| 00:02 | this is lecture six of neuroscience. we are on the action potential |
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| 00:09 | We'll actually spend some time reviewing some the material on neuronal membrane address. |
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| 00:16 | , we will start talking about the of the action potential and the major |
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| 00:22 | species and the players involved. Then come back on thursday. We'll talk |
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| 00:30 | membrane equivalent circuits. So it's important understand the passive membrane properties. We'll |
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| 00:35 | talking about active member and properties. we'll review membrane equivalent circuit. So |
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| 00:40 | you're in physics or engineering or computational developments and interesting concept to understand biophysics |
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| 00:49 | the plasma membrane within these equivalent electrical . Uh and we'll study the action |
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| 00:58 | in great detail. And the last is going to be back propagating action |
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| 01:03 | back propagation. So along with these there is a couple of flash supporting |
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| 01:09 | documents and there's also a slide on action potential that I prepared for you |
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| 01:14 | I'd like for you to follow for exam questions. So, point all |
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| 01:18 | this information out to you as we today. But just so you |
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| 01:22 | all of your information on the exam on the syllabus. All of the |
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| 01:27 | are in the lecture notes on All of the class supporting lecture materials |
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| 01:33 | also you have those links and that . If you can click on for |
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| 01:37 | or additional articles that will be reviewing the next few lectures. Okay. |
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| 01:43 | without further ado let's talk about some the concepts we discussed last lecture and |
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| 01:53 | concepts are likely uh to be exam . And what we started talking about |
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| 02:00 | we started talking about how there's a of charge across the possibility of bilateral |
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| 02:05 | plasma membrane or negative charges accumulated on outside and positive, negative on the |
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| 02:11 | and positive charge accumulated on the And so we need these fast fluctuations |
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| 02:17 | charge to handle many different things. one of the things that we |
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| 02:20 | well, why do you have these membranes? Why do you have neurons |
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| 02:26 | fire fast action potentials and have these at the level of the plasma |
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| 02:30 | And we said, well let's look this circuit which is reflexive circuit and |
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| 02:34 | circuit for example, and some of reflexive behavior that would be reflected and |
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| 02:39 | through the spinal cord would be stepping a nail that's reflexively. You will |
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| 02:45 | your foot, reaching onto something you reflexively withdraw your hand. But |
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| 02:50 | particularly discuss the patella tendon reflex, simplest kind of reflex that's Mona's |
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| 02:56 | And we discussed three subtypes of So there's three subtypes of cells will |
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| 03:01 | on the task, you have to no morphology. They have to know |
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| 03:04 | function. You have to know the they release and when they're excited for |
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| 03:09 | . And so we talked about this of the quadriceps muscle. Mona's synaptic |
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| 03:15 | through sensory neurons, ganglion cell and motor neuron. And then we also |
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| 03:21 | it for this reflex to be effective have to relax opposing muscle which is |
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| 03:26 | hamstring and that happens through involvement of synopsis or Paula synaptic activation here activation |
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| 03:33 | the inhibitory interneuron that now silence is motor neuron that would otherwise activate that |
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| 03:39 | . But now that muscle is So all of these great exam questions |
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| 03:44 | . Then we said, okay, these uh ions that are separated by |
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| 03:49 | membrane, they cannot cross the apocalyptic layer. So we need channels, |
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| 03:54 | need membrane channels that will allow for crossing of these ions. And we |
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| 03:58 | the basic concept of the building blocks the amina assets forming the polyp a |
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| 04:04 | . Secondary tertiary coordinate structures are finally the subunits that come together in the |
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| 04:10 | structure forming the channel the channels that talking about when we're talking about resting |
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| 04:16 | potential and the action potential, the gated channels, they're gated by |
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| 04:21 | There's nothing that binds to these Were talking to. There's no ligand |
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| 04:26 | binds to these channels that opens the . So the voltage is going to |
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| 04:30 | how open or how close these channels . They're sensitive to voltage and each |
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| 04:36 | of the ions has its own specific . So these ionic channels. Both |
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| 04:42 | ionic channels of sodium it specifically will for the flux of sodium potassium potassium |
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| 04:48 | for it and so on. So is, as discussed some importance given |
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| 04:55 | the size of the ion but also size of the ion compared to the |
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| 05:00 | of the waters of hydration for smaller will have larger waters of hydration. |
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| 05:06 | also talked about how these amino acids they form themselves into the channel subunits |
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| 05:12 | charged and some of them will leave or positively charged residues to some of |
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| 05:17 | amino acids in these chains. In case of the sodium channel will have |
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| 05:22 | negatively charged amino acid residue that's sitting in the most inner lumen of this |
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| 05:27 | is a short interaction with the sodium stripping of the waters of hydration and |
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| 05:32 | the sodium ion is one of the this amino acid residue serves here. |
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| 05:39 | we talked about arms law. The Ir we talked about how you can |
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| 05:45 | arms law. I. Is equal times V. Otherwise it with the |
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| 05:50 | . Is equal V over R. jeez the conductance and resistance is inverse |
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| 05:56 | the conductance. So that's where you high equals G. D. And |
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| 05:59 | will come back later and in this . And so we have these channels |
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| 06:04 | these channels will allow the passage of main ionic species. And we also |
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| 06:09 | the pumps. And if you recall channels and the flux of ions through |
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| 06:13 | channel will be determined by two The chemical force and the electrical force |
