| 00:02 | Now this is lecture six of Neuroscience we will continue talking about the resting |
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| 00:06 | at the time. The fact is you have ions that are equally distributed |
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| 00:13 | plasma membrane. And these ions cannot cross through proposal membrane. Therefore, |
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| 00:19 | need channels. Those channels are gated either voltage liens if they're receptor channels |
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| 00:26 | mechanical gated channels. And these channels of amino acids will also have some |
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| 00:33 | or positively charged amino acid residues which contribute to their cell activity for specific |
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| 00:40 | . Now, ionic pumps are different the channels because ionic pumps will use |
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| 00:46 | and they will transport ions across plasma and against their concentration gradient. So |
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| 00:54 | sodium's concentration gradient, putting more sodium the outside and against potassium's concentration |
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| 01:01 | putting more potassium on the inside of cell and they will be consuming a |
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| 01:06 | of a TP. So if we just to look at the uneven distribution |
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| 01:11 | charge where we have more concentrated uh like sodium on the outside versus the |
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| 01:18 | , then let's look at the situation there are forces like a chemical force |
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| 01:24 | is driving this ion down this concentration . So in this situation, you |
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| 01:30 | sodium fluoride in the side and you a membrane, if you insert the |
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| 01:35 | and those channels are open in the and they're selected for sodium and |
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| 01:40 | it will allow for the flux of and fluoride across the plasma membrane. |
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| 01:45 | if it was just purely based on concentration gradient, then these two sides |
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| 01:50 | become equal molar as the concentration is the left and the right side of |
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| 01:55 | membrane equal uh themselves out. So have this concentration gradient, but apart |
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| 02:04 | the concentration gradient, you also have , plasma membranes are electrically charged. |
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| 02:11 | you can think of a battery which the positive and the negative end and |
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| 02:17 | measure the voltage. So across the you have electrical potential or voltage. |
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| 02:25 | if you take a volt meter and put to the positive and negative |
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| 02:29 | it's like 10 bucks at Home you can check your batteries if they're |
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| 02:33 | good. Uh It's a good way know it actually because because you end |
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| 02:38 | spending a lot of money on the like they're expensive. Uh If you |
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| 02:44 | have something that you use, be and a lot of times you don't |
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| 02:47 | if they use that or not and throw it out and put new |
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| 02:50 | So volt meter will allow you to one end and that end and it |
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| 02:54 | like AA AAA will say 1.5 that's the, that's the potential, |
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| 03:00 | the electrical potential. So because you an even distribution of charge across the |
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| 03:05 | membrane, the plasma membrane has a in electrical potential as well. |
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| 03:10 | in the battery, we have the , the negative and the Anno the |
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| 03:15 | end. And so anions negatively charged will be attracted to the positive end |
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| 03:21 | the battery and positively charged ions like C will be attracted to negatively charged |
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| 03:28 | in the uh in in this So in in this situation, uh |
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| 03:35 | have fluoride, for example, and you will have sodium on this side |
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| 03:42 | positively charged. And sodium will also repelled by the positive charge. Driving |
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| 03:48 | across the membrane fluoride will be repelled negative charge will attract by the positive |
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| 03:53 | possible in this direction. Therefore, addition to the chemical gradient or concentration |
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| 04:00 | , there's also electrical potential and you call the electrical impulsion and attractions that |
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| 04:07 | into play when setting up the rusting in potential. Ohm's law, the |
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| 04:12 | of Ohm's law D is equal. is voltage I is current, R |
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| 04:19 | resistance. The voltage and neurons is in millivolts. Those are the relevant |
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| 04:26 | . R resistance is measured in mega because neurons are really small, only |
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| 04:31 | micrometers in diameter and the smaller the , the higher it is the resistance |
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| 04:37 | is measured in milliamps and PICO AM . So high is current. Uh |
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| 04:47 | is inverse of conductance or the opposite conductance. So, if there's a |
