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BIOL 4315/6315 Neuroscience Tue Th - NEURO Lecture 08 Action Potential III and Backpropagation
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00:02 this is lecture eight of neuroscience. I'm gonna go over some of the
00:08 that we already discussed in the previous . Two lectures but last lecture in
00:15 had several topics that are important that into the general understanding of how the
00:22 can produce such diverse patterns of action frequencies. And we talked about membrane
00:33 circuits and I've asked you to know symbols for the resistor variable resistor conductor
00:41 the battery and for the capacitor, should know these, be able to
00:53 them and also understand that the membrane be represented by membrane equivalent circuits and
01:00 you will have several features in But membrane has capacitance properties. It
01:06 resistance or conductive properties. Each island its own battery and you have the
01:12 that uses a TP to pump against rate. The second concept that we
01:20 was the resistive capacitive properties as continuation the number of equivalent circuits. These
01:28 the fact is that you have these capacity properties. And we talked about
01:34 if you have instrumentation, instrumentation will these square way perfect deep polarization,
01:43 polarization. The cell is not going respond immediately. It's a great capacity
01:48 be loaded very fast. But it 5 10 15 milliseconds for the charge
01:55 completely we distribute itself on the two of the capacitor and so therefore you
02:00 this smooth resistant capacity. The properties the cell is reflected in the cellular
02:06 . And we talked about the fact resistance is inverse to inversely proportional to
02:15 radius of small cells will have very resistance and capacitance. The greater that
02:22 area, the greater the south. letter is a capacitor, more area
02:27 area to store the charge. So is number two Important concept. We
02:33 . Number three Important concept that we is illustrated here will also find it
02:41 in today's notes. It's what we the ivy plots. So by convention
02:50 nana amperes, negative nana amperes here current by convention, inward current by
03:02 , no vault. So the why is current and the X axis is
03:14 V. M. In this case of the number. And that's why
03:18 called ivy plots. Current voltage plots you know put -100 here -50
03:28 0 50 100. And we talked the fact that some of the cell
03:41 and some of the channels in those can have these comic or linear
03:49 V. Curves. But we also out that the linear two D curve
03:57 same change in voltage. Same change voltage. 50 million balls positive with
04:06 million balls negative has the same cards this curve here, the same amount
04:14 current. So it's linear. And we talked about the fact that there
04:22 a lot of channels and therefore remembering that are non linear that means the
04:30 prefer to conduct in one direction versus . And we call these rectifying so
04:40 and rectifying. And that is that negative 50 Miller Balls, it conducts
04:50 lot of current. But in a 15 million balls it conducts very little
04:56 comparatively. So that channel prefers to ions inwardly versus outwardly. So there's
05:06 nonlinear. Then I said that for exam you should be able to identify
05:14 I. V. Plot for potassium I. V. Plot for sodium
05:20 . So it do something equivalent of . And I said this is -90
05:28 is the reversal potential for potassium. this would represent the task influxes if
05:37 cell gets d polarized cassie um is to be going out positive charge going
05:44 this outward current if you actually hyper , the number of potential using voltage
05:50 which will come back to it, this potassium will start flexing back inside
05:55 cell. So these equilibrium potentials are referred to as reversal potentials because the
06:02 at this value of minus 90 for if you push it one direction or
06:07 will reverse and travel in one direction the other. Also drew another plot
06:13 I said that you should be able recognize Reversal potential for sodium ions positive
06:22 . And so this would represent the current because sodium current would be
06:30 Alright, positive 55 million loans and is because the driving force is
06:37 Remember that the current can also be as gm conductance times V. M
06:45 reversal potential. So conductance when the minus K. 00. But
06:52 N minus E. N. A . The same thing. If you
06:58 at these hyper polarized or for for that these potentials that are below the
07:04 potential sodium is coming inside the cell that's what happens during the action
07:10 And if you drive this potential and it it really positive potential sodium will
07:15 reverse its direction. Okay. And I drew something that look like this
07:33 I said there's all sorts of rectification there's all sorts of I.
07:39 Curves. And plus for different channels different subtypes of cells can express 12
07:47 maybe even 20 different subtypes of both channels. So this is this is
07:53 tracking how much conductance, how much is flexing through a particular channel.
07:59 the curve of that channel? Where their equilibrium potential for that channel?
