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00:00 | Right away. This is lecture seven . It's Thursday September 15 and we're |
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00:11 | continue talking about the action potential. we're also gonna talk about membrane properties |
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00:17 | more. So we already talked about membrane properties, membrane potential addressed and |
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00:24 | introduce the action potential. And I that there are several concepts that I |
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00:29 | like for you to know about when talk about membrane potentials and these are |
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00:39 | . Right? So number and equivalent . And that's the first topic that |
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00:43 | going to discuss today. We call you have this diagram to follow up |
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00:49 | the action potentials and you have all your notes in the lecture notes. |
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00:54 | so uh this is from the following lecture six and seven marked in your |
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01:02 | . This is from the following lecture . Last lecture. Now, what |
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01:08 | seeing here is some of the things we've discussed in the past, but |
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01:14 | didn't discuss the circuits behind or representation those things within the membrane equivalent |
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01:21 | So, what we talked about is said, look if there is a |
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01:27 | where you have electrical equipment and that equipment is an electorate, a micro |
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01:33 | , that micro electorate can inject We talked about current injections. So |
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01:38 | like a switch, you can turn the electrode activity and you can turn |
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01:43 | off and you can do it very . Okay, so you have current |
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01:46 | that can produce these currents into the . And we discuss that these currents |
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01:52 | square wave like and that indicates that zero. You turned on the switch |
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01:58 | you produce the square wave like Okay and that stimulation is coming from |
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02:05 | electrode. This is the micro And let's say this micro electrode is |
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02:09 | a positive charge into the neuron that attached to. But this is |
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02:16 | So instruments. Okay instruments if you on the current will produce the square |
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02:23 | like positive current on. Okay switch switch off. However what you see |
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02:33 | is the cellular member in response to current. Whether it's positive D. |
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02:38 | in this direction or it is hyper . So the response from the south |
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02:45 | the square wave like current is The response from the cell has a |
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02:52 | bit of delay before it reaches the and when the signal stops you can |
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02:59 | it also takes time for this membrane . This is cell versus instrumentation. |
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03:08 | response has this kind of a rounded before it reaches the maximal deep polarization |
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03:14 | the maximal change in the membrane And also it takes time for it |
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03:20 | re polarize. So after the stimulus . There is a delay before you |
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03:26 | full re polarization and these are the of the cells that they have and |
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03:33 | properties are resistance and capacitance. Okay you look at the plasma membrane, |
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03:40 | top diagram represents plasma membrane with potassium . So each one of the potassium |
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03:50 | . Oh maybe I shouldn't be using marker. It's a little bit too |
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03:56 | . We can also should have the in zoom. It's okay, let |
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04:04 | see if I can. A Yeah, I'm probably gonna hear about |
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04:14 | the department chair something just kidding. is my one. Let me just |
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04:25 | this uh little household issues going on a little clean up following it. |
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04:34 | when you look at this diagram in and the slide on the top, |
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04:39 | talked about the fact that you have membrane and that plasma membrane is not |
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04:44 | for the ions to freely cross. it's basically it's resistant. Okay to |
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04:50 | flux of current. And that current if you have channels in the plasma |
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04:56 | and those channels are open. So you think about these channels, those |
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05:02 | and the membrane equivalent circuits can be and things for example like this is |
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05:11 | symbol for resistor. For resistance. . Uh if you recall, resistance |
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05:22 | the inverse of conductance. So this also a symbol for a conductor. |
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05:29 | G or r if you see an going through it, that means it's |
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05:34 | variable resistor, variable conductor. And what you will learn today, sometimes |
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05:40 | channels are open and sometimes they're starting close and during those moments when they're |
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05:45 | open, the conductance is quite high maximal. But then if you're starting |
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05:51 | close the channel, that conductance starts . So there's variability in the conductance |
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05:57 | on the opening and kinetic properties of channels in the membrane. The other |
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06:03 | is you can see this symbol is symbol for battery. Okay, because |
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06:16 | talked about the electro motive force and talked about the fact that ions are |
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06:20 | . So you can see that there a battery sodium, you have a |
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06:24 | distribution of sodium on the outside versus the potassium which is dominating on the |
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06:30 | . And so you have this battery here. So each one of these |
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06:34 | , not just potassium channels, but one of these sodium potassium calcium chloride |
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06:39 | can be represented by these member and circuits which are built by basically comm |
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06:46 | resistors, batteries. And talking about of the other subject matters that we've |
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06:53 | discussed. If you remember this electrochemical force, we called driving force, |
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07:00 | is electrochemical driving force. We talked the sports being the difference between VM |
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07:07 | the equilibrium potential for in this case mine that it can be equilibrium potential |
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07:11 | sodium minor. The difference in the between the Librium potentially to measure with |
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07:17 | equation and the number of potential which measure with Goldman equation will determine the |
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07:23 | force and the size of this driving . So this is the same arms |
