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  • In the last video, I showed you what a neuron looked like

  • and we talked about the different parts of a neuron,

  • and I gave you the general idea what a neuron does.

  • It gets stimulated at the dendrites-- and the

  • stimulation we'll talk about in future videos on what

  • exactly that means-- and that that impulse, that

  • information, that signal gets added up.

  • If there's multiple stimulation points on various

  • dendrites, it gets added up and if it meets some threshold

  • level, it's going to create this action potential or

  • signal that travels across the axon and maybe stimulates

  • other neurons or muscles because these terminal points

  • of the axons might be connected to dendrites of

  • other neurons or to muscle cells or who knows what.

  • But what I want to do in this video is kind of lay the

  • building blocks for exactly what this signal is or how

  • does a neuron actually transmit this information

  • across the axon-- or really, how does it go from the

  • dendrite all the way to the axon?

  • Before I actually even talk about that, we need to kind of

  • lay the ground rules-- or a ground understanding of the

  • actual voltage potential across the

  • membrane of a neuron.

  • And, actually, all cells have some voltage potential

  • difference, but it's especially relevant when we

  • talk about a neuron and its ability to send signals.

  • Let's zoom in on a neuron's cell.

  • I could zoom in on any point on this cell that's not

  • covered by a myelin sheath.

  • I'm going to zoom in on its membrane.

  • So let's say that this is the membrane of the

  • neuron, just like that.

  • That's the membrane.

  • This is outside the neuron or the cell.

  • And then this is inside the neuron or the cell.

  • Now, you have sodium and potassium

  • ions floating around.

  • I'm going to draw sodium like this.

  • Sodium's going to be a circle.

  • So that's sodium and their positively charged ions have a

  • plus one charge and then potassium, I'll draw them as

  • little triangles.

  • So let's say that's potassium-- symbol for

  • potassium is K.

  • It's also positively charged.

  • And you have them just lying around.

  • Let's say we start off both inside and

  • outside of the cell.

  • They're all positively charged.

  • Sodium inside, some sodium outside.

  • Now it turns out that cells have more positive charge

  • outside of their membranes than

  • inside of their membranes.

  • So there's actually a potential difference that if

  • the membrane wasn't there, negative charges would want to

  • escape or positive charges or positive ions

  • would want to get in.

  • The outside ends up being more positive, and we're going to

  • talk about why.

  • So this is an electrical potential gradient, right?

  • If this is less positive than that-- if I have a positive

  • charge here, it's going to want to go to the less

  • positive side.

  • It's going to want to go away from the

  • other positive charges.

  • It's repelled by the other positive charges.

  • Likewise, if I had a negative charge here, it'd want to go

  • the other side-- or a positive charge, I guess, would be

  • happier being here than over here.

  • But the question is, how does that happen?

  • Because left to their own devices, the charges would

  • disperse so you wouldn't have this potential gradient.

  • Somehow we have to put energy into the system in order to

  • produce this state where we have more positive on the

  • charge of the outside than we do on the inside.

  • And that's done by sodium potassium pumps.

  • I'm going to draw then a certain way.

  • This is obviously not how the protein actually looks, but

  • it'll give you a sense of how it actually pumps things out.

  • I'll draw that side of the protein.

  • Maybe it looks like this and you'll have a sense of why I

  • drew it like this.

  • So that side of the protein or the enzyme-- and then the

  • other side, I'll draw it like this.

  • It looks something like this, and of course the real protein

  • doesn't look like this.

  • You've seen me show you what proteins really look like.

  • They look like big clusters of things, hugely complex.

  • Different parts of the proteins can bond to different

  • things and when things bond to proteins, they change shape.

  • But I'm doing a very simple diagram here and what I want

  • to show you is, this is our sodium potassium pump in its

  • inactivated state.

  • And what happens in this situation is that we have

  • these nice places where our sodium can bind to.

  • So in this situation, sodium can bind to these locations on

  • our enzyme or on our protein.

  • And if we just had the sodiums bind and we didn't have any

  • energy going into the system, nothing would happen.

  • It would just stay in this situation.

  • The actual protein might look like something crazy.

  • The actual protein might be this big cloud of protein and

  • then your sodiums bond there, there, and there.

  • Maybe it's inside the protein somehow, but still, nothing's

  • going to happen just when the sodium bonds on this side of

  • the protein.

  • In order for it to do anything, in order for it to

  • pump anything out, it uses the energy from ATP.

  • So we had all those videos on respiration and I told you

  • that ATP was the currency of energy in the cell-- well,

  • this is something useful for ATP to do.

  • ATP-- that's adenosine triphosphate-- it might go to

  • some other part of our enzyme, but in this diagram maybe it

  • goes to this part of the enzyme.

  • And this enzyme, it's a type of ATPase.

  • When I say ATPase, it breaks off a phosphate from the ATP--

  • and that's just by virtue of its shape.

  • It's able to plunk it off.

  • When it plunks off the phosphate, it changes shape.

  • So step one, we have sodium ions-- and actually, let's

  • keep count of them.

  • We have three sodium-- these are the actual ratios-- three

  • sodium ions from inside the cell or the neuron.

  • They bond to pump, which is really a protein that crosses

  • our membrane.

  • Now, step two, we have also ATP.

  • ATP gets broken into ATP plus phosphate on the actual

  • protein and that changes the shape.

  • So that also provides energy to change pump's shape.

  • Now this is when the pump was before.

  • Now after, our pump might look something like this.

  • Let me clear out some space right here.

  • I'll draw the after pump right there.

  • And so this is before.

  • After the phosphate gets split off of the ATP, it might look

  • something like this.

  • Instead of being in that configuration, it opens in the

  • other direction.

  • So now it might look something like this.

  • And of course it's carrying these phosphate groups.

  • They have a positive charge.

  • It's open like this.

  • This side now looks like this.

  • So now the phosphates are released to the outside.

  • So they've been pumped to the outside.

  • Remember, this is required energy because it's going

  • against the natural gradient.

  • You're taking positive charge and you're pushing them to an

  • environment that is even more positive and you're also

  • taking it to an environment where there's already a lot of

  • sodium, and you're putting more sodium there.

  • So you're going against the charge gradient and you're

  • going against the sodium gradient.

  • But now-- I guess we call it step three-- the sodium gets

  • released outside the cell.

  • And when this changes shape, it's not so good at bonding

  • with the sodium anymore.

  • So maybe these can become a little bit different too, so

  • that the sodium can't even bond in this configuration now

  • that the protein has changed shape due to the ATP.

  • So step three, the three Na plusses, sodium ions-- are

  • released outside.

  • Now once it's in this configuration, we have all

  • these positive ions out here.

  • These positive ions want to get really as far away from

  • each other as possible.

  • They'd actually probably be attracted to the cell itself

  • because the cell is less positive on the inside.

  • So these positive ions-- and in particular, the potassium--

  • can bond this side of the protein when it's in this-- I

  • guess we could call it this activated configuration.