08:06 . Um Next permeability. Remember the changes. And if you calculate this
08:20 when you incorporate potassium sodium and you even incorporate chloride permeability is the highest
08:28 of resting membrane potential is the highest . Um During the rising phase of
08:31 action potential it doesn't change much for going into between those states. So
08:41 we recorded action potentials originally the voltage recorded the equals Ir voltage was
08:52 And we were controlling the current inject and you record the responses to these
08:58 injections in the form of the But if you just record the voltage
09:03 doesn't allow you to isolate specific currents potassium and see where potassium equilibrium potential
09:11 is. Not in the norms equation in the experiment in the dish or
09:17 the whole animal. And so to that we discussed this technique that the
09:23 clan. It's another concept. Again last election, you don't have to
09:29 draw the circuit and understand the circuit . What you do have to
09:33 This voltage plan Stigned us to hold potential in any desired value -100 -90
09:43 positive 50. And by having the to clamp the voltage that's what's voltage
09:52 V equals Ir. We were first voltage. We're stimulating the current with
09:57 voltage plant. We can record I so we're clamping the voltage in
10:03 the voltage. So now we have set up and what the setup does
10:11 clamp set up not just allows you clamp the member and potential of the
10:17 value but it also allows you to individual currents. I thought for a
10:34 . I didn't start this reporting. think I did. It shows that
10:39 important. Yes. So great. that's exactly what Hoskins and Huxley
10:48 They used voltage clamp to see what the currents behind this rising and following
10:55 of the action potential. So we the voltage of different potentials. And
11:03 by word current. And they saw the inward current reversed that about positive
11:10 positive 55 inward current. The sodium inside. They also saw this prolonged
11:16 sustained during the deep polarization outward And that outward current has got bigger
11:22 bigger and bigger as they clam because use the voltage clamp to clamp the
11:27 of their desired values. And and consequence of that they were able to
11:32 the early inward currents and the late currents. And later there was an
11:41 to start recording from individual channels. we understood that once you have this
11:48 polarization to remember that these deep polarization to be positive synaptic inputs exciting the
11:55 . And if the cell is excited and reaches the threshold then it will
12:01 the actual potential. And if this of the threshold will start opening sodium
12:06 , these are multiple sodium channels, is the deep polarization. They open
12:12 they close very fast. They don't open at the same time, but
12:17 close very fast after the So this some sodium firm through all of the
12:23 sodium channels. When you're recording there be hundreds of sodium channels could be
12:28 of sodium channels in the peace of membrane from which you're recording with
12:34 And this latest stage of the action , the following phase of the action
12:39 . You see that this deep polarization deep polarization. There is is more
12:43 more activation of the outward potassium current this outward potassium current is also some
12:49 these potassium channels that open with the of these channels are different because once
12:54 open they stay open for a prolonged of time. So the opening of
12:59 channels is short and transient. The of potassium channels is prolonged and potassium
13:06 responsible for re polarizing or the following of the action potential. We discussed
13:18 structure of the voltage gated sodium channel their gated by voltage because they have
13:24 voltage sensor and the fourth trans membrane , there's six trans member in segments
13:34 one of the subunits in the There are four sub units between us
13:39 and S. Six. You have four loop, which is the selectivity
13:43 for allowing only sodium or potassium channels go through. This is a mechanism
13:50 how these channels can be sensitive to and at wrestling member and potentials inside
13:58 sylvian channel in the S four trans segment, you have positively charged amino
14:04 residues and these positively charged them. know, acid residues are attracted by
14:11 negatively charged cytoplasmic side of the However, as the positive charge builds
14:19 and again these are the synaptic inputs , the cell D polarizes as this
14:24 charge builds up and the inside of membrane becomes less and less negative.
14:30 less attractive forces for this voltage sensor is positively charged amino acid residues to
14:39 close to the membrane and as it's attracted to stay here, it starts
14:46 upwards within the channel. And as shifts upwards and in the channel it
14:52 a conformational change and the three dimensional of this protein channel and opens the
15:01 . We talked about the fact that channels have two gates. This gate
15:07 closed here, it's called activation gait this gait that's a ball and
15:12 it's called inactivation gate. And so this voltage sensor slides up, if
15:18 have enough deep polarization to -40, voltage sensor will slide up and will
15:24 the opening of the channels of these gates. But that same confirmation will
15:30 that positive activation gates to open. moves the ball and chain mechanism and
15:37 chain swings the ball and plugs up Channel poor at this stage. #
15:46 , the channel is inactivated. So channels activate fast and they inactivate faster
15:54 fast and activating channels. And in to change this configuration here in the
16:01 , you have to make sure that sensor slides back down again. And
16:06 only way to do it is to the negative charge build up on that
16:11 of the membrane that will attract that sensor positively charged to slide down.