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| 06:20 | the movement of these ions is always concentration gradient via the pumps using a |
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| 06:26 | . P. Always putting more potassium the inside. And remember high concentrations |
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| 06:31 | potassium on the inside of the cell putting more sodium on the outside of |
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| 06:35 | cells. So chemistry, if it the only driving force concentration gradient, |
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| 06:45 | we would have ions flux until they equal on both sides of the plasma |
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| 06:50 | . But we have unequal distribution of ions on two sides of the plasma |
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| 06:55 | . And that is because we have interactions with ions that carry a |
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| 07:00 | So and ions negatively charged ions will attracted by anna positively charged end of |
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| 07:08 | battery in this case. But an will be repelled by the life |
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| 07:13 | will be repelled by the catholic by negatively charged then. So it's very |
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| 07:18 | that we take into consideration that the of ions through the channels is dependent |
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| 07:26 | the chemical concentration gradient and the electrical . And so we discussed the case |
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| 07:30 | if you have high potassium concentration here this side of the membrane and you |
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| 07:35 | this potassium channel potassium is going to down its concentration gradient to the opposite |
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| 07:41 | of the plasma membrane but it is going to equalize because as more positive |
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| 07:49 | of potassium flux is to the outside more positive potassium charge than positive charge |
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| 07:55 | up on the outside of the plasma and this positive charge, the electrical |
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| 07:59 | now starts repelling the positively charged ion repelling the like charge. So that |
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| 08:07 | at which the concentration gradient the chemical is driving ions into one direction and |
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| 08:14 | electrical potential force, the charge is that ion and repelling it in the |
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| 08:20 | direction. When these forces are equal each other, there's no net |
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| 08:25 | And that potential is referred to as potential or ionic equilibrium potential. The |
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| 08:32 | happens with sodium. Not all of sodium is going to flux into the |
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| 08:36 | concentration zone. It's gonna start getting the positive charge too. So there's |
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| 08:42 | interactions. So we said that first all, the four ionic species that |
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| 08:47 | have to know well and their concentrations potassium chloride and calcium, the outside |
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| 08:53 | the cells are loaded with sodium chloride environment outside of the cell also has |
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| 08:59 | high concentration of calcium two million moller opposed to the inside of the |
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| 09:04 | The inside of the cell is dominated the castle. And so it's important |
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| 09:09 | you remember either the approximate minimum all concentrations of these ions or the ratios |
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| 09:17 | these concentrations on the inside of the because this is one of the important |
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| 09:22 | in nursed equations. So, you be asked these questions to recognize the |
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| 09:26 | calculations, although you will not have use a calculator or calculate it |
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| 09:32 | And so we discussed the fact that know that on the concentrations that are |
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| 09:37 | in the south. And because we that on the concentrations we can calculate |
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| 09:42 | potentials for each individual ion. And when we calculate equilibrium potentials or nerves |
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| 09:49 | , they're calculated for each individual ions sodium chloride and calcium. The important |
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| 09:59 | of this is e reversal of global for ion is 2.303. R. |
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| 10:05 | . C. F. We have gas constant, we have the Faraday |
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| 10:09 | , we have the temperature, we're temperature 37 C, which is physiological |
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| 10:13 | temperature and disease surveillance. So if collapse from unavailing ions such as sodium |
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| 10:19 | potassium you collapse 2.303 R. C. F. You have miller |
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| 10:24 | 61.54 million bolts and you have to a log of concentration of that ion |
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| 10:30 | the outside. So potassium K. . Is concentration on the outside versus |
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| 10:37 | on the inside. And so you'll that the same happens abbreviation here for |
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| 10:43 | and potassium. It's no different for , it becomes minus 61.54 because chloride |
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| 10:49 | a negatively charged ion, it's going be divided by minus one here and |
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| 10:53 | calcium it becomes half of this because two plus. So it's going to |
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| 10:57 | divided by two. Okay, so gonna get this abbreviation and what equilibrium |
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| 11:03 | for each ion does. It tells where those two forces the potential in |
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| 11:09 | these two forces are going to be to each other for an individual |
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| 11:13 | But as we now multiple ions across the plasma membrane and have a contribution |
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| 11:21 | the resting membrane potential and also it's action potential. And so we talked |
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| 11:26 | the golden equation and Goldman equation calculates D. M. Which is membrane |
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| 11:33 | , not E. Ion which is potential, firearms but D. |
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| 11:37 | Which is membrane potential. D. . Takes the same terms are TCF |
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| 11:42 | the nurse equation, you collapse them 61.54 and introduces more than one ionic |
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| 11:51 | . So you have the casting here sodium and it says that what's really |
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| 11:56 | is how permeable the membranes of TKS for potassium DNA permeability for sodium. |
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| 12:03 | important it says the permeability is for plasma membrane. When you're calculating the |
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| 12:10 | . R. Or the number of . And what we discussed is that |
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| 12:14 | by the neuroscience the brain rules the number and potential. The cells are |
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| 12:22 | valuable to potassium. So when the are not very active, you have |
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| 12:28 | which is high concentrations in the inside potassium channels we call them leak |
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| 12:33 | potassium leak channels that are open and allowing for potassium to slowly leak out |
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| 12:40 | potassium is leaking out. The mum is most permeable to potassium. So |
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| 12:46 | permeability. This DK value is 40 higher than P. N. |
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| 12:52 | From the ability for sodium and this addressing memory attention and what you will |