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07:27 | . V equals ir except that it D. M minus E. |
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07:32 | E equals Ir. The change of driving force and the voltage. We |
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07:36 | talked about conductance being the inverse of . If we rewrite arms law I |
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07:44 | B. Over R. And Is equal one over R. That |
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07:49 | i. Is equal conductance is the force. And we talked about it |
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07:54 | we started discussing the rising phase and following phase of the action potential and |
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07:59 | you through and I said that the flocks is the current flocks is conducting |
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08:07 | the driving force. So even if channel is open is conducting but there's |
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08:11 | driving force the driving force is The current value is still going to |
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08:16 | zero, current is not moving in one direction. Netting into in and |
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08:21 | of the south. So you have of these channels and each of these |
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08:27 | have their own conductance. So you rewrite this I. K. |
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08:32 | Tansy conductance minus conductance times D. . And you can also imagine that |
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08:40 | you know the individual channel current And know the conductance is for individual channels |
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08:48 | you will say well I want to the overall conductance for the cell for |
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08:53 | of the potassium channels. So you overall conductance for potassium is calculation where |
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09:01 | have the number of potassium channels. depends 200 500 channels times the individual |
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09:08 | through each channel. Pretty basic. you have to basically calculate the overall |
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09:15 | versus the conductance through one channel, calculate overall conductance for all of these |
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09:21 | . So let's look at how the response. We already talked about this |
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09:28 | let's look about here how the input RN what it depends on. It |
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09:37 | on the wrestling channel density. So many channels there are. And membrane |
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09:43 | area. And the resistance to the is the resistance of the membrane over |
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09:53 | pi A squared. Where A. the radius of the spherical neuron. |
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09:59 | means that the smaller the neuron, smaller the A. The larger the |
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10:05 | the systems. So if you increase A the larger cells the larger cells |
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10:12 | have smaller input resistance. So resistance radius here. The relationship for capacitance |
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10:23 | what are some of the features of capacitor? For capacitance? The symbol |
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10:29 | capacitor is this symbol here, It's to the battery but two plates are |
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10:38 | in size should be when capacitor is with charge storage as is shown |
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10:49 | And what are the features of a capacitor? The features of a good |
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10:55 | that it can store a lot of to store a lot of charge. |
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10:58 | need surface area. You have a of surface area. The other good |
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11:04 | of a good capacitor is that the plates of the capacitor that hold the |
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11:09 | and negative charge are located very close each other. So the closer they |
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11:15 | , the faster that discharge can happen exchange of charge. Okay. Otherwise |
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11:20 | they're separated by several layers. And you have separation of the capacitor by |
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11:26 | a possible lipid bi layer, The membrane. So cell membranes are really |
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11:32 | capacitors and if you look at this year, the capacitance of the cell |
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11:38 | on the capacities on the membrane times by a square. So in this |
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11:45 | the larger than your on the larger input capacity. So really large cells |
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11:51 | be able to capacity state to store lot of charge based on the surface |
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11:57 | and smaller cells will have smaller Then we come to this last uh |
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12:06 | here on the right and I'm going discuss that in a second but I |
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12:14 | to finish talking about member an equivalent . So what you can do is |
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12:18 | can represent the plasma membrane using these and equivalent circuits. The resistors of |
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12:24 | and the batteries. This is for showing the active activated circuit. And |
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12:32 | does the active circuit have? That's ions? Well, it probably has |
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12:36 | coming in from outside to inside because chloride is abundant, cellular. So |
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12:42 | the current direction of positive ions. course every battery here fluoride is not |
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12:49 | much, let's say during action And you'll see another example why it's |
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12:54 | doing much in a couple of potassium potentially is going in from inside |
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13:00 | out from inside the south to the of the south. You have an |
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13:05 | pump that always works as long as is a T. P. That |
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13:09 | always work. And notice that the here is going in the opposite direction |
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13:13 | going against the concentration gradient to sodium potassium and then finally have this capacity |
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13:20 | . So this is a really good of an active circuit using these |
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13:25 | An equivalent circuits. And there's significant to these. You can use them |
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13:32 | engineering world, you can use it electronics world, you can use it |
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13:36 | computational world. You can build models models and change something about the properties |
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13:44 | these elements. The resistance availability, open there and so on. Change |
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13:49 | things and see how it changes the member and potential. Uh And what |
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13:55 | you see in the membrane potential when altering these terms. So let's go |
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14:01 | to this other slide on top. for the exam, what I would |
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14:06 | for you to know from uh this and equivalent circuits. As you should |
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14:11 | these symbols. You should be able recognize this is a resistor, this |
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14:14 | a capacitor, this is a And no no the circuit for example |
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14:20 | it's not very complicated but it should total sense to you because of everything |
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14:25 | we're talking about resting membrane potential. we're talking about the fluxus through these |