16:17 so you have to actually hyper release the stimulation. You can see
16:22 123 and the channels are no longer . Despite that sustained deep polarization and
16:29 and that's because they're inactivated. And only way to remove this inactivation gate
16:34 too hyper polarize the plasma membrane which now attract that positively charged voltage sensor
16:41 slide down towards the cytoplasmic side, yet another confirmation will change removing
16:50 It's called Dean activation and you have have hyper polarization for this to happen
16:57 subsequently the closure of the activation So two gates to control mechanisms you
17:06 to go 1234 in order for it open again. It has to be
17:12 and re polarized in order facility in to open again. Now when we
17:18 about voltage clamp, I said that have the set up with two
17:23 one is injecting the current, another is recording the current. And then
17:29 also have this clamping amplifier. These , voltage clamp and single channel recording
17:38 channel recording whole cell or patch clamp are called are done with a single
17:45 because the circuits electronic circuits are very fast and we can get sampling rates
17:51 10 20 kilohertz or 30,000 samples per . And that's enough for a single
17:59 to inject the current and also sample current. So they're capable of switching
18:04 these modern amplifiers and holding the cell a single electrode as well as recording
18:11 current with a single electorate. This clamp technique a lot of times involves
18:19 a piece of the membrane and as isolate a piece of the membrane within
18:24 , you may have different channels like example sodium channels. And now you
18:29 start testing in a more isolated Very simple system in a way how
18:35 channels may behave and what substances may affecting uh and causing as blockers or
18:43 of these channels patch clamp. So is our cell and you can have
18:50 attached recordings. It's an interesting motive where your electrode sections on. So
18:58 i if you recall they show you electrodes that are glass electrodes that are
19:04 to amplifiers. These electrodes are also to little suction tubes that are connected
19:10 the syringe that experimental controls. So suck up to the cells literally uh
19:17 a syringe. And sometimes even with own lips because the most sensitive control
19:23 the pressure when you're dealing with these sensitive minute systems is actually your lips
19:29 even your fingers. So you can things better for this particular experimental
19:34 So you suck up to the cells now you're gonna sell attached mode and
19:39 can kind of monitor what's going Like use the sellers in Montana and
19:43 be able to record the inputs that cell is recording synaptic inputs and other
19:49 . And then if you stuck up the cell and you break the membrane
19:53 you produce the section that's harder and break the membrane, you gain the
19:58 cell access which means your electorate And whatever the solution you have inside
20:04 lecture which we call inter cellular solution represent very much what is found in
20:10 cytoplasm of these neurons. Because if have a difference in osmolarity members cells
20:19 if you have a difference in in ion composition between the electorate and the
20:26 this electorate the tip of the electorate one micro meter. The palace,
20:33 amateur is 10 μ but that electorate actually humongous compared to the salad centimeters
20:41 whole electrodes. There's this ocean that connected to the little lake and if
20:47 ocean has completely different water composition and that will change completely the composition of
20:54 lake. So that's why you have match up the solutions, intracellular electric
21:00 , cytoplasmic solution and then you can all of the currents in the wholesale
21:05 whole star recording. And you will a lot of scientific literature that will
21:10 electro physiological recordings. And when you electrophysiology in the method section there is
21:15 to be a lot of electrophysiology. wholesale patch climb. This is wholesale
21:21 clamp important. This is advanced modes recording from this original current and voltage
21:27 recordings inside out recording this is really and important because we're going to start
21:34 about agonists and antagonists today. So this mode you suck up to the
21:40 , you're in a cell attached mode instead of producing this strong pulse of
21:45 , you very slowly kind of a fashion withdraw the membrane. And if
21:52 lucky you withdraw the patch of the and it has your channels of interest
21:58 that piece of the membrane. It's Inside Out because the inside the cytoplasmic
22:05 of that protein channel is exposed to outside world. So inside out inside
22:11 exposed to the outside world. Outside outside is exposed to the outside
22:18 And these types of recordings are very for any pharmacological or mechanisms of
22:27 For example, when we talked about Mackinnon and how he started deriving potassium
22:33 structure, we said that he was toxins because toxins would bind to certain
22:40 of the channel and it can block channel. So if it blocks the
22:45 , it's called antagonists. There are certain chemicals or toxins that can open
22:50 channel and keep those channels open. instead of sodium channel closing quickly,
22:55 just keeps open and that's physiologically a situation. So in this case let's
23:04 you have some suspected pharmacological botanical chemical that you think is gonna affect multi
23:12 sodium channels. But those substances do cross through the channels and they're not
23:18 soluble. So they're not crossing through membranes, you'd like to know if
23:23 substance affects that cytoplasmic side of the or the extra cellular side of the
23:31 . If its cytoplasmic and inside out if you put that substance and it
23:36 to that channel, you will see effect if it doesn't have a binding
23:41 here for that substance, that substance just be floating around having no
23:46 Then you go on the outside out , in which case you withdraw the
23:50 of the membrane and you do the pulse of suction. And what you're
23:55 for is that the plasma membrane will itself in the fashion that the extra
24:00 side of the program is on the . So, outside out now,
24:06 can take the same substance that we about that didn't have an effect on
24:09 inside. And you can say, , when I put it on the
24:13 , it actually blocked the channel. now I can tell that whatever that
24:17 binding is binding on the extra cellular of the channel. Mhm. So
24:23 will say, well, why would test benefit bonds? And inside a
24:27 of times you don't know where it going to bind. A lot of
24:30 . It could be a technique that can utilize to get to the next
24:35 and more precise steps of where exactly substances the binding and so on.
24:42 , So this patch clamp and ion physiology becomes a thing in the late
24:51 and early 60s people actually start talking channel biophysics and channel uh physiology.
25:01 . Huh. But the reality is we didn't have these tools 1939.
