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BIOL 4315/6315 Neuroscience (18563/18591) - NEURO Lecture 7 Membrane at Rest II and Action Potential I
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00:00 All right, This is lecture seven neuroscience. Last lecture we were discussing
00:07 resting membrane potential number in potential at , Which means that the cell is
00:15 very active. It doesn't mean that potential value is always at -65 million
00:23 which is the charge separation across plasma compared to the outside. It doesn't
00:29 that the 65 million malls negative value a flat line. The cells are
00:36 sporadic inputs, excitatory an inhibitory inputs that line fluctuates constantly. There is
00:44 dynamics involved and temperature changes locally at foster lipid bi layer. It will
00:51 the fluctuations and nobles as well and activity. But the trust means that
00:57 cell is not generating the action Which will be the discussion of later
01:05 . So addressed what's happening is We have an equal distribution of charge
01:21 we will talk about this ionic species come back. But one thing that
01:25 discussed last lecture was the reflex the knee jerk, stretch,
01:31 tendon, reflex. And I've asked to do a certain thing that will
01:36 you answer several questions in the you have to know the three cell
01:43 that are involved in generating this reflexive . So dorsal root ganglion cells into
01:51 and motor neurons. You have to their morphology whether they're multipolar cells or
01:57 pole or pseudo unit polar. Have know which neurotransmitter they release. Do
02:03 release glazing or glutamate or settle coding something else. And you have to
02:07 whether that neurotransmitter or that cell. excited, turn inhibitor and also the
02:15 meaning you have to understand where those are located. You have to understand
02:21 the selma's of the dorsal root cells located outside the spinal cord and into
02:28 control local network activity within spinal cord motor neurons project the motor command out
02:37 the spinal cord, their axons into muscle fibers. So we talked about
02:43 quadriceps and to have strength, flexor , opposing muscles. And for this
02:48 to be effective, you don't only the muscle but you also relax opposing
02:56 . And if you think about the and this is something that gets tested
03:01 neurologist office if they do even the checkup or if you're having some neurological
03:08 , some movement problems or something like . A little tap on that on
03:13 patella tendon here that picks up the and now by having the circuit behind
03:19 and observing a different kick in the , you can start reducing what might
03:23 wrong in this circuit might not be sensor information. And so the stimulus
03:29 to be really strong. Maybe the doesn't have inhibition and a very light
03:33 produces a very large response. You start addressing actual mechanistic pathological problems that
03:41 be happening by looking at this cap . Patella tendon and also in addition
03:47 those three types of south that I've mentioned. You should also know your
03:53 . It'll sell in the hippocampus and cortex with its projection sell excited to
03:59 and also the inhibitory cells that release . There's a great diversity of those
04:06 . So we talked about ions and talked about water some hydration last lecture
04:11 we talked about the four main ionic that are unequally distributed across plasma
04:20 And in particular those four species are , potassium, chloride and calcium.
04:34 the membrane will express channels for these . And for the next couple of
04:42 will be focusing on what we call gated channels. That means that voltage
04:47 the voltage and chemical gradient will be ions across these channels across plasma
04:54 Later in the course, when we about snappy transmission, we'll talk about
04:58 gated channels. When we talk about , we'll talk about McKenna sensory receptors
05:05 gave a challenge. So sodium is and chloride is abundant as an Aquarius
05:13 solution on the outside of yourself. dominating on the inside of the
05:18 You have high concentration of potassium, highest concentration gradient disparity exists for
05:25 .1 Michael Moller on the inside 10 , sorry millie mole on the
05:33 It's 10,000 times concentration difference. And as I discussed, it's very tightly
05:40 inside side of plaza inside of side all. There is not much of
05:44 free calcium floating around because it acts a secondary messenger. It can influence
05:51 release from internal calcium stores inside the and it can also influence the fusion
05:58 release of the neurotransmitter vesicles. calcium side or solid calcium gets bound
06:04 and calculated by calcium binding proteins and of course another. They're important element
06:11 is a pump, a pump that's a channel that's bringing in sodium and
06:18 against their concentration gradients next to one these. Each one of these ions
06:25 also see this value E N A K E C L E C A
06:30 plus which stands for equilibrium potential I also call it nerves potential value
06:38 ernst equation is used to calculate this . They also call it reversal potential
06:44 ionic reversal potential value. It's all of these terms are used interchangeably
06:50 But each one of these islands have own equilibrium potential value and you'll understand
06:56 why and how it plays into remembering and also generation of different aspects of
07:06 . Now we talked about peptide bonds you know, acid essential non essential
07:11 acids. The building blocks little bricks put together comprise secondary tertiary coordinate repentant
07:18 , prudence, protein channels that we're about are comprised of multiple subunits receptor
07:26 . Multiple subunits trans membrane proteins that g protein coupled receptors. You don't
07:35 a channel there also comprised of multiple 5674. Just depends on a different
07:42 of the channel. Each one of is selected for a given ion.
07:49 we talked about channels. Ionic channels ion surrounded by waters of hydration that
07:55 gets stripped of these orders of hydration with negatively charged amino acid residue inside
08:01 innermost lumen. This channel gets propelled to the south And sodium has the
08:09 channel, potassium chloride, calcium. some point those rules can be broken
08:14 ions that 50 channels can travel through but the ionic channels are selective when
08:19 conduct ions at a fast pace and of discharge or movement of discharge across
08:26 channels influenced the change in the potential possible number. Arms law vehicles.