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14:30 | and these channels that we're talking Remember already mentioned these channels are |
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14:38 | The permeability through these channels is And if you recall the permeability during |
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14:46 | member and potential which is uh calculated and potential using portion of the nurse |
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14:57 | and then introducing the premier ability term for potassium including potassium sodium and chloride |
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15:06 | showed potassium and sodium. And I chloride doesn't really have much influence on |
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15:11 | change in the resting membrane potential. so this top line shows that during |
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15:17 | resting or when the cell is not active and not firing an action |
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15:23 | potassium is leaking out of the cells has these leaky channels. And that's |
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15:28 | the way the nature is it has leaky channels and potassium is leaking |
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15:32 | So it's dominating and permeability and it's because equilibrium potential for potassium is |
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15:40 | And the membrane potential addresses about Which is closer much closer to potassium |
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15:46 | than for example sodium the second major . The equilibrium potential for sodium Positive |
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15:53 | positive 60. And this is an of permeability, how the conductors change |
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16:00 | through those channels, how much they conduct based on probabilities it switches where |
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16:07 | that's our sodium becomes 20 times more than potassium. And this is during |
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16:13 | rising phase of the action potential. the third uh line item here is |
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16:22 | from your building and as you can first of all, chloride across this |
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16:31 | , 0.45. And during the rising or the action potential is 0.45. |
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16:38 | doesn't change. So it tells you . Chloride really doesn't contribute the permeability |
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16:44 | chloride doesn't change and doesn't contribute to action potential. And if it has |
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16:50 | contribution to the resting membrane potential it's little. And you can calculate it |
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16:55 | you want to the D. By adding in the chloride outside versus |
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17:00 | of the selling And these permeability ratios you're interested uh the greater concentration of |
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17:07 | particular island and the greater its the greater its role in determining the |
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17:12 | number of potential. So what happens the rising phase of the action potential |
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17:19 | and potential goes up And that's because permeability for sodium has increased and sodium |
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17:24 | ruling the number of potential for just couple of milliseconds in time. |
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17:31 | we finally come to this diagram here is some of the things that we |
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17:39 | about, we'll all come together hopefully the next couple of hours. But |
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17:47 | diagram here that you're seeing is what call an I. V. |
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17:54 | And what you're seeing on the Axis is voltage in millennials membrane potential |
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18:03 | . M. Which stands voltage of memory and on the Y axis you're |
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18:08 | current I which is a nanogram So all of these channels that we're |
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18:16 | about as it relates to rest except leaky channels. They are voltage |
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18:22 | That means that the current flocks and conductance through these channels will depend on |
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18:28 | change in the voltage. They're sensitive voltage. Give it another 30 |
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18:35 | So there are channels that have linear . V. Plot. So I'm |
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18:43 | make this drawing. And this is . M. This is current in |
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18:51 | amperes. Okay this is Miller this is -80, This is -40 |
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19:07 | plus 40 Plus 80 million holes. I don't know if I'm gonna draw |
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19:15 | diagram for you. So you may to draw it with me or take |
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19:19 | picture. Uh But as you see the racing stuff, so maybe I'll |
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19:23 | to take a picture if you stop before I erase everything. So and |
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19:29 | on zoom. Can you see the ? Okay. Yes, we can |
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19:34 | it. Okay, good. Because the best I can do with the |
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19:39 | . Um Okay so here we have one nana one plus 29. After |
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19:47 | here we have minus one and we minus two. And we have a |
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19:56 | of things that are written here. of all, this is hyper |
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19:59 | right? So from resting membrane potential more negative values is hyper polarization. |
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20:06 | resting membrane potential. So resting membrane would be similar around here. This |
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20:12 | resting membrane potential value. This is little the chair. And by convention |
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20:21 | nana emperors measurers and downward deflections. is an inward car. This is |
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20:31 | . Word card and this is outward . This is awkward and what this |
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20:47 | . It actually shows a linear plot this linear plot is referred to as |
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20:59 | or linear ivy plant. I stands current, the stands for voltage. |
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21:09 | it's I. V. Plot. ivy plots are sort of like representations |
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21:16 | how voltage controls current flux through these . S. I. V. |
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21:23 | is a representation how the current depends the changes in the voltage. Just |
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21:32 | same thing. Paraphrase. So this a linear plot that you're seeing |
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21:40 | throw it in here and it's linear we're the same in the change in |
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21:46 | membrane potential and the value of the you always get the same change always |
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21:56 | the same change. And I should drive it to one here unless you |
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22:05 | so one here too, you get same change and in voltage And you |
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22:13 | the same change here also. you get 40 million volts change and |
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22:18 | have negative one and you have positive million volts change and you have positive |
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22:24 | nana empires. Everybody follows this. this is the same on both |
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22:34 | So this is linear I. Plot on the exam. I may |
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22:39 | you a question like this. I'm show you a plot that looks like |