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| 12:57 | today is that these ratios of permeability concentrations may shift a little bit but |
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| 13:06 | are pretty tightly regulated and they are um re concentrated and spatially buffered uh |
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| 13:15 | well. But the premier ability And what happens if you now put |
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| 13:23 | here and put 20 for sodium you're have a completely different value for a |
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| 13:29 | of potential. So this is how equation based on the concentration is based |
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| 13:35 | the terms from nurse equation but also permeability whether that channel for sodium is |
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| 13:40 | or not, that's what permeability And as I said addressed the cell |
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| 13:44 | is most permissible to potassium because potassium are open and they're leaking these rules |
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| 13:51 | permeability changes these rules change the actual value also changes. So this top |
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| 13:59 | again points out the nurse equation which equilibrium potential for one specific ionic species |
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| 14:07 | D. M. Which is the equation. And the membrane potential which |
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| 14:11 | calculated using multiple ionic species and permeability for those species. And if you |
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| 14:18 | to very interested in this and want run through calculations plug in chloride here |
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| 14:24 | chloride And put blow permeability for chloride one and see if it affects the |
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| 14:30 | and potential. Then put the permeability chloride at 40 and put decent one |
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| 14:35 | see what that does. You This is really kind of an interesting |
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| 14:40 | process if you want to go I'm not asking you to do that |
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| 14:44 | if you're into it it's it's a simple thing to run through just say |
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| 14:48 | it really works. So the other that I mentioned is that these local |
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| 14:54 | in whether it's potassium concentrations or some islands are tightly controlled by the cellular |
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| 15:01 | and the important players in controlling these increases in potassium concentration of astrocytes. |
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| 15:09 | the graph on the left shows ko is outside potassium concentration of mela moller |
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| 15:16 | the X axis versus a membrane potential of notables from the Y axis. |
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| 15:22 | arresting member of potential is somewhere between 70 65 minus 80. Remember the |
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| 15:29 | why I'm gonna give you a certain and values to follow is because here |
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| 15:34 | member and potential addresses indicated minus 60 minus 75. Already told you, |
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| 15:39 | gonna ask you two questions minus 60 minus 67. I'm gonna give you |
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| 15:45 | values that you should follow. And is some discrepancies. The same textbook |
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| 15:49 | give you two different values and there some discrepancy in measuring the resting membrane |
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| 15:55 | with some cells naturally will have slightly number and potential that can be more |
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| 16:01 | polarized minus 75 minus 80 or they be more d polarized minus 65 minus |
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| 16:07 | minus 55. Even in some And that can be a part of |
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| 16:11 | physiology but it also can be a of some pathology that is going on |
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| 16:16 | activity with these numbering properties and redistribution charge and separation of charges not working |
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| 16:23 | well potentially due to a failure of ionic channels. Where now you can |
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| 16:28 | this membrane potential shift potentially towards more potential. So each one has, |
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| 16:35 | equilibrium potential value is slightly different values are even given to equilibrium potential values |
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| 16:42 | sodium. You will see plus 55 62. Again, I will give |
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| 16:46 | those values that I want you to . And the reason why is because |
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| 16:51 | is the same in biology. We about those different subtypes of cells and |
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| 16:56 | said, what do these different subtypes cells do? Well. They speak |
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| 16:59 | dialects which was mostly the inhibitory cells spoke all of these different dialects and |
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| 17:04 | parameter cells were speaking the same dialect one frequency of finding action potentials. |
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| 17:10 | ? So you have these channels and of these different subtypes of cells. |
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| 17:17 | we also said that in order for to know that there are really different |
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| 17:21 | of cells we said we have to them for self specific markers. Cell |
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| 17:26 | markers or what the proteins inside the . They are variations of voltage gated |
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| 17:34 | that one cell has and the other doesn't. So that means the frequency |
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| 17:39 | action potentials they can produce is very dependent on the properties of these |
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| 17:46 | And genetics and translation from the code these channels, right into the protein |
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| 17:52 | the messenger. And a you have subtypes of these channels slightly. Therefore |
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| 17:58 | have slightly different functionality. Therefore you have slightly different measurements of membrane potential |
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| 18:04 | potentials for these islands and also depends the environment. So we'll study the |
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| 18:10 | ear, the cochlear and the inner and the organ of corti and that's |
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| 18:15 | by n dola. So as opposed cerebrospinal fluid, which we're talking |
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| 18:20 | which is high in sodium chloride. indolence is very high in potassium. |
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| 18:25 | the rules there are different. so there are these micro environmental changes |
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| 18:30 | happen now. If there is a of activity, what happens during heightened |
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| 18:35 | of activity? There is accumulation of on the outside. And this shows |
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| 18:41 | regular concentration of compassion on the outside about $5 million 3.5 to about 7.5 |
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| 18:49 | . The number of potential is pretty polarized. Close to address. But |
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| 18:55 | shows that if you increased this extra potassium concentrations in 10 2030 million moller |
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| 19:05 | million moller you have polarized the sell 2030 40 million balls. Thanks. |
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| 19:15 | you'll learn that if you d polarize cell close to $40 million, the |
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| 19:19 | will start producing action protections and so don't want that if there is something |
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| 19:24 | on with local environment where there's heightened of potassium in the synapses around the |
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| 19:31 | . You you don't want that sustained of potassium there locally because it's gonna |