08:36 voltage current resistance conductance is inverse of . Therefore, if you plug in
08:45 G in here instead of one over . You get G. V.
08:52 I voltages and miller volts relevant skills the brain. Milli amperes micro amperes
09:01 arms for resistance and neurons and PICO nano siemens for conductance is of individual
09:08 or individual neurons. There's irrelevant scales measuring these events, diffusion is a
09:20 gradient. You have the diffusion of . So if you have a lot
09:24 sodium fluoride on one side and you the channels for sodium fluoride purely based
09:30 the concentration gradient. If you're just at the chemistry, you're going to
09:34 equal molar or equal concentration distribution across plasma membrane but ionic movement is also
09:43 by electrical potential and each one of island's essentially has a battery representation electrical
09:51 . You have battery and cat eye such as sodium will be attracted to
09:58 end of the battery. To capture and Lauren will be attracted to pano
10:03 positive. And then the battery discharge is again across plasma membrane. And
10:10 is the charge separation. Is the between the voltage on the inside of
10:15 cell versus the voltage on the outside the south. So what is the
10:20 on the inside of the cell? voltage on the inside of the cell
10:23 accumulation of this negative charge across plasma . What is the voltage on the
10:28 of the cell is presumed to be or zero neutral zero mobiles charged.
10:36 the difference across Bosnia membrane. The versus out of 65 million difference at
10:44 . Some basic information current flows direction net movement of positive charge of Catalans
10:51 into con direction and ions move opposite direction. If you increase the charge
10:59 , that means if you accumulate more more negative charge, you hyper polarized
11:05 from -65 to -75 to -80. you decrease this negative charge and you
11:12 it with respect to the outside of cell, you are causing a deep
11:17 of this plasma membrane. It's really to plasma number but equilibrium potential for
11:25 ion exists because ions have a Yeah. And equilibrium potential is a
11:33 at which point the chemical forces that driving these ions across plasma number and
11:40 the channels to equalize the number of they encounter. Electro motive or electrical
11:49 . And that electrical force a lot times is created by the movement of
11:55 same ionic species across plasma membrane, charge now becomes repellent to a positive
12:02 . So let's look at the situation as an example, potassium and a
12:07 , let's say, protein, negatively protein that's stuck in potassium channel opens
12:12 potassium goes down its concentration gradient from there's a lot of potassium to where
12:18 a little potassium but it never equalizes both sides. Because as this positively
12:26 catan and potassium moves across the it accumulates the charges accumulating on the
12:34 member and on the outside and the of the positive charge now starts repelling
12:40 charged potassium from further coming down its gradients. At this point, the
12:48 force that is pushing the number of pushing to equalize with the other
12:55 The same number of ions is the force. And the electrical potential electrical
13:02 pushing it is exact same and they're equal to each other and they oppose
13:12 other. And ions will still be across the membrane and the channel.
13:17 there isn't going to be any net movement. That means there isn't gonna
13:21 net movement to the inside or outside this equilibrium potential value. That's the
13:27 potential value is the value. And the membrane it's a value at which
13:36 chemical force is the same and opposite the electrical force. And there is
13:43 not bionic movement. Right? You see that there is an accumulation of
13:50 across the plasma membrane here. But you go on the extra cellular
13:55 or even if you go on the inside of the side, a plasma
14:01 the cell, the core of the charge neutral, just like the outside
14:06 charged, mutual. So all of charge accumulation and separation of charge and
14:12 distribution of charge by physically is at level of the plasma membrane. And
14:20 we're discussing is we're discussing another important that's called the driving force. Driving
14:27 is the difference between the voltage of membrane and each ion if the Librium
14:33 . So, we'll come back to uh concept of the driving force,
14:38 just introducing it in the slide will back in the following slides.
14:44 if the ionic concentration is known, can calculate equilibrium potential for each
14:55 So, how do we know you're the concentration inside accents or inside
15:01 I'll show you the movie a little later, but it's basically taking these
15:06 squid axons and squeezing the tube like giant action, squeezing the inside of
15:13 tube and measuring the concentration of ions the inside and then taking where the
15:18 live in the ocean and taking taking acquis environment that's inside the squid which
15:27 going to be a lot more saline ours. So it will be high
15:33 of sodium chloride. And so these the only concentrations also differ by species
15:38 environments and which surround those different You can get all of the
15:43 Now you know what's the concentration of your four major ions. Now you
15:50 use a equilibrium potential formula to calculate . This is an example of
15:55 sodium is abundant on the outside. will be driven into the cell down
16:00 concentration gradient with a sodium ions and charge accumulate on the inside of the
16:06 sodium gets repelled by its own electrical reaches its equilibrium potential value. So
16:17 pumps are different and the pumps always against concentration gradient. We use
16:22 T. P. And the export sorry. The import to potassium stand
16:31 the export sodium always against the concentration is always never changes the direction.