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22:50 | right? Or something similar and I'm ask you to think about something. |
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23:00 | the reversal potential or equilibrium potential for is -90 million volts. Mhm. |
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23:10 | is the driving force? If the of potential is that -90 million balls |
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23:17 | and the equilibrium potential, potassium is v. M. is -90. |
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23:29 | is VM -E. K. equal zero. So this would be a |
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23:40 | of a potassium channel that has linear plot because potassium will be going out |
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23:52 | the cell right once you raise this . And we also call these reversal |
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24:00 | . Because if you were to hyper the plasma member, remember, this |
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24:05 | sort of a normal regime around resting potential and there might be enough of |
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24:12 | negative input. There might be something on with this potential becomes even more |
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24:17 | but it's not going to stay there physiologically the state the state of the |
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24:24 | is to fluctuate around this resting membrane back and forth produce an action potential |
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24:30 | come back to this resting membrane So it's not going to stay in |
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24:35 | negative potentials for a very long time it's more likely to be more and |
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24:40 | hyper polarized negative potentials and it's definitely going to stay in the positive potentials |
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24:45 | a long time. Because if you the cell and have it be polarized |
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24:51 | million balls for a long time, cell will die. So it's non |
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24:56 | . But this is what happens The other plots that I may show |
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25:01 | and recall that whenever I'm talking about values for the reversal or equilibrium potentials |
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25:12 | other such things you should go back this and these are your values I'm |
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25:21 | on the exam follow these values equilibrium for potassium minus 90. I started |
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25:27 | it reversal potential. Also started calling reversal potential because if the cell actually |
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25:32 | polarizes below minus 90 potassium instead of there's a lot of it on the |
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25:40 | instead of cohen going outside is gonna coming back inside. Okay so the |
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25:46 | of the current flow through these channels now. I'll show you another plot |
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25:54 | on the exam question that looks something this and I'm already gonna give it |
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26:00 | by putting this value here. But say it's gonna look like something like |
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26:07 | And I'm gonna ask you do you this is likely to be the potassium |
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26:13 | . V. plot or sodium ivy sodium. And you would answer that |
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26:18 | by first of all seeing where is current? Where is the current |
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26:23 | zero. The current value is zero positive 55 million balls. Which is |
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26:30 | reversal potential for sodium or equilibrium potential sodium. If the cell is addressing |
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26:37 | in potential year sodium channels open, is sodium doing? There's a lot |
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26:43 | sodium on the outside of the sodium is coming inside its inward inward |
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26:48 | And then at positive 55 if you to de polarize or have the cells |
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26:54 | these really positive potentials sodium current would reverse and it would become outward, |
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27:01 | inward, that's why we call equilibrium reversal potential. So it's a real |
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27:08 | of the direction of the flux of ion outside to inside versus inside to |
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27:13 | of the cell. Mhm. So I should take this picture because it's |
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27:19 | to turn into an abstract piece of . Um Okay. How many of |
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27:32 | ivy plots, how many of these gated channels? There's a plasma member |
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27:40 | having one south? Yes. That's a guess for each for each |
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27:48 | and that's a good guess for the ions. However there's and it's a |
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27:53 | good guess but there's variability in these and there are subtypes of both educated |
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27:59 | and one cell may have 34 subtypes both educated sodium channel. Four or |
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28:04 | subtypes of both educated potassium channels and on. So a single cell and |
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28:10 | guess is great because this is the ions that we're talking about but the |
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28:16 | will have variations of this channel. remember we talked about roderick mackinnon and |
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28:21 | said how important amino acid structure is we talked about splice variants and things |
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28:26 | spliced differently sometimes with slight variations in structure. Well that structure of the |
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28:32 | and three dimensional structure is also going determine how much of the ions are |
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28:37 | through, how fast it's opening. it's a channel we actually have |
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28:41 | How fast it's opening. How fast those gates closing the gap? So |
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28:46 | cell may have up to 10, even sometimes 20 different subtypes of voltage |
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28:54 | channels and not all of them are have the linear I. V. |
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28:59 | right? Some of the channels are to conduct their little inwardly but then |
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29:07 | will conduct a lot of it So you can see that this 40 |
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29:12 | volts negative change just produces this level current but positive produces a much higher |
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29:19 | of current. So this is outwardly outwardly rectifying channel. We call it |
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29:25 | rectifying. It prefers to conduct. there are some channels that do the |
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29:32 | . They prefer to conduct a lot current inwardly. And they're not so |
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29:37 | at conducting that current outwardly. So can see that there's a lot of |
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29:43 | here in current. Okay. And very little change if you came up |
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29:50 | , you wouldn't even see this Okay, so this current this current |
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29:55 | to conduct inwardly or inwardly rectifying currents just by definition what they are. |
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30:04 | you will see these kinds of variations I. B. Plots. And |
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30:09 | what do you do with 10 or or 20 different ivy plots? I'm |
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30:16 | gonna use my fantasy here. You have to drive. Yeah this is |
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30:34 | you have. So he's a two cells present in a very abstract ways |