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| 19:37 | d polarizing the surrounding cells. So really graph illustrates that if you increase |
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| 19:44 | society the potassium the number of potential going to de polarize and so you |
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| 19:51 | to prevent that and astrocytes have a morphology for spatially buffering these increases in |
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| 20:00 | concentrations if you recall astrocytes have their or end feet sitting around the |
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| 20:08 | So they control the synaptic transmission and regulate the molecules around the synopsis and |
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| 20:16 | the south they also have their own sitting on the blood brain barrier. |
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| 20:20 | they're controlling what is one of the for molecules to pass into the brain |
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| 20:26 | the blood or not. And they these extensive processes spatially so they can |
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| 20:35 | up this abnormal increased concentration of potassium redistribute through its own So Mazz and |
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| 20:45 | quite widely spatially and beyond that. we study synaptic transmission we'll also learn |
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| 20:52 | many neurons and cells in the brain cells also have gap junctions electrical junctions |
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| 21:00 | those are different from chemical synopsis or junctions and the electrical synopsis will allow |
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| 21:07 | astra side to connect another astra side pass that ion pass that positive charge |
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| 21:14 | an adjacent astra side which has processes are spatially widely distributed. So very |
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| 21:20 | these local increases in potassium will be buffered by the specific networks. And |
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| 21:29 | is important because if you don't spatially and you have sustained increases locally and |
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| 21:35 | concentration you do polarize the cells by . You make that network very |
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| 21:41 | And if you sustain it for a time and you keep raising this concentration |
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| 21:46 | reach this point of hyper excitability excitation over and inhibition is no longer able |
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| 21:52 | control the cell activity. Huh? uh in your book as I mentioned |
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| 22:06 | may have mentioned not the great uh of discovery write ups. They feature |
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| 22:14 | scientific discoveries and people behind these And I like to talk about robert |
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| 22:22 | because your book talks about robert Mckinnon I'd like to talk about roderick Mackinnon |
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| 22:29 | I want you to take the story here together with the knowledge or the |
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| 22:34 | that you're seeing a slide. And story is that Roderick Mackinnon is a |
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| 22:39 | doctor so he's an M. But he decides that his passion or |
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| 22:48 | quest that he wants to pursue the if he wants to the discovery he |
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| 22:55 | to do is related to potassium channel . So he leaves medical practice and |
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| 23:05 | into a lab and starts working using flies as a model electrophysiology side directed |
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| 23:17 | to genesis toxin injections in order to deriving the three dimensional structure of the |
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| 23:25 | channel. So we're talking about eighties and uh many of his medical colleagues |
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| 23:33 | like you're not happy to be a doctor here, you're gonna be a |
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| 23:37 | now says. Yeah that's my that's really tickles me. That's what I |
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| 23:42 | to answer. I want to know structure of the fashion challenge. So |
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| 23:46 | you're looking forward in your careers in , you are on a quest. |
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| 23:52 | you don't have it yet, don't about it on that quest for that |
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| 23:55 | . If you're on a quest for change too, it is important that |
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| 24:00 | don't perceive your quest as a quest degrees that you perceive your quest as |
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| 24:07 | quest to get to the final answer . The point that you want to |
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| 24:13 | at the pathway in the career? forward their steps back that people fall |
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| 24:21 | , bring themselves back up the year 10 years later surprised everybody if it's |
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| 24:27 | long but it's never straight. The is never straight. You move |
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| 24:34 | You come to intersection medical school, school graduate school, this school, |
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| 24:38 | school. What is the pathway? is the quest? So search deep |
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| 24:45 | yourselves what the quest is. And roderick Mackinnon, his quest is the |
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| 24:52 | of the potassium channels. So he I'm gonna be now working as a |
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| 24:59 | . This is what he wants to . Uh Is it better to be |
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| 25:05 | . D. Versus P. D. I don't know is it |
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| 25:07 | to be a nurse rather than the cutting somebody's brain? I don't know |
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| 25:11 | each his own. It's what you're about is where what you're driven about |
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| 25:18 | is one of my great colleagues says is the shower test in the morning |
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| 25:24 | you wake up and you're in the ? What are you thinking about? |
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| 25:29 | can be thinking many different things of , but what you're thinking about as |
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| 25:34 | relates to your professional development or some quest that you want to do, |
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| 25:39 | very easy to think about fishing and know things like that. Fun things |
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| 25:43 | but what is what is kind of is what we want to do, |
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| 25:46 | is what is it? And you have to find motivation and have to |
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| 25:51 | motivate yourself and other people can help motivate and also other people can try |
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| 25:57 | break you down. So you have stay strong on that, on that |
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| 26:01 | , on that, on that goal you're trying to achieve. So let's |
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| 26:06 | a little bit about what roderick latino . So he takes flies and uses |
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| 26:10 | flies as a model. So any quest in this case starts with some |
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| 26:15 | of a model. It can be model of the salad, can be |
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| 26:17 | computer model, it can be a that can be a monkey, can |
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| 26:22 | a humor to a certain extent. the work that he's doing is doing |
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| 26:27 | the fruit flies because fruit flies multiply it's a great system then vertebrates. |
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| 26:35 | you don't have to do the same of a red tape regulatory lab work |
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| 26:40 | you do for the for the rodents higher order species. So he works |
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| 26:48 | shaker flies and he's looking at this and as I told you this channels |