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| 04:54 | of resistance, then the conductance is . If it's little resistance, then |
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| 04:59 | conductance is high. OK. And is measured in nano, nano semen |
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| 05:06 | PICO semen. So those are relative that are important for these measurements for |
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| 05:12 | and neuroscience that we're talking about. you can rewrite arms law, E |
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| 05:18 | IR you can put I is equal over R or because R is one |
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| 05:26 | G it's current is equal conductance times change of voltage. We write home |
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| 05:32 | in those terms too. Now, movement of current is really flow of |
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| 05:39 | cross plasma membrane is in the direction the movement of the positive charge. |
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| 05:44 | these are some of the basics again our review. So when we again |
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| 05:50 | a electrode in these neurons, we a negative potential compared to the outside |
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| 05:56 | the south. And the membrane potential this build up of charge and an |
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| 06:03 | separation of charge at any moment and and potentials, they have constant |
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| 06:09 | So resting number and potential, even it's listed at minus 65 millivolts resting |
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| 06:16 | and potential, this ring number and is constantly going to fluctuate, it |
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| 06:22 | become more depolarized. Remember in this is depolarization or if this number and |
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| 06:30 | goes to more negative values, it hyper polarized Uh And so it's constantly |
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| 06:38 | . It's at any moment in which means that one moment in time |
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| 06:42 | might be minus 65 another minus And that's just normal because cells receive |
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| 06:48 | input, spontaneous activity, both excitatory inhibitory. Also, there are small |
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| 06:54 | fluctuations in temperature across different parts of membrane. We're making a little piece |
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| 07:00 | the membrane, more excitable, therefore over the other one that is |
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| 07:04 | it is dependent on electrochemical properties. so the chemical gradient and electrical |
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| 07:12 | the chemical gradient is such that the of the cell is dominated by |
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| 07:17 | There's 20 times more potassium, 100 the inside of the cell versus five |
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| 07:23 | , 22 1 on the inside of cell. And the ratio or 100 |
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| 07:28 | versus five millimoles inside the cell. outside of the cell is dominated by |
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| 07:34 | chloride as well as calcium. You to look at the greatest disparity in |
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| 07:40 | concentration gradient outside versus inside. It's calcium that has two millimolar on the |
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| 07:48 | and it has 0.0002 millimolar on the of the cell. So there's 10,000 |
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| 07:55 | times CALS on the outside versus the of the cell. So we were |
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| 08:00 | purely dependent on the chemical concentration Then calcium will have the biggest driving |
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| 08:06 | down this concentration gradient. It's 10,000 times calcium on the outside versus |
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| 08:13 | But addressing membrane potential. That is the case because these channels that we're |
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| 08:20 | about, they have to be In order to consult to conduct calcium |
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| 08:25 | addressing me and potential. The nature it such a way that potassium channels |
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| 08:31 | open and potassium is leaking. And potassium s are dictating primarily the resting |
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| 08:37 | and potential. Ok. The charge up that you see here exists just |
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| 08:46 | the inside and the outside of the . If you move further into the |
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| 08:52 | of the cell, or if you outside of the cell away from the |
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| 08:57 | membrane from the phospholipid bilayer, these are charge neutral. So the charge |
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| 09:04 | and charge build up is really existing the plasma membrane. And it's also |
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| 09:11 | because that's where it's going to uh very quickly charge across plasma membrane and |
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| 09:17 | the actual potential. So this potential then that you're seeing is similar to |
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| 09:24 | battery sort of a like a negative of the battery on the inside of |
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| 09:29 | plasma membrane and the positive on the . So this creates a battery |
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| 09:35 | OK. So now we have concentration but we also have a battery, |
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| 09:40 | channels that is the case are most and dictate resting membrane potential. A |
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| 09:47 | pumps will be working and using a in order to maintain that resting membrane |
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| 09:54 | as close to minus 65 as So every time it fluctuates away the |
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| 09:59 | TP A tries to rebuild that uh into its resting number and potential |