25:07 action potential. Then you have the time that develops fastest circuits. You
25:11 the ability to start using like voltage and things like that in the forties
25:17 now. So now you're coming to end of the fifties, you are
25:21 a biochemist become interested. There's something this membrane called channels and what properties
25:26 they have? And a very instrumental in describing the ability of this
25:37 Tetrodotoxin to bind to sodium channels was Toshio Narahashi from Japan. And so
25:46 for a minute, I'm gonna um you watch a little bit of the
25:57 And my disclaimer is always the Simpsons that Simpsons will offend everybody at some
26:04 , including themselves. So uh the why I'm showing this cartoon is not
26:11 it's offensive. I think it's because fun. But I'm showing this cartoon
26:16 if you remember this cartoon and the of this cartoon, then we should
26:19 able to also remember the neuroscience. and uh it's pretty cool. The
26:26 story is that uh Simpson senior decides he wants to try some new foods
26:33 town. And so he ventures out a japanese restaurant and he orders everything
26:42 on the menu and tries all of sushi and he's very satisfied. So
26:48 is the back story. Hopefully the work, she's here for me,
27:00 for you. If it is cut property. It's yes, yes it
27:04 poisonous, potentially fatal. But if properly it can be quite tasty.
27:09 must get the master. Oh miss . Your hair smells master, you
27:20 needed in the kitchen. I said for me. Done it. But
27:24 , we need your skilled hands. skilled hands are busy. You'll do
27:30 mm poison poison, tasty fish concentrate. Mm Fan tastic. Beautiful
28:05 isn't, God's sake. Don't eat bite. Couldn't possibly Mr Simpson,
28:12 shall be blunt. We have reason believe you have eaten poison poison.
28:18 should I do? What should I ? Tell me quick. No need
28:21 panic. There's a map to the on the back of the menu dumped
28:27 new homer. What'll it hurt Homer. I never heard of a
28:31 pork chop. Your wife agreed that should break this to you. No
28:35 doc. I can read mars like book. Oh it's good news,
28:42 it? No. Mr Simpson. in fact you've consumed the venom of
28:46 blowfish and from what the chef has me it's quite probable you have 24
28:52 to live 24 hours. Well I'm sorry I kept you waiting so
28:56 . Oh Mark, I'm gonna I'm gonna die. Well, if
29:02 one consolation is that you will feel pain at all. until sometime tomorrow
29:07 when your heart suddenly explodes. A little death anxiety is normal.
29:11 can expect to go through five The first is denial. No way
29:15 I'm not dying. Second is anger little do after that comes fear after
29:21 . What's after? Fear bargaining You gotta get me out of
29:24 I'll make it worth your while. , acceptance. Well, you all
29:28 go sometime. Mr Simpson your Astounds me. I should leave you
29:33 alone. Perhaps this pamphlet will be so you're going to die. But
29:47 marine world has other, more exotic in store known to hardly anyone in
29:52 West. For example, the the poisonous puffer fish of which there
29:57 about 100 species worldwide. You need license to sell puffer fish in
30:03 but as a buyer you need one . Iso Okamoto has a fuego restaurant
30:14 of course a license. He's not to buy the increasingly popular nontoxic farmed
30:20 , which can be recognized by its fins. Nor is he interested in
30:24 small species caught in the wild from waters. This true connoisseur is only
30:31 for one thing toxic wild as fresh possible and that means tora fugu,
30:39 puffer fish. The Kobe beef of cuisine, A single specimen of this
30:59 , which is only found in the of Japan may well cost €100.
31:12 of Tokyo's historic districts is located around temple. Most wild fugu restaurants are
31:19 be found here. There are about restaurants specializing in Fugu in Tokyo
31:25 From the outside they're usually easy to and they're always highly specialized. One
31:36 them is Ricky's oh tomatoes restaurant where even prime ministers drop by war says
31:43 name, the pure fish place. also need a license to prepare
31:49 The poison in Fugu is tetrodotoxin. 1000 times more potent than cyanide and
31:56 is no antidote. The poison paralyzes victims but leaves them fully conscious,
32:04 preparation is critical. The skin and of the fish are poisonous and they
32:09 not contaminate the non toxic meat on muscles. High concentrations of highly poisonous
32:26 are found in the innards, especially liver and ovaries. So now uh
32:34 kind of a common thing that we'll a little bit about these toxins that
32:39 found in the fish sometimes are found shellfish and clams and stuff that this
32:46 do not synthesize the toxins. So toxin is present in them as part
32:52 the other microorganisms as part of the . And so and they said it's
32:56 present the sea of japan. There be something environmentally. There actually causes
33:03 , these fish to to contain the that produced into the toxin. Okay
33:12 we'll talk about other toxins that are by clamshells. You may have heard
33:18 botulinum toxin also that's produced by micro , bacteria and and all expired canned
33:26 . Typically that's something to know that animals sometimes not all animals are animals
33:33 produced venom and they actually produce you know, but there's other animals
33:40 will carry bacteria and sometimes it's seasonal . So uh now the reason why
33:49 have to be very careful is you want the tetrodotoxin from the organs to
33:54 into the sushi meat and so you to separate it. It takes seven
34:01 I believe to get this license to a master chef to have a license
34:07 prepare food footage. So you will , well why why is this
34:14 And it is done poison? The triggers numbness in the mouth and is
34:23 . But here they do not. little bit of the toxin. That
34:28 be a laugh like trace amounts on fish. It's a thrill I guess
34:33 culinary thrill to experience tingling and numbness your mouth as you're eating this
34:40 Uh, I don't know if it any other intoxicating properties as far as
34:45 for your what Now the people seem happy and I would really like to
34:50 the, you know, a su temple someday. I've only lectured about
34:55 for the last 15 years and, and try some of this from,
35:02 know from a licensed place, I it's a, it's a really interesting
35:07 way to, to consume. It's around for a long time in the
35:12 and it used to cause a lot deaths in the past. Now you
35:16 have maybe one or two once that probably, you know, shaft and
35:22 slicing liver or something. And but used to be where you know,
35:29 was a thing you did and like out of it a lot. So
35:36 and it's a, for some people a, it's a family tradition and
35:42 would really love to tried and I tried to see anybody serves it here
35:48 Houston. I'd like to go to original place tried. Okay, so
35:57 is uh, now you understand why showed you the Simpsons because if you
36:03 it through the toxin it can kill and how it can kill you is
36:09 binding to sodium channels. Right? that's why we watch the Simpsons.