16:38 reverses and uses energy 80 p. do so. Yeah. So now
16:44 you look at these major ionic sodium potassium calcium chloride, what you're
16:49 is known concentrations of these ions on outside in mill imola and on the
16:57 and we talked about how the greatest or the greatest disparity and ratio exists
17:04 calcium. Uh huh. And additionally can also this is useful for calculating
17:13 equilibrium potentials and you will need a for the exam that you'll need to
17:18 the variables and the values that we're . Yeah. Answer properly the questions
17:23 the movement potential but It's five million on the Outside for potassium. 100
17:32 the inside. So another way of it is 1-20. The ratio of
17:38 on the outside vs. Inside. 20 times more potassium On the inside
17:43 the selling outside. There's 10 times sodium on the outside of the cell
17:49 inside of yourself. Huh? So can either use the minimal of
17:54 you'll see how it plays in the time or you can use the ratios
17:58 the science inside versus outside. And you can see at 37 C because
18:05 temperature dependent equilibrium potential potassium is sodium is 62, calcium 123,
18:14 chloride -65 minus 70. And by way these values are slightly different in
18:22 books. Like for example, here will say wait a second, you're
18:26 us that reversal potential for potassium is 80. And here it says reversal
18:31 for potassium is 102 You're telling us five inside and 100 outside and this
18:39 here there's 135 inside and three So I'm not going to confuse you
18:47 I'm not going to ask you three 65 480 was 80, 70.
18:58 And why is there a difference? measurements are taken from slightly different neurons
19:04 they're slightly different neurons express different subsets these channels and they have maybe variations
19:10 the local concentrations of the ions Mhm. So this is this is
19:16 this is the disparities and you'll see of these disparities and that's not something
19:21 worry about. What is to worry is understanding the main variables that learns
19:28 equation. So you have 23 Ion stands for equilibrium potential 2.3 artie
19:37 zf log of ion concentration on the of the cell versus ion concentration on
19:43 inside of the south R stands for constant and t. Is absolute temperature
19:49 in this case we're using 37 why? Because of his body Physiological
19:56 temperature is 37 7. Again, is the charge of the ion or
20:02 valence of the ion. So it's for mono Vaillant. It's too,
20:09 -1. For an eye on this haven f is an electrical or Faraday's
20:17 . So you have two constants gas third a constant. You know the
20:21 which is 37° centigrade. And if plug in a single mama valent Cat
20:31 on such as potassium or sodium, can abbreviate and collapse this 2.3 artie's
20:42 , Into what becomes 61.54 million So this this whole 2.3 are TCF
20:51 available value 61.54. And then you to plug in the outside concentration of
20:59 versus the inside concentration of potassium. you do that here on the
21:04 there's little potassium 1 to 20. this ratios I was just discussing and
21:10 slide about But you can plug in actual values, you can plug in
21:15 million mole and 100 million moller into nearest equation. Or you can just
21:19 in the ratio. It will mathematically make a difference, 1-20. So
21:24 you plug in This calculation here, log 1/20 is -1.3 And you multiply
21:36 1.3 times 61.54 million balls. You have a equilibrium potential value for
21:46 So this is how you calculate You don't need to know how to
21:49 the narcissist equation. You don't need use the calculator to calculate it,
21:55 you have to be able to recognize differences. Let's say if the noticed
22:01 is written where there is 20 potassium the outside and zero on the
22:11 you know this is wrong. So have to know these values. You
22:15 have to know the calculation. You to recognize what is the RtC.
22:20 . You also have to recognize that you pull again a cat eye on
22:25 into the Z into the valence value get 61.54 abbreviation, 61.54 for Mama
22:32 Ion. This value becomes -61.54. ? Because you plugged in chloride which
22:42 -1 for violence In the case of which is a dive. A
22:49 A tie on. This abbreviation becomes Because you have to divide it by
22:57 , You plug in valence of 2-plus over here. So when you do
23:04 calculations and the abbreviation for each eye You have the individual equilibrium potential values
23:11 region for these ions. Okay, is a liberal potential values freeze on
23:18 centigrade. Okay, this is for eye on each ion has its own
23:24 potential. But the plasma membrane contains for multiple ions which means that the
23:33 potential of the plasma members. We're talking about equilibrium potential. When we
23:37 about the equilibrium potential for one we're talking about that specific ionic channel
23:42 the chemical and electrical forces opposing each . That's what we're talking about.
23:47 we talk about the equilibrium potential, we talk about the number of
23:51 it has to do a lot with chemical and electrical forces but it's not
23:56 on just one high on. And to do the calculation for Goldman equation
24:03 for the membrane potential we use the one equation And goldman equation has two
24:10 from nerves equation. The main difference that it has a permeability ratio it
24:19 the permeability value. What is the for potassium? What is the permeability
24:26 sodium? The other thing that it , it's not for one ion.
24:31 is a calculation that incorporates potassium and if you want to you can add
24:37 too. And this tells you that overall VM. It's not E.