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31:00 | will have multitudes of voltage community channels multitudes of the curves. Some of |
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31:06 | are linear only curves. Some of are non linear. They're rectifying |
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31:11 | But when we talked about the dialect we said that these different subtypes, |
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31:16 | the inhibitory cells that produce such diverse of action potentials. That diversity comes |
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31:25 | the fact that you have different voltage channels expressed by those cells. And |
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31:30 | different voltage gated channels will have slightly properties. These I. D. |
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31:36 | properties and these I. V. properties will play into the overall response |
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31:42 | the cell as it produces. You the stuttering frequencies of action potential. |
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31:46 | bursting or continues are fast or slow so on. So maybe I should |
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31:53 | a picture of this. Okay, Alright, the ivy plots a lot |
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32:18 | the things that we're talking about. they're gonna be coming back up in |
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32:23 | next hour and a half that we still about to cover of the new |
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32:29 | . But in general we want to about action potential. The rising phase |
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32:34 | by sodium, the following phase dominated potassium. So we talked about the |
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32:40 | of the action potential. Okay, I'm gonna before I go here, |
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32:47 | actually gonna go back into your action diagram and I'm gonna one more time |
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32:54 | about it. Everything that's here. should know that everything that's on the |
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33:00 | . UK chloride. RMP wrestling number potential threshold of action potential generation. |
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33:07 | that's happening here in the number of fluctuations. So the cell gets excited |
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33:11 | inputs in the dendrite glutamate. The potentials to de polarize the cell and |
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33:17 | the potential a little bit and then inhibitory inputs come in and they hyper |
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33:22 | yourself. And if there's enough of , excitatory input, so single excitatory |
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33:27 | in the C. N. Produces a deep polarization of about 0.5 |
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33:34 | balls. So one active synapse deep of 0.5 million balls. 10 active |
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33:41 | , deep polarization of five million 20 active synopsis 10 militants, 40 |
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33:50 | synopsis 20 million volts. Which is be enough to drive the cell to |
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33:56 | threshold for action potential generation. So are greater inhibitor excited during inputs inside |
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34:02 | cell. And graded deep polarization and polarization until you reach the spawned and |
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34:08 | the response becomes all or not All none means that you will always produce |
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34:14 | action potential of -45. You can it. People are as the cell |
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34:18 | always produce action potentials. If you the stimulus, it will produce |
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34:23 | Eyes action potentials throughout the sustained stimulus the cell gets exhausted, which does |
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34:29 | to during the continuous stimulus. So we talked about this resting membrane potential |
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34:36 | the south membrane is doing a random . A little bit up, a |
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34:40 | bit down a little bit up a bit down. If it's up and |
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34:43 | the threshold will fire the action Now this stay here, potassium is |
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34:51 | most permissible to plasma member, potassium dominating here at rest. Once we |
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34:58 | the threshold for action potential, we bolt educated sodium channels. Why do |
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35:03 | open them? Because we de polarize south through the synaptic inputs enough into |
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35:10 | positive potentials from negative 65 to It's enough now to open the sodium |
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35:19 | educated channels. So the change in voltage is what opens voltage gated channels |
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35:25 | then the membrane is dominated by leaking conductance. Is once the sodium channels |
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35:33 | sodium takes over sodium with deep polarization more sodium channels which causes more deep |
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35:40 | which opens more sodium channels. So is a positive feedback loop. And |
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35:45 | would say great sodium is dominated and conductance drives the overall D. |
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35:52 | member and potential to its own equilibrium value. It's saying I'm gonna take |
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35:57 | home to Vienna which is positive 55 it fails to do so and fails |
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36:04 | do so for two reasons. There's about the channel kinetics sodium channels when |
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36:09 | open actually very quickly closed and you about this today. Number one. |
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36:13 | channel kinetics number two as the number potential D polarizes. What happens to |
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36:19 | driving force. The driving force is difference between E. N. |
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36:24 | And the number of potential the driving shrinks here for sodium and those are |
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36:29 | two reasons why the peak of the potential does not reach the E. |
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|
36:34 | . A. Or equilibrium potential for . Now at this point when the |
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36:39 | member it is so deep polarized, is a huge driving force for potassium |
|
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36:45 | this is their russell potential for potassium number one, number two sodium channels |
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|
36:52 | started closing because of the kinetics. they open they have to close, |
|
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36:57 | starts ruling the game and potassium selfishly to drive the DM number and potential |
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37:06 | its own equilibrium potential values and it succeeds. So it almost succeeds. |
|
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37:12 | you have this hyper polarization that's lower the firing of action to control the |
|
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37:18 | membrane potential and you have this re that slowly happens. The sodium potassium |
|
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37:24 | slow wear that are working with against gradient with A T. P. |
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37:31 | A. T. P. During area here, under the curve of |
|
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37:37 | action potential, which is pretty much to the threshold with the action |
|
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37:43 | it's an absolute refractory period, which that you cannot devote another action potential |
|
|
37:50 | the absolute refractory period. But during re polarization, when it crosses back |
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|
37:56 | about -50 or so close to this is relative refractory period. And |