36:15 now, uh, what the story in your path of discovery, We
36:20 at roderick Mackinnon. This is Hashi story is that he basically had tetrodotoxin
36:28 isolated not surprisingly tetrodotoxin in japan from fish, but he doesn't know exactly
36:35 it does. He has a violent . This is 19 late fifties,
36:40 sixties, there's no voltage clamp in , the setup. I'm showing you
36:47 was just, you know, it so forward thinking it's like, what
36:50 these people doing there poking some electorate something doing something. So there's no
36:55 plan. So he comes with this from Japan flies to the United
36:59 spends about a year here doing some work and gives an opportunity to get
37:04 the voltage clan. And why is clients important for him? Because he
37:10 that the toxin they have the he discover but that he hasn't isolated the
37:16 and they can apply the toxin that can record currents and they can see
37:21 voltage and they can see that action get blocked. But he wants to
37:27 experimentally that tetrodotoxin blocks action potentials by voltage gated sodium channels. And so
37:35 carries the substance of the United States the voltage clown, You de polarize
37:41 cell and you see this very strong current followed by an outward current.
37:46 applies that through the toxin and he the specifically blocks the voltage gated sodium
37:54 and it doesn't stop the outwards. blockers are also called antagonists. It's
38:01 that closes the channel blocks the channel otherwise impedes with the downstream function of
38:08 channel, especially when later we'll talk g protein coupled receptors and there are
38:15 different toxins that combined to sodium channels there's actually different subtypes of both educated
38:22 channels that you learn about today It doesn't block the outward current
38:28 The toxin. And you have another ethyl ammonium. So you have either
38:34 substances natural substances or chemical substances ta a chemical substance that's a specific blocker
38:41 potassium outward currents. And you can it doesn't protect the more current.
38:47 . You have all of the tools hand. You have the pharmacological tools
38:51 , agonists, antagonists were just looking antagonists. You have a way to
38:55 the currents the voltage clamp. You see where these currents reverse. You
39:00 the kinetics and the properties of these where they open, where they
39:05 how fast they open, how fast close. And these toxins, as
39:10 mentioned, are not unique to puffer uh and sacks a toxin which is
39:19 clams and mussels. Again, these are sometimes geographical in the sense that
39:26 in the south we don't have much the clam and mussel culture, but
39:32 and northwest, that's a very big . And during certain times of the
39:37 when the temperature is hot, there's tide, there's certain algae blooms and
39:42 microorganisms in the water and it's not to eat especially raw plants and muscles
39:50 may contain some of these toxins and toxin would also target sodium channels.
39:57 here in the south as you the rule is that you don't eat
40:01 in the months that have are in just the opposite. Right in the
40:13 here when the temperature goes up, gets really hot and the animals are
40:18 breeding and for a number of reasons told not to eat crawfish. And
40:23 are the months that don't have are them. If it was the months
40:27 have are in them, we would completely out of business. But the
40:30 that don't have are in them is july and august and you're not advised
40:36 may thank you. So it's not during those periods of time to
40:43 So there's also seasonal variations and geographical and the different toxins that these animals
40:50 pick up in the sea of japan the Chesapeake bay and so on.
40:55 cocoa toxin comes from Colombian frog. then you wonder, well, what
41:00 these toxins do? They can bind different parts of the channel? Because
41:04 channel is a three dimensional structure. can affect Inactivation of this channel.