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| 26:54 | very complex strings of polyp appetites that built into the ordinary structures. So |
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| 27:01 | using what is called gene mutations through directed me to genesis. It's a |
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| 27:06 | that you can try to mutate a sequence in this protein. And as |
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| 27:12 | mutate the gene with a certain code a specific sequence of the protein. |
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| 27:18 | can see what part of the proteins important and is the whole building as |
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| 27:26 | as the whole channel. As important every amino acid in its position is |
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| 27:31 | for this channel. Mostly. What want to know is the structure of |
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| 27:34 | channel. Because we want to understand this channel opens and closes and what |
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| 27:39 | the pharmacological toxicological, the physiological molecules regulate the opening and closing of this |
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| 27:47 | . Okay, so he actually uh his gene mutations, he's trying to |
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| 27:55 | the innermost lumen of the channel that's most important thing and he's targeting basically |
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| 28:01 | sequences of amino acids that control the and closing of this channel. I |
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| 28:06 | you that these channels that we're talking are voltage gated. But these channels |
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| 28:11 | are voltage gated, they can be by molecules abnormally. So normally physiologically |
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| 28:17 | binds to these voltage gated channels. if a spider bites you, if |
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| 28:23 | snake bites you, if there is chemical toxin they have the ability to |
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| 28:30 | to these volt educated channels. the other thing that you notice is |
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| 28:35 | that certain mutations in this potassium channel shaker flies, they call shaker flies |
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| 28:42 | they're shaking and it's a model for seizures and in fact people that have |
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| 28:52 | have mutations and voltage gated potassium So then you ask this question |
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| 28:59 | how close is this? If I'm at a fruit fly some sequence of |
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| 29:05 | acids, I'm using some toxin that's nature. There's some spiders, it's |
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| 29:11 | to find. Mhm. What what it do for humans? And then |
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| 29:20 | ask yourself, wait a second. we have like 70% home ology and |
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| 29:27 | and gene codes with warms? we do. So what does that |
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| 29:33 | ? That means that there are certain amino acid sequences. That means that |
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| 29:40 | amino acid sequence that is important in potassium channel that is gonna get bound |
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| 29:46 | that spider toxin? That sequence may in high order species all the way |
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| 29:52 | humans and do not all of But so there are conserved amino acid |
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| 29:57 | between the species actually. And so becomes important if you see a certain |
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| 30:02 | and that sequence is inside the channel means it's important for flies. It's |
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| 30:06 | for rats. It's important for it's important for humans. Okay. |
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| 30:13 | he is deriving this three dimensional structure he doesn't see the channel, he's |
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| 30:18 | all of these techniques. Again, didn't say I want to be an |
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| 30:22 | physiologist and see record when the channel open and closed. I want to |
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| 30:27 | a molecular geneticist and just induce a in this in this channel. He |
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| 30:33 | I have a problem. I want know my class what is the structure |
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| 30:37 | this channel? So I'm gonna use tools that I have available to |
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| 30:41 | I'm gonna learn this new technique if have to to use the tool to |
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| 30:45 | to the answer of my question, doesn't quite get to the answer by |
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| 30:50 | these techniques. So he decides to labs again and this time he decides |
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| 30:55 | he is going to be a And if he's going to learn this |
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| 31:00 | the time, very hot technique called ray crystallography. And people look at |
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| 31:10 | , this is like you are you uh specialized in engineering. And you're |
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| 31:16 | that I'm gonna be an endo Now, you know of course, |
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| 31:22 | know the beauty is that with PhD you have proven you can actually jump |
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| 31:27 | fields with M. D. You do that. You have to if |
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| 31:33 | a dentist, you're doing dental That means you're not doing root canals |
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| 31:38 | goes under endo Dantas. You wanna an orthodontist. You have to go |
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| 31:41 | endo dental training school I think two additionally practice. Then you're under Dantas |
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| 31:48 | close related areas right? Still drilling just you just cannot get deep enough |
|
| 31:54 | you're not endo Dantas because then you're about nerves and tissues and other |
|
| 32:00 | But in science you actually can do and you see sometimes really interesting careers |
|
| 32:07 | really interesting scientists are the ones that able to dynamically reposition themselves based on |
|
| 32:15 | happening with discoveries based on new technologies out but also being unafraid to reposition |
|
| 32:22 | . It's very easy to get stuck that same rut and do things for |
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| 32:27 | many years. The same way. the same techniques using the same person |
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| 32:31 | the lab the same post doc has there 20 years using the same technique |
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| 32:35 | 20 years. I have plenty of like that and we learned it 20 |
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| 32:40 | ago. They're still just improving it little bit but it's the same. |
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| 32:44 | he says I want to visualize this structure and what X ray crystallography does |
|
| 32:49 | a really sophisticated science especially at that . You trap a single protein inside |
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| 32:55 | crystal and that crystal is translucent and you expose the X ray beams you |
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| 33:05 | onto that crystal and you actually can the structure and visualize the three dimensional |
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| 33:12 | of the programs. And that's what Mackinnon does. And so he describes |
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| 33:17 | poor loop or the hairpin loop, really important, that controls the passage |
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| 33:22 | the ions inside these channels. And describes the structure of these channels. |
|
| 33:29 | , it would be really interesting to to somebody like roderick Mackinnon now because |
|
| 33:34 | you have been following anything about protein or may have even heard it because |
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| 33:40 | was on NPR was on the national maybe a year and a half ago |
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| 33:45 | so that there are no artificial computer driven algorithms that can predict a |
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| 33:54 | much better than X ray crystallography, and faster than with way less |
|
| 34:02 | So would you really go and learn field now or would you say I'm |