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| 10:08 | So because we have two interacting we have the chemical force or the |
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| 10:13 | gradients and we have the electrical force the charge uh of the ions. |
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| 10:20 | can imagine the situation where, for , we have a lot of potassium |
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| 10:25 | the inside of the cell. And also have a certain molecule that is |
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| 10:30 | and permeable. So it kind of to the other side or down |
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| 10:34 | we have a lot of sodium on outside of the cell. And |
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| 10:38 | we have some sort of a negatively protein that kind of cross through the |
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| 10:43 | number. So in this situation, have a drive which is concentration |
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| 10:50 | there's more potassium. So if you a potassium channel, that potassium from |
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| 10:55 | concentration area is gonna be driven across membrane into a lower concentration area, |
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| 11:01 | chemical gradient is going to try to this potassium. So it becomes equal |
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| 11:04 | both sides in concentration. Yeah. what happens is that shortly after this |
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| 11:10 | starts crossing into the other side, accumulation of positive charge on the membrane |
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| 11:17 | this side becomes more negative. And this positive charge that is built up |
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| 11:23 | this side of the membrane starts pushing , it's becoming repulsive to potassium. |
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| 11:30 | charge is pushing the positive charge back the south. The same happens with |
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| 11:37 | sodium channel opens up sodium will flux its concentration gradient until there is a |
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| 11:43 | up of positive charge on the inside the membrane. And that positive charge |
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| 11:48 | is becoming repulsive to sodium entering inside the cell. So the moment at |
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| 11:55 | this chemical force, chemical force driving the concentration gradient, one direction and |
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| 12:04 | electrical force which is discharged now repelling ion in the opposite direction. The |
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| 12:13 | value at which these two forces are and opposite to each other. That's |
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| 12:19 | equilibrium potential value. There's also no flux of ions across plasma membrane. |
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| 12:25 | means that there's no more ions flowing this direction versus that direction doesn't mean |
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| 12:30 | no ion flux. Ions are still sodium going in and out, going |
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| 12:36 | and out. However, at this , there is no net ionic |
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| 12:40 | there's no more sodium going in versus going on because there's two forces. |
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| 12:46 | is a chemical and another electrical is pushing the odds across, but now |
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| 12:52 | equal and forces opposite in direction. when you have the equilibrium potential to |
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| 13:01 | equilibrium potentials. For each ions, have to know the ionic concentrations that |
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| 13:06 | watch a short video that will show how we actually historically found out ionic |
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| 13:12 | inside of the cells versus the outside the cells. And nerve equation takes |
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| 13:19 | consideration the ionic concentrations. This is concentration of potassium on the outside of |
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| 13:25 | cell versus potassium on the inside of cell. But in order to calculate |
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| 13:30 | exact value of the tib potential, have to know more than just the |
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| 13:36 | of the on the outside and the . And so we use this formula |
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| 13:42 | is the nernst equation which is equilibrium for each ion such as EK. |
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| 13:50 | it's rewritten here for E ion, is 2.303 RT ZF log ion concentration |
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| 14:01 | the outside versus ion concentration on the . So we have to take into |
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| 14:07 | R which is a gas constant T is a temperature Z which is a |
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| 14:13 | . So most of these s we're about are monovalent kays or an ion |
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| 14:20 | . And then we have divalent calcium the only 12 plus. So this |
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| 14:24 | the Z, the valence value and is the Faraday constant or the electrical |
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| 14:30 | . Then you have log of the of insert your favorite ion E |
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| 14:36 | potassium on dioxide versus potassium on the . E sodium, insert your sodium |
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| 14:43 | on the outside versus the inside based logarithms and these concentrations to solve for |
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| 14:51 | potentials. So to solve for each potentials, this is the solution for |
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| 14:56 | ion all of that 2.3033 in the slide RT ZF collapses into 61.54. |
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| 15:07 | the value is the mill of all now that when you collapse across |