41:10 it doesn't inactivate fast enough, which that the 13 channel will be over
41:15 and you can exhaust the cells by the channel completely open or you can
41:20 the cells and the communication between the by blocking sodium channels. The
41:26 The facts are actually the fact that channels that we're studying in neurons produce
41:32 potentials in the brain. But sodium are found throughout in the heart and
41:38 the muscles and tetrodotoxin would eventually cause diaphragm time in the muscles of the
41:46 to collapse or be responsive due to of sodium channels and an ability to
41:52 , causing basically a suffocation on and . Now when we talked about robert
41:58 , we said the key used different to deduce the three dimensional protein structure
42:04 the channels. Because you want to where these agonists and antagonists are binding
42:08 their binding on the outside on the , whether they remember insoluble whether they're
42:14 inactivating the channel faster or whether they keeping the inactivation gate open all of
42:21 things. You can start actually deducing three dimensional structure. It allows us
42:26 study effects of channel brocade, all these toxins. So we can actually
42:32 what effects that have on different And not only just to do it
42:37 to see what it does, but to formulate therapeutic and pharmaceutical preparations using
42:47 aspects of these toxins that you can synthesize these chemicals or biological equivalence of
42:55 nature is potent. So the little can kill you. Some of these
43:03 can be very potent. Um and doesn't have to be a big,
43:08 know, shark or big bear strangling or something. It's just that there
43:15 these molecules small toxic molecules and venoms stuff and such that can can be
43:22 to us. This is the structure tetrodotoxin. This is taxi toxin so
43:29 some would have a similar structure. is the th theatrical ammonium, this
43:36 cocaine structure of cocaine has a low binding to sodium channels as well.
43:43 the fact is that there's a lot biological chemical and even illicit uh drug
43:50 legal drug molecules that will be targeting of the channels that we're talking about
43:55 these are some of the mechanisms of actions. Okay this is a reminder
44:01 the ivy plots and this is just illustration showing you how the currents in
44:08 particular recording reverses zero millet balls. you have a linear on the baby
44:16 . You see the currents are going this direction negative deflection and then they
44:21 and start going in the outward Okay this I. D. Plots
44:31 here shown for acetylcholine receptor channel. talk about acetylcholine receptor channels in the
44:39 section when we talk about synaptic we talked about acetylcholine receptor channels,
44:44 muscular junction but also in the central synopsis. Okay so this is a
44:51 diagram you welcome to to copy this take a snapshot of this. Uh
45:01 the following slide also shows you the . Okay the paradigm of this nonlinear
45:09 which you will find in the channels they will be rectifying because they have
45:13 dimensional structures that have the gates that and open and it influences their
45:20 D. Plots. Hmm lidocaine has binding side right here on the
45:29 Six trans member ring segment. So substances in this case lidocaine will target
45:37 parts of this three dimensional structure. one channel can be bound by many
45:43 biological natural substances or synthesized chemical Uh I guess in local anesthesia in
45:55 1860s, cocaine was somewhat used but common day local anesthetic of lidocaine.
46:03 does lidocaine do? So if you a cut, let's say or you
46:08 some stitches on your arm or if doing dental work and somebody's gonna drill
46:13 teeth, they inject lidocaine local anesthesia is light again that stays. And
46:21 will buy the voltage gated sodium channels it will block the signal transmission from
46:27 nerve endings and the perception of pain the periphery. So when you go
46:35 dentist office and you're getting a killing root canal, a crown, they
46:43 drill. They will inject, you go numb, your lip gets all
46:48 and then you come home and you don't feel the pain. The numbness
46:54 away and typically what happens in the hour. So you go, wow
47:02 hurting. So then you are you know by dentists. Take some
47:07 take some Tylenol, whatever works for is a regular painkiller. But that's
47:12 what lidocaine does. It basically blocks perception of pain. The pain signaling
47:17 the periphery. And uh as a anesthetic, this is an interesting system
47:25 you'll see it in a lot of literature. You see it in
47:29 frog size is a system and if look at this diagram here when we're
47:35 about the patch graham. When you a piece of the membrane, it's
47:40 that it will have one channel May 10 different you have 100 different
47:47 And one patch of the membrane in electorate may have a combination of sodium
47:53 potassium channels in that patch of the . They have a combination of three
47:57 subtypes of sodium channels. There are subtypes of even the same bolt educated
48:03 channel. Okay, so you have of this variety. But a lot
48:09 times you don't have enough of that in the system. So if you
48:14 to mammalian brain and the neuron take patch of the membrane and you try
48:19 record some current using all of the that we used voltage plan pharmacology,
48:25 antagonists. And you can't tell the . It looks like there might be
48:30 voltage created subtypes of sodium channels, you can't tell the difference. For
48:35 . And that's because you don't have of the signal. It's called signal
48:39 noise ratio. You have noise coming other signals from other channels. Then
48:45 may go through this system which is side. So you can isolate that
48:49 and you can genetically over express Using aside system this is a good
48:55 because as opposed to tend micro meters diameter for neuron, these eggs are
49:03 millimeter in diameter, you can place very large lecture you can over
49:08 So you can isolate a channel of and over express it where it's the
49:12 dominant thing in this new sides. you can produce your stimulation with your
49:20 and you can record the properties of different channels. So in a way
49:24 can amplify what you cannot see in complex system in your patch pipettes.
49:32 can go to choose sides and amplify express that one channel over express the
49:39 . Amplify the channel and amplify the function. Now you understand the kinetics
49:44 this current for this particular potassium Now you can go back in that
49:49 complex system in that patch with a and say know exactly what to look
49:56 because these are the qualities of this that I've amplified it. I see
50:01 now can actually recognize it as a more sensitive or smaller scale.