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38:02 | you stimulated the cell or the cell an incoming stimulus to continue stimulus, |
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38:08 | can produce another action potential during relative period. But not during the absolute |
|
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38:14 | you cannot have another action potential on of this action potential. So that |
|
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38:19 | doesn't work. There's not the way brain cells work. And some of |
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38:25 | South will have very short relative refractory which will allow them to produce action |
|
|
38:32 | . High frequencies and some of them take time to recover longer and the |
|
|
38:37 | potential frequencies are going to be Mhm. These are the reversal |
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|
38:44 | This is the reversal potential for So in fact if you were at |
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38:49 | membrane potential and you were just talking driving force, what ion has the |
|
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38:57 | driving forces, resting membrane potential? the driving force. It's council. |
|
|
39:04 | Because calcium reversal potential is positive. and plenty. But it's a cell |
|
|
39:10 | permeable to calcium. No. Is a lot of calcium on the |
|
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39:15 | 10,000? More than that. It's permeability is key. Calcium channels are |
|
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39:20 | open or they're not located with sodium potassium channels are. So where you |
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39:26 | action potentials you will have a lot voltage gated sodium and voltage gated potassium |
|
|
39:33 | where you release neurotransmitters, external You will have a lot of voltage |
|
|
39:37 | calcium channels. So this is important cell has its own strategy of placing |
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39:46 | channels, voltage gated channels and receptor channels in different locations along you know |
|
|
39:52 | own cell body and processes. Did have a question. So for the |
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39:59 | relative doesn't harm the cell or having stimulus during that period. No, |
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40:09 | the cell does need time to So you cannot have neuron sustained stimulus |
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40:16 | a long time neuron without starting to some other physiological problems. And the |
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40:24 | have the way to come back and this, you know, shifts to |
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40:29 | potentials back to resting moment of If they lose the ability to maintain |
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40:34 | separation of charge, that's probably something a dysfunction in the channel and now |
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|
40:41 | can have a shift in resting membrane , it becomes more positive and those |
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40:46 | will need less excited to start And this could be a part of |
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|
40:51 | too. It's also part of as I mentioned themselves, will have |
|
|
40:58 | , that's -70 -65. But if took to, you know, equivalent |
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41:03 | circuits same age, the same but one of them had a mutation |
|
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41:08 | channels, you're likely going to see membrane properties change the resting membrane potential |
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|
41:13 | a shift the action potentials. They be faster than maybe slower, depending |
|
|
41:19 | on the on the dynamics of these can be longer, you know, |
|
|
41:24 | say if sodium channel remains open longer action potential would be longer. |
|
|
41:32 | very good questions. Okay, this the undershoot that I'm talking about the |
|
|
41:40 | over drives by dominating the membrane. , another important concept to understand this |
|
|
41:50 | clan. And I mentioned this yesterday this diagram looks scary but it's actually |
|
|
41:59 | . It's pretty old and rudimentary. there isn't gonna be scary questions in |
|
|
42:04 | diagram either. But do we need understand and know what voltage plant |
|
|
42:10 | Absolutely. Because this will really explain us how the action potential that we're |
|
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42:18 | member and potential that we're measuring. can also measure individual currents for sodium |
|
|
42:22 | potassium and to measure individual currents for and potassium and other ions. We |
|
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42:28 | to be able to manipulate the member potential because these channels are vaulted |
|
|
42:34 | So if we could manipulate the membrane , if we could manipulate the voltage |
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|
42:39 | the membrane experimentally, we could answer lot of questions about the kinetics of |
|
|
42:44 | currents and which currents are dominating during rising phase, the falling phase and |
|
|
42:49 | on. And so the the setup is illustrated here is sort of a |
|
|
42:56 | to the squid giant axon watch the on that. It has two |
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|
43:03 | One of these green electorate is a electrode. So this electrode or measurement |
|
|
43:10 | measures the membrane potential inside the south . And it is also connected to |
|
|
43:16 | voltage clamp amplifier. So measures BM volt meter and it's connected to an |
|
|
43:25 | . This green plate here is a electrode or the ground basically. Which |
|
|
43:30 | it's zero from the outside as we . The difference between outside and inside |
|
|
43:34 | the cell address is about 1965 million or so. So yeah, this |
|
|
43:39 | elector. Now the number two. information from the cell gets fed into |
|
|
43:44 | voltage clamp amplifier which has a certain voltage. What is command voltage, |
|
|
43:55 | voltage is what you are commanding it be. So, this is the |
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|
44:00 | to command that. I want this to be minus 90. I want |
|
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44:04 | membrane voltage to be minus 60 I it to be zero. You're the |
|
|
44:11 | , you're commanding this voltage. in other words, also because its |
|
|
44:16 | clamp your clamping a voltage at the member and potential value minus 17 minus |
|
|
44:23 | minus 30 and so on. So information is going in here and you |
|
|
44:28 | commanded desired potential. So you're commanded certain desire potential. And in order |
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|
44:35 | hold that certain desired potential, there's be fluxus of currents that this axon |
|
|
44:41 | in the dish. But if this was activated by the stimuli, there |
|
|
44:45 | be fluxus of currents across. Or you added some chemicals that would be |
|
|
44:49 | of these ionic currents across. So voltage clamp amplifier is injecting current. |
|
|
44:57 | it's always saying basically keep this at seven to keep this at minus |
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|
45:02 | This guy, the green guy is this one is dictating or commanding clamping |
|
|
45:08 | planted at minus 70. This one measuring measures minus 70 minus 70. |
|
|
45:14 | , minus 70 minus 70 Good, have a deviation to minus 40 flux |
|
|
45:20 | happened, inputs came in channels So in response to that, when |
|
|
45:26 | becomes different from the command potential, when the member and potential now shifted |
|
|
45:31 | minus 40 I said at the state 70 then the clamp amplifier will inject |
|
|
45:36 | current into the axon through this second . This feedback arrangement causes member and |
|
|
45:43 | to become the same as the command . So you're holding it minus 70 |
|
|
45:49 | goes to minus 40 you bring it to minus 70. So you can |
|
|
45:53 | see what's going on at minus 70 what current might be flexing at that |
|
|
45:57 | potential value. So the current flowing into the acts on invested across this |
|
|
46:04 | can be measured here. And any current deviations which might be because |
|
|
46:09 | ions flexing, not because of the , because of the synaptic inputs on |
|
|
46:13 | flexing all of these deviations can be . These deviations will be actual currents |
|
|
46:20 | these currents can be action potential currents you snap the currents and uh in |
|
|
46:27 | way this is a negative feedback like the air conditioner temperature goes up |
|
|
46:32 | have it set at 70 goes up 75 says, no go back to |
|
|
46:37 | A. C kicks in negative This is the same way minus 70 |
|
|
46:42 | minus 40. No go back to 76 if it goes from minus 70 |
|
|
46:47 | minus 80 it's a smart system. like a sea that can either cool |
|
|
46:53 | heat at the same time. because I guess it's called, what |