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| 15:11 | you have 61.54 log of potassium outside inside sodium chloride. Now, this |
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| 15:18 | changes here from 61.54 because remember it's ZF RT over ZF. And so |
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| 15:25 | have a divalent cion here. So divided by two. That's why that |
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| 15:30 | is half of 61.54. It's 30.77 . And that's to derive the calculation |
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| 15:39 | calcium. Now, uh hm if plugging in these values here for outside |
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| 15:45 | versus inside concentration, you can plug the actual millimolar values. And so |
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| 15:50 | spoke about that, that each one these ions will have their own millimolar |
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| 15:54 | . So 100 potassium on the inside versus five on the outside or a |
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| 16:03 | of outside versus inside ion. So doesn't matter in this situation, you |
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| 16:07 | plug in either 1 to 20 for or five millimolar on the outside of |
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| 16:13 | versus 100 millimolar on the inside of , either one and then you |
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| 16:20 | take a log of 1/20 multiply that by 61.54. And you have the |
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| 16:27 | from equilibrium potential value for potassium. each one of these ions has its |
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| 16:35 | equilibrium potential value. So as you see, potassium has a minus 80 |
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| 16:43 | , the cooler room potential value, has positive 62 millivolts, calcium positive |
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| 16:51 | plus fluoride minus 65 millivolts. So question that you may have at this |
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| 17:02 | is do I have to know how use this Nernst equation? Do I |
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| 17:12 | to use a calculator? Will I to use a calculator to calculate equilibrium |
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| 17:19 | for these uh ions? And the is no, but you should know |
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| 17:24 | terms of these equations, the Nernst and then the second equation that we'll |
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| 17:28 | about the Goldman equation. So you know the terms, what the, |
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| 17:32 | are the variables that we're talking What's Koo versus K I potassium on |
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| 17:41 | outside versus inside? What is RT ? What is Z value stand |
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| 17:47 | Why would that calculation be, have insert calcium ions be able to recognize |
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| 17:54 | ratios? I want you to know actual concentrations of these four ions from |
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| 18:00 | term or the ratios doesn't matter. if you know the concentrations, you'll |
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| 18:05 | know the ratios but not the other around. OK. So knowing concentrations |
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| 18:10 | really good knowing their individual equilibrium potential uh eic values that's also going to |
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| 18:18 | on the test. And you'll understand because it will come back into play |
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| 18:22 | we talk about the actual potential. , four important points, I guess |
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| 18:28 | we want to make about currents and membrane potential is that you can have |
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| 18:33 | changes in ionic concentrations outside versus inside that can result in pretty significant. |
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| 18:39 | just a few millimolar change in the may result in a few millivolt or |
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| 18:46 | 10 millivolt change in the membrane And we'll talk about how that is |
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| 18:52 | and how it affects memory potential. net the difference in electrical charges view |
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| 18:59 | and outside of the membrane surface. this is where the charge build up |
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| 19:03 | this is where there's going to be discharge. Also rate of movements of |
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| 19:08 | across plasma membrane is proportional to what call a driving force. So, |
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| 19:15 | far, what we've described is we the equilibrium potentials for each ion. |
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| 19:20 | I told you that you can use N equation to find the equilibrium potential |
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| 19:24 | each ion. But how do you the membrane potential? And why is |
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| 19:29 | important? Why is the difference between membrane potential and the equilibrium potential for |
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| 19:35 | ion is important? How is that play into the dynamics of action potential |
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| 19:40 | we talked about? Now if the difference is non equilibrium potential can be |
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| 19:46 | . And we already see that because know these concentrations we walk through the |
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| 19:52 | . OK. So now when you channels in the membrane, there is |
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| 19:59 | important thing, these channels have to open when the channels are open, |
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| 20:05 | , they're called they're permeable. They high permeability. If they're wide |
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| 20:12 | If the channels are closed, there's permeability through those channels. So there's |
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| 20:18 | things come into play. First of , how many channels you have and |