50:09 so what we've learned in the first is that neurons are individual is that
50:21 are quite diverse. There's exciting or neurons that excitatory neurons of the synapses
50:29 cause the synaptic deep polarization. These polarization are strong enough. The cell
50:36 produce the action potential. If the receives from another side inhibitory inputs,
50:44 going to be hyper polarization and the will be less likely to produce action
50:50 when the cell produces the action potential produces the action potential in this particular
50:58 . This particular location is called axon segment and the reason why the self
51:04 the action potential. They're not in dendrites and not in the soma.
51:08 then the accident initial segment is because accident initial segment is loaded with voltage
51:15 sodium and voltage gated potassium channels. ones that we've been talking about that
51:21 produce the action potential. That action that it is produced at the accident
51:27 segment we'll get regenerated. Each note wrong deal once it gets generated here
51:34 the card is kept insulated by the sheets here and the C.
51:39 S. It's the legal tender sighs only at the note of wrong there
51:45 will have this regenerative event where you have the same amplitude action potential regenerate
51:53 note of run there with the purpose reaching the axon terminal where the synaptic
52:01 is going to take place. This action potential in the axon terminal will
52:07 the release of neurotransmitter and communication onto other. South. Huh. And
52:14 reason why the action potentials will regenerate each note of run there is because
52:21 note of randhir is loaded with voltage sodium and voltage gated potassium channels.
52:28 ones that you generate action potential. the cell starts devising its own strategy
52:35 where it is going to place different protein channels that will receive signals different
52:44 gated channels such as sodium and potassium produce action potentials to re generate action
52:51 to regenerate them again at the external . And when we talk about synaptic
52:57 , the cell places voltage gated calcium of the external terminal which are absolutely
53:04 for the vesicles fusion and neurotransmitter So there's a strategy here, there's
53:10 uneven distribution of these channels and you them strategically. You want the after
53:18 initial segment to produce action potential. want action potential to be regenerated
53:24 you're gonna load these particular sides of cell with high concentrations of sodium and
53:30 channels. When we looked at the diagram by Ramon alcohol, we said
53:38 Ramon E actually drew these lines and said that den brides are gonna be
53:43 the inputs and Selma's. And then drew the axons. And he says
53:48 there is a rose running along these . So he says that there is
53:52 principle of dynamic polarization that neurons communicate information in one direction. That's what
54:00 postulated. So he postulated that the is not going to come back through
54:06 accent back into the soma. And doesn't. But what he didn't see
54:12 that beyond this action potential that we forward propagating action potential, there's also
54:20 small back propagating action potential back propagating that gets produced attacks on initial
54:31 So what are the reasons and how that same area in the accident?
54:40 segment produced two spikes, one moving forward propagating and one back propagating action
54:48 . Why would that be of interest us? And so for this material
54:57 , you have class supporting lecture documents this article And it was revolutionary in
55:09 . It's called who let the spikes and who let the spikes out.
55:16 exactly what the article is about. let the spikes out, Who would
55:22 generated the spike the action potential in excellent initial segment. It's written by
55:28 friend and colleague, chris Della professor neuroscience at Tufts University and at the
55:35 , his postdoctoral mentor, john who was at stanford University, brilliant
55:42 , also had one of my graduates a PhD from my lab that went
55:47 to stanford to do his postdoc with Connor and john Huggins, mentor David
55:53 . Uh Fang and Fang just received faculty position in texas here. So
56:02 uh there's some interesting intersections here that in life when you train people and
56:08 make friends in science. But what that time, Chris dull and john
56:15 explained is how it's possible to produce action potential back propagating action potential as
56:20 turns out that this action initial This is will be all of the
56:25 . Polarizing inputs coming in and then still have inhibitory inputs. A lot
56:30 inhibitory inputs are going to be around soma trying to stop the excitation from
56:34 . Polarizing the cell here, but D polarizing signal if it comes
56:40 it's gonna reach the acts of initial . It turns out that acts on
56:43 segment expresses two subtypes of both educated channels. The first subtype is
56:50 A. D. One flew into a purse Odion v voltage gated 1.2
56:57 a subtype marking for that channel. those are high threshold sodium channels,
57:02 means they require high levels of voltage in order to open. Remember these
57:08 voltage gated channels next to it and little bit further away from the selma
57:14 the direction of the axon. We another sub pack of voltage gated sodium
57:19 and that's N. A. 1.6 and N. A.
57:22 1.6 of the low threshold. That that they require low amounts of changing
57:30 in order to open. So what is that this D polarizing signal that
57:35 in from the dem rights. If strong enough and passes through the inhibition
57:39 enters into the axon initial segment, signal is not going to be strong
57:45 to open N 81.2 because they need . It's high threshold, it's going
57:51 bypass this positive current until it hits area that contains maybe 1.6 and maybe
57:58 on the low threshold sodium channels. will open up along the flocks of
58:04 and generating this explosion which is the potential and it's a forward propagating action
58:11 . So you've now activated low threshold gated sodium channels and produced forward propagating
58:18 potential. And because of this explosion there's additional positive charge in the
58:24 That positive charge in the areas the summit with this positive D.