|
|
46:57 | it the nest? Uh It's a control that they see that that they |
|
|
47:03 | these days. And I think it's enough to recognize that the temperature went |
|
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47:07 | and down and kicking either cool or heater, which in Houston, I |
|
|
47:11 | it happens. We have this, know, we go from summer into |
|
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47:15 | sometimes, or from winter into summer there's this few days where you're using |
|
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47:21 | C and then wait a second, getting cold and you're turning on the |
|
|
47:26 | . So, but this is a feedback system, it's just feeding, |
|
|
47:29 | know, to the deviations from this of potential from this clan. |
|
|
47:34 | the voltage clamp techniques for studying membrane in the squid, giant axon, |
|
|
47:40 | how it started. That's how it with two electorates. Modern electrophysiology, |
|
|
47:45 | did with one electrode. We can it in much smaller accents and much |
|
|
47:50 | cells. Remember, these are one in diameter. So you can see |
|
|
47:54 | clearly with the naked eye and even with tweezers. Yeah, So now |
|
|
48:04 | and Huxley, if you remember where 1939 were the first ones to publish |
|
|
48:10 | action potential. And in 1963, and Huxley received Nobel Prize in Physiology |
|
|
48:17 | Medicine for their work on the action . So they published the first one |
|
|
48:22 | 1939. You can see that 24 later they did a lot of work |
|
|
48:29 | that period of time. And after recorded action potential, they had this |
|
|
48:35 | . Well, what are the important is because remember they squeezed the ions |
|
|
48:40 | of the squid and then later they the ions out of neurons. And |
|
|
48:44 | said, oh these are the other . And then we're able to poke |
|
|
48:50 | and finally recorded this action potential. we know this. So, you |
|
|
48:55 | this potassium chloride, no calcium, islands are important, which are the |
|
|
49:00 | that produce that blip on the silla . And to answer that question, |
|
|
49:05 | needed the voltage clamp because the voltage allows you, as I mentioned, |
|
|
49:10 | clamp these potentials different what we call values. So Hodgkin and Huxley contributed |
|
|
49:19 | our understanding to this day of the and dynamics of sodium and potassium during |
|
|
49:26 | action potential. They came up with model Hodgkin and Huxley model for action |
|
|
49:33 | . If you're in computational neuroscience, know about it or you learn about |
|
|
49:37 | and most likely even use it What they did with voltage clamp has |
|
|
49:43 | here is that they d polarize the 2 -26. Again, this is |
|
|
49:49 | that the cell wouldn't do physiological, cell wouldn't sit at positive 50 to |
|
|
49:54 | positive 65 for the sustained period of . But this is an experimental manipulation |
|
|
50:01 | allows us to basically discover how these and how these channels were potassium and |
|
|
50:07 | channels. And if you recall this deflection and in current is inward |
|
|
50:15 | As I mentioned on the plots its current. So once you clamp the |
|
|
50:22 | from resting to -26, you do it. You're seeing this little blip |
|
|
50:28 | is an inward current and that blip away. And then you have the |
|
|
50:33 | outward current and blue and then you polarize the south zero. Lock it |
|
|
50:40 | zero. You can see even larger current but you also see much larger |
|
|
50:47 | term. The features of this inward are interesting because it turns on and |
|
|
50:52 | turns off and then the dominating current the outward current. You do polarize |
|
|
50:58 | this case to positive potentials. Now have positive 26. You start seeing |
|
|
51:03 | decrease in the similar current. This current, the sodium coming inside the |
|
|
51:11 | . Why are you seeing a decrease positive 26 As opposed to zero. |
|
|
51:19 | is this current here smaller than it a zero value because you reduce the |
|
|
51:26 | force for sodium and it's positive Which I told you go by my |
|
|
51:35 | , which is positive 55 which is close to positive 52. But is |
|
|
51:40 | any inward car that positive 52? , but there's still stimulus here. |
|
|
51:45 | still be polarization and there's still massive con And then look at positive |
|
|
51:53 | You see this little blip here. this little blip here is this current |
|
|
52:01 | now has reversed that sodium that's And why has it reversed? Because |
|
|
52:09 | 65 is on the other side of equilibrium potential which is positive 55. |
|
|
52:16 | you still those currents have very sustained prolonged potassium conductance. So what they've |
|
|
52:25 | is that there is an early and conductance in the late sustained conductance. |
|
|
52:32 | early inward transient conductance of sodium rushing the south. And this late outward |
|
|
52:40 | is potassium leaving the cell. And you look then in this diagram illustrates |
|
|
52:48 | lot of these teachers here that this the stimulus of the polarization and you |
|
|
52:54 | see that during the rising phase of action to control you have several |
|
|
53:02 | These are voltage gated sodium channels. simplicity reasons. You have three channels |
|
|
53:07 | are shown here in reality activates tens hundreds, sometimes thousands of channels depending |
|
|
53:14 | areas of the number and so But you can see that the sodium |
|
|
53:18 | almost immediately after the deep polarization they . The deep polarization is still happening |
|
|
53:27 | . In fact there's even more deep here than when this channel was |
|
|
53:33 | But this channel is closed already. this is the transient nature of the |
|
|
53:39 | gated sodium channels as they open for close you'll understand why in a little |
|
|
53:46 | . And if you average over these trace system, this is the average |
|
|
53:50 | conductance or the some of the channels it looks smoother this inward current coming |
|
|
53:58 | . And these dash lines that you're here also correspond hear to potassium |
|
|
54:04 | So this is during the deep What you're seeing is that potassium slowly |
|
|
54:10 | activating individual channels. The other teacher that in biology, not all channels |
|
|
54:18 | participate in national potential will open at same time and close at the same |
|
|
54:21 | , slight variation and duration, which gonna be open the time at which |
|
|
54:27 | going to open. Uh And if look at the potassium channels, once |
|
|
54:34 | open the potassium channels, they're long and sustained conductance is so very different |
|
|
54:41 | these transient opening and closing civilian channels potassium channels. As long as there's |
|
|
54:47 | polarization is prolonged sodium potassium current So, if you were to break |
|
|
54:55 | the action potential, that rising phase dominated by sodium influx and then the |
|
|
55:03 | phase is dominated by potassium b flux potassium beating yourself and this is inward |
|
|
55:10 | sodium coming in. This is potassium current the same as as you're seeing |
|
|
55:17 | . And that's what Hodgkin and Huxley . And to do that, you |
|
|
55:21 | a voltage plan. So what questions you expect from this diagram, what |
|
|
55:27 | voltage clamp do. It allows you clamp the potential allows you to isolate |
|
|
55:33 | current conductance is but you're not going be asked to draw this diagram. |
|
|
55:39 | is it a positive and negative feedback ? Well, it acts more like |
|
|
55:42 | negative feedback system. So and the is, is a negative positive feedback |
|
|
55:48 | for sodium influx during the rising phase the action potential is it's it's positive |
|
|
55:54 | system because more sodium radicalization, more more deep polarization, but it's ineffective |
|
|
56:00 | positive feedback system because these channel they shut down very quickly as soon |
|
|
56:05 | they're open. So in order for to understand really how voltage gates the |
|
|
56:14 | , we have to understand the sodium structure. And so we'll spend about |
|
|
56:19 | next 10 minutes or so talking about , you recall the obvious polytrack tied |
|
|
56:25 | you know, I have to change building blocks. We have sub units |
|
|
56:30 | gated sodium channels have 1234 subunits. one of these subunits has six trans |
|
|
56:39 | segments as 12345 and six as As you can see Sheldon Purple has |
|
|
56:48 | lot of positive charge. So it contains a lot of positively charged amino |
|
|
56:53 | residues. What does that mean? we talked about the positively charged amino |
|
|
56:58 | residues inside the inner channel lumen that interacting with sodium ion stripping in the |
|
|
57:03 | of hydration and propelling it through the . Is because as you make this |
|
|
57:09 | ordinary complex structure three dimensional structure of proteins, you'll have some amino acids |
|
|
57:15 | will be positively charged. It will negatively charged. And it just so |
|
|
57:19 | that S. Four and the voltage sodium channels at the accumulation of these |
|
|
57:24 | charged amino acid residues hanging in this . Four. And that becomes important |
|
|