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| 34:07 | be artificial intelligence guy in neuroscience, gonna use that and do something |
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| 34:13 | And that would be something that I'm you about is a dynamic adaptation to |
|
| 34:17 | is happening in the modern world, you need to pick up the skills |
|
| 34:21 | move. Another example is, you , some parents learn how to use |
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| 34:27 | and others didn't make the grandparents for guys, but and others didn't and |
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| 34:33 | ones that learned how to use email of like had this other aspect of |
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| 34:38 | that they pursued and have the communications they had over the grandparents that didn't |
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| 34:43 | the email didn't go on social media something like that. So the communication |
|
| 34:49 | also very different. Uh So it what you do is what you |
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| 34:56 | And he he decided that his again it's not a degree, it's |
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| 35:02 | a position. It's a it's a a specific puzzle that he wants to |
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| 35:08 | . And he'll do multiple techniques he use to his advantage to solve this |
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| 35:14 | will switch the institutions go from D. Two P. H. |
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| 35:18 | . To do this kind of Okay so today we will start talking |
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| 35:24 | the action potential and you will learn lot about the action potential. We've |
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| 35:31 | dwelling in this resting membrane potential world you will learn that the rules of |
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| 35:37 | changes for the rising phase of the potential which is dominated by sodium |
|
| 35:44 | You have overshoot which is above zero evolves. The following phase of the |
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| 35:50 | potential was dominated by potassium heat And then there's this undershoot which goes |
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| 35:56 | the resting membrane potential and you have re polarization that is slow. That |
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| 36:00 | partly rebuilt by N. A. . Uh pumps 80 P pumps. |
|
| 36:07 | the first one I showed you the number in potential is that if the |
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| 36:11 | is resting number and potential and you an electrode you will see a shift |
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| 36:15 | minus 65 million balls and it will around that the resting number and potential |
|
| 36:20 | fluctuating up and down because as the receives glued in eight inputs, there's |
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| 36:27 | potentials that are excitatory and they'll de the cell a little bit. It |
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| 36:32 | gabba inputs, synaptic inputs that are that will hyper polarized itself. So |
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| 36:38 | cell will keep walking in the surround walk recall around wrestling number of potential |
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| 36:44 | it gets a strong enough input excited input and it fires an action potential |
|
| 36:50 | you record an action potential, inter early. It's a very fast |
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| 36:56 | 1 to 2 milliseconds in duration and 100 millibars and sides. So it's |
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| 37:04 | very very fast electrical spark that neurons . You can record these parts again |
|
| 37:11 | intracellular recordings with the whole cell recordings will discuss. But also you can |
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| 37:17 | it extra cellular, you're recording it cellular early. You notice that this |
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| 37:21 | shows that you're still a scope or volt meter will show completely different values |
|
| 37:26 | micro balls. And that's not because action potential is a micro balls |
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| 37:32 | It's because you're outside the building and listening what's happening inside the building. |
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| 37:38 | you're only picking up a fraction of signal when you're performing extra cellular |
|
| 37:47 | Uh let me lead you to your folder and you have supporting class lecture |
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| 37:58 | . And we're gonna watch this really old video if you're a college student |
|
| 38:03 | me especially, you need graham early your life grammar lee is a digital |
|
| 38:08 | assistant and the carefully pods, body and habits are so very different from |
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| 38:19 | of humans that there might almost be from another world. So perhaps it's |
|
| 38:25 | surprising that it took a long time scientists to discover that there are fundamental |
|
| 38:30 | between the nervous systems of pods and . Yet it was the recognition of |
|
| 38:40 | useful difference in their nervous system, enabled scientists to undertake research that has |
|
| 38:45 | to a growing understanding of the mechanisms our own nervous system. The breakthrough |
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| 38:52 | the nerves that control the contraction of mantle muscles used in jet propulsion. |
|
| 39:00 | this archive film shows by simultaneously contracting mental muscles. Even a moderately sized |
|
| 39:06 | can inject a huge amount of water great force. In the mid 19 |
|
| 39:16 | , the british zoologist Professor James Young engaged in a study of the squid's |
|
| 39:23 | . Young observed an array of large structures, each as much as a |
|
| 39:28 | in diameter, in the squid's as these structures were never filled with |
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| 39:33 | . They could not have been blood from their similarity to surrounding nerve |
|
| 39:39 | Young thought they must be single giant axons. They're transmitted nerve impulses |
|
| 39:44 | a concentration of nervous tissue called a to the mantle muscles using electrodes. |
|
| 39:56 | stimulated the surrounding nerve fibers and found he could only produce large muscle contractions |
|
| 40:01 | the mantle when the large tubular structures intact. So these were indeed, |
|
| 40:13 | axons. Scientists quickly appreciated the significance Young's finding for here at last was |
|
| 40:22 | axon. Large and robust enough to with the techniques available at the time |
|
| 40:27 | one that survived for several hours when from the nucleus, the intracellular contents |
|
| 40:38 | the giant axon could be removed and , leading to the discovery that sodium |
|
| 40:43 | were more concentrated outside the nerve and potassium ions more concentrated inside. |
|
| 40:51 | refilling the empty axons with solutions of known chemical composition experimenters were able to |
|
| 40:58 | the mechanisms of iron transport across the . Chemical experimenters remember we talked about |
|
| 41:10 | transport and I said in early they used to inject the dye and |
|
| 41:15 | follow a faster travels. So this some of those experiments were performed in |
|
| 41:20 | very similar way, just looking at ionic crossing of the membrane, but |
|
| 41:25 | looking at the ectoplasmic, able to the mechanisms of iron transport across the |
|
| 41:36 | . The giant axons are large enough robust enough for fine electrodes to be |
|
| 41:41 | through the cell membrane and into the plasm. In these early techniques, |
|
| 41:53 | fine glass tube was first inserted into axon and secured with thread. Then |
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| 42:19 | tube was used to introduce a fine electrode from which the voltage between the |
|
| 42:24 | and the outside could be measured. the formation of the Nerve Impulse was |
|
| 42:30 | too rapid for detailed study with any the electrical measuring devices of the late |
|
| 42:40 | , Improvement of electronic equipment, such the detail study with any of the |
|
| 42:45 | measuring devices of the late 1930s, wasn't until the 1950s following the wartime |
|
| 42:53 | of electronic equipment such as the cathode Oscilloscope, that major progress was |
|
| 43:02 | Scientists found that the nerve impulse was as a characteristic wave of electrical potential |
|
| 43:08 | that this all or nothing action potential generated mainly by transient movements of sodium |
|
| 43:14 | potassium ions across the nerve membrane. on the squid giant axon unravel the |
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| 43:23 | of the formation and propagation of the action potential. This understanding led directly |
|
| 43:29 | the development of drugs that block action formation and so act as local anesthetics |
|
| 43:36 | used routinely as painkillers in dentistry and surgery. Anybody care to guess what's |
|