24:44 . Which is a liberal potential for DNA equilibrium potential sodium. This is
24:49 . The number of potential value is using the same abbreviation from the previous
24:54 . T. Z. F. have to monitor villain Catalans here and
25:02 what else? The premier ability. then this calculation shows that if the
25:08 membrane I am from the ability to is 40 times greater than it is
25:15 sodium, then this is the solution you will get. Okay. What
25:22 that mean? That means that at you sunk in that electorate and you
25:29 negative 65 millibars. When the selling is not very active At resting number
25:34 potential. The foster lifted by. is 40 times more permeable to potassium
25:41 . This is the biophysics. The of this plasma membrane neurons are slowly
25:47 potassium. That's what the cell number his most permeable to. So addressed
25:54 is dominating the membrane potential and sodium very little permeability. That for the
26:05 rule or ratio if you make 40 more permeable to potassium versus sodium changes
26:13 the number of potential changes. And you enter the action potential phase two
26:18 the cell phone, Ostpolitik beyeler becomes impermeable to sodium and and some other
26:25 things started happening. No. Let me put this in perspective in
26:32 class supporting materials. You have this that I drew last year and I
26:40 recommend that you draw the slide and these values on there. So maybe
26:45 can dedicate a half a page to page of your mouth because this is
26:49 you're going to be able to understand . And also answer quite a few
26:56 about the action potential. I'm going show you the slide now, but
27:00 going to come back and talk about slide in the minutes. The reason
27:06 I want to show you the slide first of all, you have the
27:10 scale and the Y scale is in balls. Uh and you're measuring
27:18 which is member in voltage remembering potential in voltage member of potential. This
27:24 zero value here. Our Mp stands resting membrane potential, Resting membrane potential
27:35 -65 million ball value here. It's line here. But as I
27:42 address the cell is fluctuating a little here. It's deep polarizing, a
27:48 bit hyper polarizing a little bit. it will be following this this line
27:52 this. Okay, up and down and down up and down.
28:00 You have the equilibrium potential values here have a equilibrium potential value for calcium
28:06 chloride and for potassium. So once equilibrium potential value which is calculated by
28:16 equation is for one eye on value it's electrical forces for the ion oppose
28:25 are equal to the chemical gradient. potential for VM is dictated by several
28:34 ionic species which saw example address that's by sodium and potassium and it's very
28:41 dependant on permeability cell number and may a lot of different channels and say
28:47 have a lot of sodium channels that channels are closed. So the cell
28:51 is not permissible to sodium. So becomes not a significant contributor, chloride
28:58 not a significant contributor. But those change this resting number and potential.
29:04 the cell is receiving excitatory glutamate vitamin synopsis are activated to sell D
29:13 , the cell D polarizes, the becomes more positive. If the cell
29:19 receiving inhibitory inputs, inhibitor synapses are , the cell will be hyper polarizing
29:27 the cell will go down to the again, positive emphasis will come back
29:32 the positive If it reaches this value is actual potential threshold value of about
29:39 million barrels produced this event and not or non event which is the actual
29:48 event. Mhm. If the excitation strong enough and excitatory synapses are activated
30:02 it drives the member and potential to threshold value for action potential. The
30:08 will produce is very fast deep polarization the form of the action potential produces
30:15 very fast deep polarization will have a influx of sodium and then it will
30:22 followed by the following phase of the potential. The potassium e flux is
30:29 to be dominated, it's going to to this level here and get slowly
30:34 to the resting number and potential. the help of that make a
30:39 Okay, so now you understand what's potentials? Each eye on each island
30:48 its own channel. Each island has own equilibrium potential. But these ions
30:53 part of the bigger game. And membrane, they're part of the several
30:57 that are flexing back and forth. part of the several channels that have
31:02 own opening and closing kinetics and some them could be open and some of
31:07 could be closed. Sure. Okay for for you for a second,
31:22 slide kind of overviews everything you've learned resting, remember and potential bonds law
31:33 circuit the main cells in that the building blogs of this protein
31:39 the selectivity of these channels, the , the diffusion concentration gradient and also
31:48 electrical gradient. Of course, the of the neurons potential just overlap with
31:58 of the slides. Yeah, Goldman , this is the major difference.
32:07 ? This is the nicest equation where have each ion with its concentration and
32:13 equation where you have to you can a third eye on and see how
32:17 changes the calculation here, How it the voltage value if you want
32:23 And also the permeability ratios for these . A very important thing that we
32:30 where we discussed astrocytes was that We said when we talked about,
32:35 are responsible for synaptic transmission for uh of the neurotransmitters. So their end
32:46 are involved in synoptic regulation, Synoptic regulation. And the other end processes
32:52 feet are involved in blood brain barrier what things enter into the interstitial space
33:00 the brain. Yeah. Astra sites these very extensive branches and as specific
33:09 are interconnected with other ostracized, it's the brain through gap junctions. So
33:14 electrical junctions that allow for the passage ions across different cells. It's very
33:22 because the ionic concentrations, if you an ionic concentration you can change the
33:30 potential overall membrane potential of the sell quite a bit. And this is
33:35 example of how extra cellular potassium, is potassium k Plus on the outside
33:41 milan moller and normal potassium is about to 5 million moller in this range
33:48 in the outside concentration this outside normal of potassium, the membrane potential value
33:56 close to -70 -65 million balls. if you change the Miller moller concentration
34:04 potassium to 10 million balls, Look where you are already, you're about
34:10 million balls. So if you change concentrations of ions locally on the outside
34:19 the cells, you can change the membrane potential Which is influenced by other
34:26 and it will now influence other aisles well. If you change this to
34:31 million more Year at -40 million what does that mean? The cell
34:39 be starting to fire action potentials. if you increase potassium concentration from the