58:29 charge. And now it's enough to the high threshold voltage gated sodium
58:35 And when those channels open up they a very small back propagating action
58:41 So if the forward propagating action potential in the water of 100 million
58:46 the back propagating action potential is on order of a couple to few milli
58:51 and amplitude. And so this back action potential would also like to go
58:57 the accent. But it can because a much larger deep polarization here,
59:01 lot more positive charge here. So fights this positive charge and actually back
59:08 into this almost and back into the . So you have basically two subtypes
59:15 channels low and high threshold. The threshold produced. The forward, the
59:20 threshold produce the back propagating action And the following slide tells you why
59:28 should care about it. No wait second. So what is back propagating
59:42 is a small current traveling back into selma and into the den,
59:47 What is forward propagating spike in the of forward propagating spike is to release
59:53 , that Texan oil terminal. So is the purpose for this back propagating
59:58 going into the selma and done And it's very important if this cell
60:03 and this cell response was producing an potential, the back propagating action
60:10 If it's within a certain time very fast time window will inform these
60:15 inputs that were tuned together. Because this cell fires all of these incoming
60:21 from different cells coming into the cell the cell doesn't produce an action
60:26 that means these inputs may be But if the cell produces an action
60:31 and produces the back propagating spike it these inputs hey we're communicating here and
60:37 closer does in time the better is learning between the pre synaptic inputs and
60:43 posse synaptic cell response. So it's to as plasticity or spike timing dependent
60:50 because it depends on the time that between this pre synaptic campus and the
60:56 synaptic response and the forward and back action potential. So in other words
61:02 very important for plasticity plasticity. The that you strengthen the synapses and make
61:09 more effective and reactive in time more in time. This is all a
61:17 of learning paradigms. So the more , the more communicative are the
61:23 the more likely they will tune to other and strengthen each other and if
61:28 circuits are incoming and communicating to the but the cell is not responsive.
61:34 circuits are not going to learn how communicate and how to do this temporal
61:41 and temporal plasticity in a very fast . I have this challenge of the
61:47 but I will save that for the section actually for you to look at
61:52 challenge of the day. The final of slides that I would like to
61:59 point out is that each cell is different cell can be different more
62:07 but most importantly it's different because of expression after certain molecular markers and when
62:14 talked about certain molecular markers we talked for volume in our field indian we
62:19 that they have certain elements inside of . But these cells will express diverse
62:26 of ion channels in each subtype of cell can express the same channel neuronal
62:33 in a pickle done rides this external itself all over the selma including the
62:39 to so it has a different expression of the same channel. That means
62:46 cell is going to have different biophysical because of the expression channel that expression
62:52 of these channels see certain cells will channels only in the axons, the
63:01 . So you have up to maybe even 20 different step types of
63:08 voltage gated channels in each cell. you can have an 81.2 and 81.1
63:15 81.6 and maybe there's an 81.12 and 11. Some of them are found
63:22 the heart muscles, others in the the different subtypes and then the brain
63:29 subtypes of cells will produce these different . Now, you know that these
63:33 channels, the subtypes of channels, I told me, some of them
63:37 high threshold, some of them have threshold. It means some of them
63:41 a lot of currency open. Others little currency. Open. Each one
63:46 the cells by having these different subtypes sodium and potassium calcium channels will have
63:55 different subset of ivy plots that these can produce. So one cell may
64:04 like this, another solemn may look another abstract variation of what I just
64:08 here and by having this diversity of channels and ion channels having a functional
64:18 open fast, stay open long, fast and so on. But having
64:26 , that's how neurons are capable uh these eclectic electric behaviors and they're not
64:38 eclectic as they are diverse. And obviously the cells that will have really
64:45 sodium channels and can close and open very fast will produce really fast patterns
64:52 frequencies of action potentials. Maybe the that have more potassium channels And also
64:58 as many fast sodium channels will produce patterns of action potentials. And so
65:05 having this ion channel expression that is in different subtypes of cells. These
65:13 channels having their own kinetics and ivy and properties can account for this diversity
65:24 the patterns that can be produced especially the inhibit their internet on center plant
65:31 again, this diversity is necessary so we can process very complex and very
65:37 inputs and come up with very diverse complex outputs, intellectual or eventually everything
65:46 a motor output. You can have great thought in your head, but
65:51 it's just in your head and you're gonna put a motor function to write
65:55 down or speaking to somebody, it's in your head, it's there,
65:59 produced. But to put it you need a motor pattern. So
66:04 of all of these, all of basically diversities of channels, properties of
66:12 membranes will allow us to produce these complex dialects as we call them,
66:19 neurons to produce really complex computational patterns the brain. Alright, so this
66:26 ends our first section of the course this is the material that you will
66:34 responsible for your midterm one. Uh lecture went better than yesterday's lecture.
66:45 didn't have some of the videos opened they were playing really strange commercials when
66:50 did. So, I may share lecture today with also monday section and
66:56 tell them that you have a better of the monday version. But you
66:59 a better lecture today. So you have to look at the monday
67:03 I mean, I will post you lecture from yesterday. So thank you
67:08 much. Review the material for the . Review and I'll see you all
67:13 thursday. Thank you for being here class. I appreciate you being here
67:17 same produce on zoom. Take care
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