering that exists around the engine.

But with Channels and Pumps, the core elements of the motor,

they exist within the cover itself, it's amazing.

[SOUND] Often in working with electrically active membrane,

one talks about the Transmembrane Voltage.

This is a concept as well as a measurement.

The idea of the Transmembrane Voltage, is that you have two electrodes.

One electrode, which I've diagrammatically shown as B,

is on [COUGH] one side of the Membrane.

The other's on the other.

If B is on the inside.

A is on the outside and the Transmembrane Voltage,

is always the potential of B minus the potential at A.

That is to say the positive measurement is the one inside, and

the negative measurement is the one outside.

That is shown in a more elaborate fashion in this diagram.

The positive measurement is made from the electrode inside, and

the negative measurement from the electrode outside.

Here are two terms that you hear used widely in Bioelectricity.

It's worth taking a moment to discuss them.

What is the difference between Passive and Active?

Is this the same, as the difference between dead and alive?

Well, there's some similarities.

But no, it doesn't mean the same thing.

Passive simply means, the same quantities,

such as the resistance are existing over time without change.

Active means that following some trigger, or

in the presence of some trigger, the properties may change rather dramatically.

Now, suppose you imagine an alligator.

I've watched alligators before, growing up.

Alligators, can sit there a long time doing nothing or, at least,

you can't tell they're doing anything.

They are passive.

None of their visible properties, at least, are changing over time.

On the other hand, given the right stimulus, the right food,

the right object in their proximity, alligators can become active very quickly.

Alligators can run fast, they can bite hard, and

the change from Passive to Active.

Occurs, it seems, at the blink of an eye.

So an alligator, when Passive, is not changing with time, but

the alligator is not dead.

I recommend to you, that if you see a Passive alligator, you keep your distance.

Because the alligator can change from its Passive state,

to its Active state very quickly.

So the same terms are used, for electrically reactive tissue.

Sometimes it's Passive, meaning its properties are not changing with time.

Sometimes it's Active.

Meaning its properties are changing with time, just like the alligator moving.

But in all cases, the tissue is still alive.

Now, dead tissue is Passive.

But, living tissue may be either Passive or Active.

So, no, the terms are not the same.

It is, Passive versus Active is not, the same as dead versus alive.

Changes can be quite dramatic.

If you think of a resistor,

we're accustomed to resistors having constant values.

But, in a Biomedical, Bioelectric system,

you might have a resistor that's a million ohms, but then changes to be 1000.

And then changes back to be a million.

So, that's what's meant to be Active.

Conclude this discussion, with a picture of the Duke University chapel steps.

See you again, in the next subsection.

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