57:29 | sensing the voltage and opening this channel between us. Five and a |
|
|
57:35 | As was discovered by roderick mackinnon and channels. He talked about the hairpin |
|
|
57:41 | . So this is the hairpin which is from one subunit is one |
|
|
57:47 | loop, another subunit, another pin . 3rd 4th. This is the |
|
|
57:53 | filter, the innermost lumen of this that will be controlling and selecting for |
|
|
58:01 | in in this case this voltage sensor here as four and it actually is |
|
|
58:10 | here because it is drawn by the currents that are accumulated. This is |
|
|
58:26 | inside of the south and addressing member potential. The inside of the south |
|
|
58:35 | is negatively charged and the outside is charged and opposites attract each other. |
|
|
58:43 | so this positively charged amino acid It's like a voltage sensor. It |
|
|
58:48 | attracted by a negative charge that's accumulated the membrane and in this position developing |
|
|
58:57 | and the gates for the sodium channel closed. So this is an illustration |
|
|
59:03 | the gates and will have more details that. So what happens to bad |
|
|
59:11 | the projector is too high up. what happens during the resting number and |
|
|
59:20 | . You have negative charge when you polarize the cell, you have accumulation |
|
|
59:26 | positive charge. Guess what happens when accumulate this positive charge? And they'll |
|
|
59:31 | , well, what are these deep is coming from? Remember we talked |
|
|
59:34 | membrane going up and down synaptic excitatory inputs, We polarize the |
|
|
59:39 | So if they do polarize the cell , if there is enough of the |
|
|
59:43 | charge accumulated here, it's going to this positively charged amino acid residues And |
|
|
59:52 | amino acid residues are going to this sensor in S. four, it's |
|
|
59:59 | to slide up through the coating as is being repelled by the build up |
|
|
60:06 | positive charge here. It now slides through the protein and it changes confirmation |
|
|
60:13 | that three dimensional structure. So you a confirmation all change in the protein |
|
|
60:19 | which through this conformational change allowed for channel to open the gates. So |
|
|
60:25 | need deep polarization, its voltage gated is getting this channel, what is |
|
|
60:31 | the gate open or closed voltage. there's also certain kinetics in this |
|
|
60:38 | And these kinetics are such that if have this sustained deep polarization here from |
|
|
60:45 | to -40 number, that voltage gated channels will be very well activated at |
|
|
60:51 | threshold of action potential which is So if you d polarize the south |
|
|
60:57 | if you climb the potential at you should activate sodium channels now voltage |
|
|
61:03 | sodium channels and what you're seeing is as we saw in the previous |
|
|
61:08 | these individual channel traces and these channels up and they don't always open at |
|
|
61:14 | same time. And once they open the very closely very quickly closed within |
|
|
61:20 | a millisecond or so. So you four positions here. When the sodium |
|
|
61:28 | is closed, sodium channel actually has types of gates. These arms that |
|
|
61:34 | closed, we call them activation gates this ball and chain here that's hanging |
|
|
61:42 | in the cytoplasm, we call it activation gates. So this is in |
|
|
61:48 | gate. So when there is enough polarization and you have the vaulted sense |
|
|
61:54 | sliding up sodium channels, you have opening of the activation gates and now |
|
|
62:02 | have the flux of sodium through the channel. And number two showdown is |
|
|
62:08 | in. So you have this deflection in a single channel, inward current |
|
|
62:13 | coming in through the sodium channel. this ball and chain is not just |
|
|
62:21 | there. You change the conformational change confirmation structure of the protein. You |
|
|
62:27 | the gates here and this ball and because it says this conformational change actually |
|
|
62:34 | and plugs up the channel. So closes the challenge. So as soon |
|
|
62:39 | you open the activation gate, the gets inactivated with the second gate. |
|
|
62:50 | kind of just happened. It's it's a little bit of a charge, |
|
|
62:53 | it also has to do with the three dimensional structure. Like when you |
|
|
62:58 | the gate, something else may So in this case you open the |
|
|
63:02 | and this other gate comes up and and and and plugs it up. |
|
|
63:08 | just a complex movement of this three structure. Uh Now you have this |
|
|
63:16 | inactivated and this is the reason why doesn't reach its equilibrium potential value because |
|
|
63:25 | soon as it opens, it as soon as it opens, it |
|
|
63:30 | . As soon as there is a in the channel opens the gate, |
|
|
63:34 | same conformational shift encourages the second mechanical in the channel closure of that |
|
|
63:42 | So how do you deign activate? do you remove the inactivation gate or |
|
|
63:50 | activate this channel? The only way do it is still hyper polarized the |
|
|
63:55 | again to -65 million bowls. Because when you do that, this |
|
|
64:04 | sensor, we just slid up open the gates and plugged up the |
|
|
64:07 | . Now, if you re polarize cell and make the inside of the |
|
|
64:12 | , negative voltage sensor will be attracted slide down to the negative voltage back |
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64:18 | its position and as it slides down the position you have a confirmation will |
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64:25 | again, which kicks out the ball change Dean activates the channel and closes |
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64:31 | gates. And so the sodium channel to go through 12341 and four is |
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64:38 | same. But you cannot leap 13 you cannot leap 124. You have |
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64:46 | have this re polarization Dean activation, sliding again of that voltage sensor Dean |
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64:54 | and closure of the channel. So funny squiggles that I drew on the |
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65:01 | and I said some of them are . Some of them are nonlinear. |
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65:05 | that's because channels will have slightly Vietnam and different subtypes of voltage gated serving |
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65:15 | will have slightly different channel kinetics, that maybe they're inactivated faster or maybe |
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65:22 | takes longer time for that ball to because they have slightly different structures or |
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65:29 | and it may take away that linear and make it more nonlinear. And |
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65:36 | kinetics of these sodium channels and potassium are not the voltage gated channels and |
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65:42 | not only influence the ivy plots that talked about, but also the overall |
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65:47 | and properties, passive number and properties their ability to produce the active firing |
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65:55 | the action potentials during the stimulation. lecture we will discuss this voltage clamp |
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66:04 | and wholesale patch clamp technique and brief , you shouldn't worry about it if |
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66:09 | not into biophysics too much. Just that voltage clamp is really important to |
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66:17 | out these individual currents and without it couldn't do it. We could just |
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66:22 | the voltage forever, not knowing what are contributing to the changes in that |
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66:28 | . We'll talk about these different We'll talk about tetrodotoxin. What Some |
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66:35 | review. Some of the things that already talked about. So some of |
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66:39 | things will come up. Talk about anesthetics briefly and the back propagation of |
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66:46 | action potential. So this is the for next week. And then if |
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66:51 | recall on thursday, a week from , we have our midterm exam review |
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66:57 | then you have your midterm exam coming uh the following Tuesday. All |
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67:06 | great. Thank you very much for here. Thank you. Everybody on |
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67:11 | . I took these pictures. Is gonna be helpful if I upload these |
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67:16 | ? Okay. Thank you very I appreciate you appreciating my artistic |
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