34:46 of the south to 10 15, million moller you will generate abnormal firing
34:52 neurons. You will generate abnormal synchrony high potassium. A lot of models
34:58 different lives biochemistry, electrophysiology neuro lives used potassium chloride, high potassium florida
35:04 listen. A lot of selectivity. potassium is also a model for generating
35:12 activity by d polarizing the south and abnormal synchronization. So astrocytes come in
35:19 handy because they're supervised the synaptic transmission they supervise these local changes in the
35:28 and as soon as their local rises potassium concentration on the outside astro science
35:36 slurp up this potassium will distribute it its own cellular network and then we'll
35:47 it through the interest connected a specific networks, essentially balancing out any local
35:55 persistent increases in ionic concentrations. Its buffering it spatially buffers potassium concentration from
36:04 active neurons, very active brain regions networks and buffers it to essentially avoid
36:13 situation where there's too much potassium and much deep polarization of the plasma membranes
36:19 too much firing of neurons causing abnormal . See how this is still part
36:28 the Galileo function. Astra sites we about, we call them I think
36:35 chores and again, you know if don't do household chores, you know
36:41 car chores like fill up the gas kind of driving when it runs out
36:45 gas. So it's very important course and it tells you that ostracized and
36:52 can now influence and regulate neuronal excitability they do so by spatial buffering of
36:59 that do so. So you later neural transmission by cycling with the neurotransmitter
37:07 and before we move and go back talk about action potential. Again I
37:13 to highlight this person that I highlight year and uh I wish actually I
37:20 just read his story. Maybe it's little bit longer or maybe you talk
37:26 it in a much longer way. Dr roderick Mackinnon uh medical doctor is
37:34 medical doctor and uh Harvard and he very interested in the protein channel
37:45 So you have to realize that what understand about these channels that they're building
37:51 amino acids that the form the structure tertiary co ordinary that they have all
37:56 least of the units that we can protein structures and now we can calculate
38:04 without even visualizing them. I don't if you guys heard but last year
38:08 was a big breakthrough, artificial Being able to calculate protein structures better
38:16 X ray crystallography. Which actually allows to visualize the protein structures. That's
38:22 significant. But at the time when Mackinnon did his MD, he wanted
38:28 know what structure and function those channels . Imagine you have this long string
38:35 amino acids. Now you kind of wound it up into sausages, sheeted
38:42 up, flat it up, you , made sub units. Not all
38:48 these sequences are very important. And roderick Mackinnon wanted to know which sequences
38:58 important, which sequences controlled the opening closing of these channels. And so
39:06 you have simpler systems. You can to fruit flies and say, what
39:10 you doing with fruit flies and studying channels and making things like shaker
39:16 It's a potassium mutation in the in fly that makes the fly shape.
39:21 fly is essentially a productive. What that have to do with the
39:27 Well, we have conserved amino acid which means we have certain parts of
39:35 very large protein. Certain sequences could conserved in fruit flies and warms in
39:44 and other higher low order species shared . So if you identify sequence in
39:53 fruit fly and the potassium channel that important. Let's say that that sequence
40:00 the keyhole. The whole door is large. How do you unlock the
40:10 ? It's just a little tiny mechanism the actual key. Not even the
40:16 hall. Just make sure that the of the key match precisely. It
40:21 the door. And then do you the entire door when you open the
40:27 ? You lower the handle when you the handle. Some doors have springs
40:33 they will help you open them. this whole sequence of the pro dam
40:41 very important to have the door. the key, the one that unlocks
40:47 door or the handle, the one opens the door are the most important
40:53 . And if you find that they're and you mutate them and you see
40:57 change in the channel function then you're something. So he used side directed
41:03 genesis to do these mutations. And potassium channel. We also use toxins
41:10 electrophysiology use toxins because nature makes very molecule, spiders, snakes, fish
41:21 , whatever it is and those molecules staining molecules will bind us. Their
41:27 sequences. They could be the key fits into the keyhole and keeps the
41:33 shut. It interferes with the mechanism opening the door. It could be
41:38 opposite, it could be binding on side of the door and preventing it
41:44 closing completely. Just keeping it barely . This is what these toxins do
41:49 binding to different sequences. Then the . And we'll talk about this later
41:54 the course when we talk about agonists antagonists. But agonists of the substances
41:59 will open the channels, antagonists of substances that will close the channels.
42:05 you will use electrophysiology because if you the channel there's gonna be more current
42:12 more conductors through that channel. So , you know that you either mutated
42:17 or you've used the toxin to a sequence, a certain part of this
42:23 that now opens the channel a lot . Use another mutation, you use
42:28 talks and now you found a part this long protein structure that is responsible
42:34 keeping the channel closed, not closing keeping it closed. Then you find
42:40 part of this pro to him that responsible for closing or opening the
42:47 So this is what roderick Mackinnon And he's using these models and he
42:54 a lot and uses mathematical models to predict the structure of the potassium
43:02 But then he says, I'm still happy because I want to visualize the
43:10 . So he then gives from this , flies gene mutations and establishes a
43:19 Brexit crystallography and his colleagues tell him , you know, you're kind of
43:25 little bit nuts because you did. . You kind of did a one
43:32 and postdoc and professorship with the structure mutations. Now you're going to do
43:38 PhD imposed up in professorship in X crystallography which is completely different from electrophysiology
43:46 molecular biology, its biochemistry, it's a tiny protein inside a crystal.
43:53 projecting X ray across that protein inside crystal, visualizing its structure. Using
43:59 of the other tools including mathematics, finally derive the precise structure with the
44:08 included. And so when his colleagues , you know, what are you
44:12 , you're like entering completely new He says don't worry about it.