| 43:44 | routine local anesthetic used in dentistry? surgery like if somebody has to. |
|
| 43:53 | . Yes, very good. So block sodium channels and in the periphery |
|
| 44:04 | will basically block the activation of the endings and your teeth so you don't |
|
| 44:10 | pain so somebody can keep drilling and and drilling. Hopefully not. But |
|
| 44:14 | do. They will inject you if drill you, they'll inject you if |
|
| 44:20 | cutting uh repairing a wound for example needs to be cut or something needs |
|
| 44:26 | be done. A local surgery doesn't general anesthesia. Local anesthesia, we're |
|
| 44:33 | about local anesthesia. This is definitely from general anesthesia. When you put |
|
| 44:38 | to sleep. It's different molecules and involved. 2 19 thirties, they |
|
| 44:44 | recording this in the squid squid. squid is spraying water on them but |
|
| 44:50 | what's giant. And the squid is axon so they can see it. |
|
| 44:54 | can stimulate if they don't see action . Until 1939. I said it |
|
| 44:59 | the first one and then forties and when they started routinely recording them. |
|
| 45:04 | I have to realize when somebody started that in 1939 there was no other |
|
| 45:11 | , There was no other guy. else. This was the guy Hodgkin |
|
| 45:16 | Huxley were the two guys, there no two more guys in Australia. |
|
| 45:20 | more guys in Germany and you know guys in in in uh Middle |
|
| 45:27 | Someplace it wasn't. So it was was just advancement. The really incredible |
|
| 45:32 | . People were waiting even in the , people were still waiting to get |
|
| 45:37 | these setups to do certain experiments because were they were quite limiting the axon |
|
| 45:43 | done because it's one millimeter in one millimeter equals to 1000 micro |
|
| 45:55 | So you can see it one you can touch it, you can |
|
| 45:59 | it with scissors and stuff. N. S. Axons are one |
|
| 46:07 | meter. This is 1000 micrometers Our axons and the C. |
|
| 46:14 | S about one micrometer. Therefore you see vertebrate cns axons. These recordings |
|
| 46:23 | the networks of south that we're talking . You don't see that until another |
|
| 46:28 | years or so passed in the 70s 80s. Just historically you have to |
|
| 46:34 | how these things came about and what had to deal with to uh to |
|
| 46:41 | all of this great information. So you recall, we talked about generating |
|
| 46:49 | for tom Charles and we talked about the cell receives positive input and this |
|
| 46:54 | positive charge which represents glutamate excitatory If the cell reaches a certain |
|
| 47:01 | this electrode here recording electrode will produce potentials whenever I'm showing you diagrams here |
|
| 47:09 | show these square waves for these perfectly waves. These are produced by |
|
| 47:17 | This is when you flip the switch and turn it off in a very |
|
| 47:21 | manner. So you turn on the positive charge here, you turn it |
|
| 47:27 | , you sustain positive charge and you it off. The cell doesn't respond |
|
| 47:31 | the square wave like fashion because one the things that we didn't discuss but |
|
| 47:35 | do it in the next lecture cell has certain properties. It has resistance |
|
| 47:43 | and it has capacity its properties. because of the resistance it's resistant to |
|
| 47:49 | immediate charge switch. And capacitance means the charge has to redistribute from the |
|
| 47:54 | plates of the capacitor. Because of you don't see the square wave like |
|
| 48:00 | in the cellular response. You see surrounded appearance. It takes some milliseconds |
|
| 48:06 | this charge to reach its maximum change plasma membrane unlike the electrode which is |
|
| 48:12 | and off switch for the charge. other thing is that if the cell |
|
| 48:17 | stimulated but does not reach what we the threshold for action potential, the |
|
| 48:23 | response in this passive we call electra properties just showing you a little bit |
|
| 48:30 | resistance. Capacitive properties on the flat . If you inject more current and |
|
| 48:37 | current, injecting more positive current here activating more excitatory synapses, inject more |
|
| 48:45 | . You will drive yourself through the for action potential. And it produces |
|
| 48:50 | certain pattern and frequency of these action . And if the same salad receives |
|
| 48:55 | stronger input from their electoral the square it is going to respond. And |
|
| 49:02 | greater frequency or number of action potentials it produces over the same time period |
|
| 49:09 | . So the concept that we already and is reflected here that the strength |
|
| 49:17 | the stimulus of the input is reflected the number or the frequency of action |
|
| 49:25 | that is produced beyond the fact that can produce different dialects and different |
|
| 49:29 | The number of action potentials. If increase the stimulus, the number of |
|
| 49:33 | potentials also goes up this important uh that we have to know and understand |
|
| 49:46 | I talk about action potential in a that you're all going to understand |
|
| 49:50 | And then it's gonna be very easy you to know it for any other |
|
| 49:54 | matter or in the future is this again that I talked about. V |
|
| 50:00 | Ir eyes equal V over R. eyes equal G. V. Except |
|
| 50:09 | the conductance here the current for this ion or any ion is dependent on |
|
| 50:16 | conductance for that eye on just the ion. So geez the conductance times |
|
| 50:21 | driving force already mentioned that concept of driving force in the previous slides. |
|
| 50:27 | the driving force is the difference between . M. Which is membrane potential |
|
| 50:34 | equilibrium potential for a given ion in case for IKEA. So, I |
|
| 50:41 | walk you through the slide. I you to understand the slide really |
|
| 50:46 | So, visa equilibrium potential values for equilibrium potential values for sodium. These |
|
| 50:53 | potentials that we discussed, the cell fucking electrode inside the cell and all |
|
| 51:00 | the channels, sodium and potassium channels all closed. Okay, if the |
|
| 51:07 | are closed, is there any Is there anything being conducted through these |
|
| 51:11 | ? There's no conductance conductance? potassium And what is the current for |
|
| 51:18 | ? The current for potassium is zero the conductance is zero. Now, |
|
| 51:24 | there driving force for potassium? When membrane potential is at zero. The |
|
| 51:30 | force is the difference between membrane potential the equilibrium potential for potassium Equilibrium potential |
|
| 51:38 | potassium is -80 -19 Member and potential zero is the big driving force with |
|
| 51:46 | . Yes, it's a huge driving . This is the separation the difference |
|
| 51:51 | , huge driving force for potassium. is there any conductance for potassium? |
|
| 51:56 | . So despite the fact that you huge driving forces potential to drive the |
|
| 52:02 | , it's electrical potential to drive the , all the channels are closed conductance |
|
| 52:08 | potassium current to zero because current is times the driving force. So now |
|
| 52:16 | de polarize the VM into negative let's say about -20 Equilibrium potential for |
|
| 52:26 | is -80. The difference between GM and he came on. The safety |
|
| 52:33 | still there about 60 million balls. have a driving force and now you're |
|
| 52:39 | potassium channels now you have to conduct with capacity. So I have greater |
|
| 52:46 | and zero. And of course you a driving force of about 60 million |
|
| 52:52 | . So you have potassium current very here with these potentials minus 20. |
|
| 53:02 | Mercedes the reversal potential for potassium. is when the number of potential reaches |
|
| 53:10 | , the Volt meter is showing -80 G K. This potassium flocks still |
|
| 53:17 | on so the conductance is greater than may be minimal or small but is |
|
| 53:22 | than zero. The driving force is -80 -180 zero. The overall |
|
| 53:36 | This is you the reason for it because the current is flux inc there's |
|
| 53:42 | of potassium going in and out. is no net flux in one direction |
|
| 53:47 | the other. So the eye on Current is zero because the driving force |
|
| 53:54 | zero. So now you understand also this concept of concentration gradient and electrical |
|
| 54:05 | that is basically embedded in a liberal announced equation. And now you have |
|
| 54:12 | driving force which is the difference between overall number and potential which can shift |
|
| 54:17 | . D polarized, more hyper polarized equilibrium potential. Is that equilibrium |
|
| 54:22 | And the difference between the shifts and member and an equilibrium potential can reduce |
|
| 54:27 | driving force can increase the driving force each one of these ions has an |
|
| 54:33 | potential. So if sodium channels were here, there would be a huge |
|
| 54:39 | force for sodium. The difference between and 62. In this case the |
|
| 54:45 | channels are closed and this is what of a happening addressed the trust potassium |
|
| 54:54 | dominating potassium is leaking through the plasma . Mhm. A trust there is |
|
| 55:01 | no current for potassium because there's no for us. But there's conductance for |
|
| 55:07 | . And if you're now de polarize cell to minus 75 it's the situation |
|
| 55:12 | going to be different. It's gonna conducting for protection because there's gonna be |
|
| 55:15 | small driving force, the difference between 80 and let's say minus 75 60 |
|
| 55:20 | 20 and so on. So the driving force concept during the resting membrane |