44:15 one of the visual as his So he does extra crystallography and all
44:20 his studies. All of this persistent over decades uh leads to this beautiful
44:26 potassium channel here, the innermost limit . And he also describes the hairpin
44:35 which is the selectivity filter inside the that allows for the passage of different
44:43 species that we talked about, such potassium or sodium. And the reason
44:47 I like his story is Path of the book. This is the section
44:53 Path of Discovery. The reason why like his story is because he has
44:58 goal. He has a passion and goal is to understand and to visualize
45:08 channel to understand the structure. To this function structure equals function, adjusting
45:14 , adjust function and so adjusting function structure to. They're both interconnected that
45:21 his goal. And that is his . In other words, his fashion
45:25 not to have an MD tag on flap coat are PhD tiger on his
45:32 coast or a second huge d tire his lap code and some people would
45:39 , well you know, you went medical school now, you're well you're
45:44 deep into science. So he's passionately a goal and that's why I use
45:49 example also because in career paths you wind in different ways. People pick
45:54 a new career in their 60s, don't retire at 70 anymore. They
46:00 new hobbies at 75. That means things and and pursuing your goals and
46:07 your passions and you have to think things further in the future when you
46:14 about yourself, if your career this and graduate next year, I don't
46:18 the forget it. You know like have to do it now. My
46:21 told me I need to have PhD now law medicine, you know,
46:28 a goal and it may take you this way, stopping, turning
46:35 going this, going this way, . Skyrocketing. When you hear stories
46:41 all of the entrepreneurs and billionaires in the world. You on mosques and
46:49 such. You hear those stories, , Second register 3rd Richardson, how
46:56 he done amazing right. He's an or something or not. Your
47:04 Unbelievable. What about 15 stories of failures before that one success About 15
47:15 nighters to earn a grade that leads to a degree that leads you to
47:21 amazing opportunity that you never had. you don't highlight the failure story is
47:28 lot of times. But in life a list and the mistakes a lot
47:33 times will make you more experienced. . And it will work smarter and
47:41 harder. But you have to go the grind and you have to do
47:46 yourself and make mistakes and reading other stories. And I'm going to help
47:51 . But having a perspective and that , having multiple avenues forks into leading
47:59 the end goal and having time that potentially no limitation is a good and
48:09 way to look in the future. . All right. So now we
48:16 on to the action potential And now going to learn everything about from resting
48:21 potential state where the cell is not active to when the cell gets
48:26 The rising phase, the overshoot about Noah balls. The falling phase.
48:31 undershoot, which is below the resting and potential. And rebuilding of this
48:37 into the resting membrane potential with the of the epa's palms. Remember,
48:43 the morse code, It's the morse . If you think of everything that
48:50 at the resting membrane potential, everything happens, what we call sub
48:55 Sub threshold is that threshold potential value action potential generation -45 syllables. Everything
49:02 happens in this range between -45 and cells don't go lower than about -90
49:09 balls. And they never stabbed minus 90 because minus 15 minus 60 65
49:19 , 60, 70, 66 surround walk the resting number and potential.
49:24 you can imagine that this random walk an analogue code. These are synaptic
49:32 . And the action potential is all none. So what? It is
49:38 or 1 But in digital code. . And it's a very fast digital
49:44 . So you have essentially both sort analog coats of threshold and the digital
49:51 . That is the action potential Alright, for this. I'm gonna
49:57 into when I thought I had it , wait a second. I must
50:12 been here almost closed it. so when we start talking about the
50:21 potential, I really like this old . If we should watch and understand
50:26 it's sort of all began understanding of action potential is in studying national potential
50:39 business. Right. Mm. The bonds, body plans and have it
50:53 because of humans and all those behaviors another world. So it's not surprising
51:00 took a long time for scientists discovered there are fundamental similarities treatment other systems
51:07 parts. And yes, it was recognition of a useful difference in the
51:16 system which enabled scientists to undertake research has led to a growing understanding of
51:23 our own nervous system. The breakthrough that control the contraction of the mental
51:30 using precautions. This archive shows a thing is tracking mental models. Even
51:39 moderately science with inject a huge amount water with great force. Uh
51:46 mm wow. In the mid the British geologist Professor Jay Z's young
51:53 engaged in the study of Squeeze Young observed an array of large tubular
52:01 , each as much as an enemy a squeeze mantle. I think structures
52:06 never fill with blood. They were being blood vessels from their similarity surrounding
52:13 young, they must be single Giant axons transmitted million ounces from the
52:19 of the tissue called hysteria Gambia to natural models. Mhm. Using electro
52:30 stimulated the surrounding fighters and how that can only produce large muscle contraction in
52:36 battle for the large tubular structures remain . So these were indeed giant
52:49 Mhm. Scientists proof we appreciate the of young findings. But here it
52:57 an excellent bars and robust enough to with the beginnings available at the time
53:02 one can survive for several hours when from many years. The interest in
53:12 context of the giant Verizon could be and analyzed, leading to the discovery
53:17 sodium islands were more concentrated outside the cell and potassium islands more concentrated
53:26 I'm refilling the empty Exxon solution to chemical composition experimenters were able to unravel
53:32 mechanisms of iron transport across the Okay, yeah. The general tax
53:42 large enough fast enough the final electrodes be inserted through the cell membrane.
53:47 into the accident in these early But fine glass tube was conserved.