|
| 55:29 | values that we discussed. It's just special situation here in the sense that |
|
| 55:35 | the plasma membrane is the most The potassium channels. You have the |
|
| 55:40 | . There's a much greater conductance for than sodium. And you really are |
|
| 55:45 | recording large current fluctuations because there's not driving force for potassium during the rising |
|
| 55:52 | of the action potential sodium channels open and as there is deep polarization, |
|
| 55:58 | more sodium channels opening up more deep , more sodium channels opening up. |
|
| 56:02 | so sodium conductance becomes the most dominant the rising phase of the action |
|
| 56:10 | Um as the membrane shifts of these potentials, it's coming close to the |
|
| 56:17 | potential for sodium and further away from potential for potassium. And at this |
|
| 56:24 | of the following phase, potassium also dominates again, it takes over. |
|
| 56:31 | , if you talked about permeability, cell member of this most permeable potassium |
|
| 56:36 | permeable to sodium. Most permissible And this reef polarization process that happens |
|
| 56:43 | is dominated by potassium addressed and dominated the A. T. P. |
|
| 56:50 | pumps a TPM A. K. I'll leave the concept of the voltage |
|
| 56:59 | for the next time. I'm gonna you up with this wide and we'll |
|
| 57:04 | with this lie today community. Strange how it doesn't fit, still |
|
| 57:23 | fit mm hmm. So I'll use like this. I actually showed it |
|
| 57:29 | the class yesterday and said what happened my slide? I guess I just |
|
| 57:32 | to press the minus button for everybody see clearly without my slide was getting |
|
| 57:37 | off. Okay, so this is exam slide. Remember I showed you |
|
| 57:42 | slide on the said it has all types subtypes of glia. Good take |
|
| 57:47 | notes. It's a great study tool you. You can actually start writing |
|
| 57:52 | things about the functions of real functions cells. But you can also start |
|
| 57:56 | things like for example the legal tender . De milo nation multiple sclerosis. |
|
| 58:03 | illegal tender sides. You may want put Schwann cells pollination and P. |
|
| 58:08 | . S. Parenthesis, shotgun marriages . Okay. And I told you |
|
| 58:14 | you have like a page on Alzheimer's or something, keep that because we'll |
|
| 58:18 | adding information. This is probably one the best tools for your exam. |
|
| 58:23 | as far as action potentials. And values that I have outlined here on |
|
| 58:28 | scale are the values that I am to ask you questions on not the |
|
| 58:33 | textbook. Not the two values that the same textbook. It's the ones |
|
| 58:37 | I've put up here. Okay, the slide. So first of |
|
| 58:43 | what I put here is this is arresting member and potential. It's about |
|
| 58:48 | . And you can see that cells positive input. The number of potential |
|
| 58:53 | receives negative input. It goes So these are synaptic potentials. The |
|
| 58:57 | potentials a lot of positive input coming , a lot of synapses activated, |
|
| 59:03 | going to reach the threshold for action generation, which is -45. If |
|
| 59:09 | reach this threshold, these all inputs graded and additive or subtracted. If |
|
| 59:14 | reach the threshold for action potential you generate an all or non event |
|
| 59:20 | if you repeat this action potential, will also the next one will be |
|
| 59:24 | 100 million volts and size, 100 size, 100 kilovolts and size. |
|
| 59:28 | all or not. So you cannot many action potential, medium and large |
|
| 59:32 | all or none because of the dynamics the sodium influx in the cell number |
|
| 59:37 | becoming most permeable to sodium. So you see it wrestling number of potential |
|
| 59:44 | is close to equilibrium potential for So for the individual in equilibrium potentials |
|
| 59:49 | chloride put political potential, potassium sodium calcium that these are the scales. |
|
| 59:58 | the actual values that I'm gonna ask questions about by asking questions. And |
|
| 60:08 | we reach this threshold and as you polarize to 45 miller volts negative 45 |
|
| 60:16 | volts, you start opening voltage gated channels. The action potential resting membrane |
|
| 60:22 | is dictated by voltage gated sodium But it's accommodating all excited to inhibit |
|
| 60:29 | inputs. That's why it's walking a bit more excited, a little less |
|
| 60:32 | , more and more excited. Once reaches this value, it opens up |
|
| 60:38 | educated sodium channels more deep polarization more more deep polarization. More sodium more |
|
| 60:43 | more so it's a positive feedback More sodium channels opening up and with |
|
| 60:50 | which is dominating this rising phase of potential. What sodium is trying to |
|
| 60:55 | is trying to take it away from 70 which is close to equilibrium potential |
|
| 61:00 | potassium because the rest is dominated by potassium, it tries to drive this |
|
| 61:06 | and potential to its own equilibrium potential But two things happen that it never |
|
| 61:16 | the equilibrium potential for sodium that goes positive values 10 15 2025 30 |
|
| 61:22 | Sometimes we never do that. Middle of all value two reasons for |
|
| 61:30 | . As sodium channels. Oakland will sodium channel kinetics on thursday and you |
|
| 61:36 | learn that sodium channels if you open , they have a mechanism that will |
|
| 61:41 | them in one or two milliseconds. you open the channel, some channels |
|
| 61:46 | open for a long time, sodium close right away so sodium channels start |
|
| 61:52 | , They open and they start Number one, Number two, is |
|
| 61:58 | is happening to the driving force is number of potential dif Ola rises to |
|
| 62:03 | sodium driving force. It goes down . The driving force for potassium is |
|
| 62:09 | that great because it's close to equilibrium here. The driving force And here |
|
| 62:13 | sodium is huge. It's saying, me go, let me run to |
|
| 62:17 | equilibrium potential. But as it is and running and running to that equilibrium |
|
| 62:24 | . The driving fourth is shrinking and and shrinking and shrinking. Mhm. |
|
| 62:30 | the current value is also dropping the and channels are closing. These are |
|
| 62:35 | two reasons why sodium fails to drive number and potential to its own equilibrium |
|
| 62:41 | values. And then potassium says number potential. You all the way over |
|
| 62:48 | , look at my driving force, on my way. Civilian channels you |
|
| 62:55 | . I'm gonna take over and dominate now you have potassium influx potassium leaving |
|
| 63:00 | potassium ions and channels are doing is this membrane potential Dm into its own |
|
| 63:08 | potential values. But it also doesn't succeed, it has this more negative |
|
| 63:16 | polarization past the wrestling memory potential. doesn't quite succeed because of two |
|
| 63:22 | it's leaking channels. They get activated also driving force reduces. And also |
|
| 63:29 | pumps start telling it, I'm gonna against concentration gradient. They never stop |
|
| 63:34 | against concentration if that helps re polarize membrane puts the resting number and potential |
|
| 63:41 | continues doing the random world excitatory inhibitory inputs and excited. Whoa ! |
|
| 63:47 | lot of excitatory inputs, threshold boom potential. So, during this phase |
|
| 63:56 | rising and falling action potential, once crosses the threshold in that direction going |
|
| 64:06 | and before it reaches re polarizes back the threshold value of action potentials. |
|
| 64:12 | nothing that you can do to the membrane to make it fire more or |
|
| 64:16 | make it fire another action potential on of the existing action potential. It's |
|
| 64:21 | on an event. And the reason the sodium channels open the closed. |
|
| 64:27 | actually have to re polarize the membrane the sodium channels to regain their structure |
|
| 64:32 | to be able to open. We'll about in the next lecture. Once |
|
| 64:36 | slide down in this position, you're relative refractory period. And if you |
|
| 64:41 | the cell is the cell received a strong input during this relative refractory |
|
| 64:46 | It can produce another action perfection. cannot during the absolute and it can |
|
| 64:53 | forced to produce one during the relative factory period. Remember I told you |
|
| 64:59 | of the cells have very fast frequencies action potential 600 hertz. So they |
|
| 65:06 | very short refractory periods, relatively factory . And there's something about the dynamics |
|
| 65:11 | the channels opening and closing very That allows them to produce these fast |
|
| 65:17 | . And then there are some cells will have longer refractory period, different |
|
| 65:22 | dynamics and they will produce lower action . They also produce different patterns of |
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| 65:27 | potentials. So, I will end today. Um and uh when we |
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| 65:36 | back on thursday, we will review of this material, we will use |
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| 65:41 | diagram again and press on to some concepts on action potential. Thank you |
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| 65:48 | being in class despite a few of . I like it. Thank you |
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| 65:52 | for being online on zoom and I'll everyone on thursday. |
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