53:59 acts on and security friends. Oh. Mhm. Then the deal
54:23 used to introduce a fine wire electricity was devoted to the inside and the
54:29 the measure. But the formation of early impacts was far too rapid for
54:36 study with any of the electrical measuring of the late 1930s, It wasn't
54:42 the 1950s following the wartime improvement of equipment such as the capital greatest
54:49 The major progress was made. Scientists that further in urban cross was transmitted
54:57 an accurate way of electrical potentials. this all or nothing action potential regenerated
55:04 by transient movements of Syrian cassie. Times across the middle membrane deception squid
55:13 that song unravel. The mechanism cervical and propagation of the action potential.
55:20 led directly to the development of drugs block action potential formation and those actors
55:26 and effect painkillers and dentistry and minor . Yes. Yeah. Mhm.
55:42 . Okay. Right. Pretty This is how it all came
55:56 Isolating john toxins, tying them practicing fishing knots on the Exxon's
56:07 accidental transport, slow fast uh external composition inside the cell concentration of islands
56:17 the cell. Um And the recordings recordings of the action potentials with the
56:25 scopes that became finally available. Uh . So for the action potential methods
56:35 recording action potential, most of these inter cellular that we talk about
56:41 So when we talk about electricity, and neurophysiology, you cannot do these
56:49 uh inside the cells and living humans example. So you can do it
56:58 mostly in the brain tissue and you do living animal brains with certain
57:05 And you can see that intracellular recorded potential and so on the order about
57:10 million balls. So you have about million old fluctuations about one to
57:14 That's how large of a change it in the plasma membrane, then how
57:20 it is on the outside. You also record extra several action potentials.
57:24 so a lot of neurosurgical techniques. operative neurosurgical techniques. Before operations,
57:31 may utilize extra cellular recordings even in brain to shoot the humans. Those
57:37 are very small on the order of 100 microphones. So you need to
57:41 very powerful amplifiers in order to pick the action potentials or any activity from
57:47 outside of the cells. How does cell generate action potentials? Well,
57:52 already talked about the fact that the receives an input. And if that
57:59 is strong enough. And if that is excitatory, that means the cell
58:04 going to de polarize and as it polarizes, it generates a number of
58:10 potentials. So if you look at this is the current on top that
58:17 injected into the south through the micro and these traces is what we call
58:22 wave like pulses because they look We turn on current positive current and
58:31 off positive current. If the cell a little bit of this current and
58:37 can equate this small deep polarization, a little bit of this current too
58:43 or weak stimulus or weak input. cell mate d polarizes plasma membrane and
58:50 can see the cellular response of the potential is not square. And that's
58:57 the membrane has resistance and capacity of . So you have to build up
59:02 charge across possible membrane. It takes milliseconds. The membranes are very good
59:08 overall it takes several milliseconds to build the charge. But the action potentials
59:14 get generated if it's a stimulus or input is stronger. If there is
59:22 deep polarization and more active. Synopsis for synopsis are engaged, the cell
59:29 reach the threshold production potential will produce certain output, certain frequency of action
59:37 the terminal. If that cell receives even stronger stimulus, a stronger deep
59:44 that will respond by producing higher frequency action potentials. So in a way
59:50 frequency of action potential. So the of action potentials over a certain period
59:55 time, over a certain stimulus period time is equal to the strength of
60:02 response, weak response or weak Sub threshold response, stronger stimulus action
60:12 . Maybe a few really strong stimulus strong input. You generate a number
60:19 frequency of action potential. So this one way in which the cells encode
60:26 strength of the stimulus is through the and frequency of the action potentials that
60:33 produced. Uh huh. Ionic driving . Yeah, that's what we discussed
60:44 . I mentioned the driving force to when we said we're going to talk
60:47 the equilibrium potentials. We talked about equilibrium potentials. Let's look at this
60:53 example here. First of all you potassium channels and sodium channels here and
61:02 of the channels are closed and there's membrane potential across plasma membrane. There's
61:13 car influx, there's no conductance, just reversal potentials for these two
61:19 There's nothing the channels are closed. the channels are closed there's no conductance
61:25 there's no charge accumulation. Now you potassium open potassium channels and potassium starts
61:35 the cell. Okay. And that and potential from zero value becomes
61:43 Uh huh. And so you have conductance that is dominating and you have
61:51 potassium current that is greater than zero the ions are flexing. What happens
62:00 potassium ions in this flux of ions the number and potential to minus 80
62:08 revolts. You reach the equilibrium potential . That means that at this point
62:18 electrical force is equal and opposite to chemical force. And there is a
62:25 . You see the conductance of potassium actually greater than zero But the current
62:32 zero for a while because the driving , which is the difference between the
62:39 and potential and equilibrium potential for potassium zero here. If the number of
62:48 is the same as equilibrium potential for eye on there will be a flux
62:54 ion. So there will be conductance there will not be net ionic movement
63:00 means no current flow into one favoring one direction of that zero.
63:11 , let's look at the action This is just one eye on trying
63:15 reach its equilibrium potential value. I place it all within the context and
63:20 slide the PowerPoint slide that I was earlier with the driving force and all
63:24 the potential values on it. So is happening at the rest is addressing
63:30 and potential. As we discussed, cell is leaking to potassium, potassium
63:36 is high. During the rising potassium conductance goes down, sodium channels
63:43 up this influx of sodium, more goes in, there's more deep
63:48 it's a positive feedback loop sodium This was dominating over potassium and the
63:55 potential is going to positive values at rising at the very top of the
64:01 this deep polarization of action potential. you're actually reducing the driving force for
64:09 , increasing the driving force for potassium the falling phase is dominated by potassium
64:17 . So these phases four phases of potential. address. That's dominated by
64:24 rising phase sodium falling face potassium and to rest which is dominated again by
64:32 channels being leaking. Mhm. Next we'll discuss voltage clown and I will
64:43 today's lecture by going over this one time. There's an action going on
64:55 the plasma membrane and it is resting at rest the plasma number and is
65:03 some excitatory inputs deep polarizes a little receiving inhibitor airports. This is the
65:10 member and potential that we're tracking. number of potential is dictated and is
65:17 on the interplay of four major ions potassium chloride, calcium. Each one
65:25 these ions has an equilibrium potential Nonce potential value reversal potential value on
65:33 sand, we can calculate the equilibrium values using the first equation. If
65:39 know that on the concentration of the RTZ f constant surveillance temperature log outside
65:48 inside. So we have the equilibrium Address. The numbering is about
65:57 This is zero millet balls Valium. is an overshoot well. This is
66:04 undershoot during the following phase of the potential. The membrane potential value falls
66:11 the resting number of potential value and gets rebuilt by sodium and potassium
66:19 So what happens here? And how this driving force influence the whole situation
66:27 rest? If you're addressing member and which is -65. The equilibrium potential
66:34 potassium is about -80 -19. So difference between B. M. And
66:41 . And P. And V. . E. K. Is a
66:45 big it's about $10 million. It's small driving force for potassium. It's
66:51 the rules of the biology that potassium are open and they're leaking. So
66:56 was dominated. Although the driving force is VM the difference between numbering potential
67:03 numbering potential is this trace here -65 . And the Vienna equilibrium potential value
67:11 sodium sodium have a big driving force . Yes it has a huge driving
67:19 . Remember the driving force is the between the membrane potential which is several
67:24 and the equilibrium potential for that ion has even greater driving force fluoride has
67:32 no driving force around its own equilibrium value. When the cell d polarizes
67:39 reaches the threshold value. This all non action potential event causes the opening
67:45 the sodium channels. sodium channels That means more sodium flexes in.
67:51 means more deep polarization. That means sodium flexes. Um more channels
67:55 More sodium flexes in. It's a feedback cycle. What sodium is trying
68:01 do is sodium is trying to drive overall number of potential. The overall
68:08 on toward its own equilibrium potential value positive 55 million balls. All the
68:15 are open and we're rushing in but doesn't reach equilibrium potential value for sodium
68:21 two reasons why. The closer the of potential value comes to the equilibrium
68:28 value for sodium. The small of driving force gets for sodium and the
68:35 force for potassium increases tremendously. These polarized potentials because the difference between where
68:42 equilibrium potential for potassium is and where membrane potential is is great at this
68:48 . Okay, so now you have situation where the driving force has
68:56 The second reason why the member and doesn't reach equilibrium potential value for
69:03 It's because of the kinetics of the channel. sodium channels actually close sodium
69:09 when they open and sodium is rushing and more channels open. They also
69:14 closed very quickly. So we'll discuss sodium channel kinetics and on thursday before
69:21 review. So, it never reaches sea equilibrium potential. But now,
69:27 is empowered which our island has the driving force. Now it's potassium sodium
69:35 are closed. So, guess what now? potassium the fluxus influx me
69:42 sodium coming in, potassium rushes out the cell. And what is the
69:47 trying to do this is entity potentially that is trying to drive the potential
69:54 to its own equilibrium potential value. , selfish sodium wants to take it
70:02 sound equilibrium. potassium wants to take down to some So it succeeds and
70:07 drags down between below the resting number potential because it's also a wiki to
70:14 . And then, with the help sodium Ak TPS pump. It gets
70:19 into the this fluctuating resting membrane potential during this rising and early falling phase
70:27 the actions. With our show the is in the absolute refractory period.
70:33 means that if you were to stimulate cell to produce a very strong stimulus
70:39 that we were talking about that produces frequencies of action potentials. If you
70:43 to do during this absolute refractory you would not be able to produce
70:47 action potential. You have to rebuild channel kinetics. You have to close
70:53 the channels, redistribute the ions across plasma membrane to prime it again for
70:59 next action potential. At this very of the following phase of the action
71:04 and the undershoot period. You have relative refractory period, which means that
71:12 cell is less likely to fire if receives the stimulus of an input.
71:16 if the stimulus and the input was enough, it could make the cell
71:21 . So it's relative the factory. during the action potential itself, it's
71:28 so you cannot have one action potential top of another action potential. And
71:32 cell dynamics. And the channel dynamics to change. We polarized in order
71:37 the south to produce the next action and to sustain the frequency of firing
71:43 these action potentials throughout the continuous Sure. So when we come back
71:48 thursday, we'll look at the sodium potassium channel dynamics. We'll discuss
71:54 clown and tetrodotoxin. So we'll do half an hour of new material to
72:02 this section and then we'll dedicate however it takes half an hour, 45
72:09 to review in any questions that you have. So if you don't
72:14 please check with casa support before you with me. If you're having difficulties
72:20 casa, please check your notes and . Uh the video points for the
72:27 , Make sure you have access to and bring your questions to give the
72:32 on thursday so we can have a session. You can guys do a
72:37 study over the weekend and face Thank you all. See you
72:42 Thank you for being on
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