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| 20:21 | how many channels are open because you have 1000 channels, but all of |
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| 20:26 | are closed and you're not gonna have ion that gets conducted across those |
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| 20:32 | So neuron membranes are permeable to more one type of ion. And that |
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| 20:39 | that each one will have their own channels. So there's going to be |
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| 20:44 | gated sodium channel, voltage gated potassium . And they're selected to these |
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| 20:49 | the membrane permeability determines membrane for town resting membrane potential. So it happens |
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| 20:58 | the nature of the the membranes that these potassium channels and this is inside |
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| 21:04 | outside. And potassium has what we leak channels. So it's rusting membrane |
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| 21:11 | . Most of the conductance is gonna because of the potassium leaking out of |
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| 21:16 | cells. It's just the way that build it is that it has these |
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| 21:20 | channels in there oozing out the highest of resting membrane potential for potassium that |
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| 21:29 | as the membrane potential changes and the ail for different ions change. So |
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| 21:35 | do we derive uh the membrane And why are we talking about |
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| 21:42 | Because in order to calculate the VM the membrane potential, we're using the |
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| 21:48 | equation, the Goldman equation is very in many ways to the nurse |
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| 21:54 | So that same 2.303 RT Z uh collapses into 61 50.54 millivolts and logged |
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| 22:04 | the concentration. This is identical to equation. But there are two differences |
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| 22:09 | that you're already seeing. First of , we have this term here added |
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| 22:13 | is PK and it stands for permeability potassium. And it's not for uh |
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| 22:21 | like PK measure and in the blood the brain analytical chemistry. This is |
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| 22:29 | . The second thing that is different the equation is that we're using sodium |
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| 22:35 | potassium, we're using two ions. equation is designed for a single |
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| 22:44 | This is the difference Nernst equation calculates potential or single ionic species such as |
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| 22:55 | membrane potential. And as we just in the previous slide, the membrane |
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| 23:01 | will flux all of these four types ions that we're referring to. And |
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| 23:07 | means that the membrane potential in order calculate the membrane potential, it's not |
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| 23:11 | just to look at the fluxus of ion, you actually have to study |
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| 23:16 | fluxus of sodium and potassium and their . And what it shows is that |
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| 23:21 | resting membrane potential, the plasma membrane 40 times more permeable. That's the |
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| 23:29 | value for potassium is 40 over the value for sodium which is one. |
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| 23:36 | rest in number and potential. There's permeability to potassium. Potassium is leaking |
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| 23:43 | . And the rest in number and you calculate using sodium and potassium concentrations |
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| 23:49 | you derive a value of about negative millivolts and across the textbooks or across |
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| 23:56 | papers that ring number and potential value fluctuate also across the different cell subtypes |
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| 24:01 | may fluctuate too and regionally because they have slightly different concentration gradients inside versus |
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| 24:10 | . Not exactly 105 but 110 versus or something like this is slight |
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| 24:17 | So you'll see differences, but I tell you what values I want you |
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| 24:21 | know and I will keep repeating those over the next couple of lectures. |
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| 24:28 | these channels are permeable and potassium channels permeable to potassium. And that's the |
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| 24:36 | determinant of rusting number and potential. there are other selective permeability channels, |
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| 24:44 | gated channels such as sodium is permeable to sodium. And we'll talk about |
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| 24:49 | in a few uh slides, potassium are often referred to as a family |
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| 24:58 | potassium channels. And we're talking about gated potassium channels. It's referred to |
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| 25:04 | family of channels because a lot of potassium channels will have a certain structure |
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| 25:09 | this inner pore loop that is acting a selectivity filter for the iss to |
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| 25:15 | able to cross through this channel. remember these channels are built from amino |
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| 25:22 | sequences and they're grouped into potassium channel because they will have conserved amino acid |
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| 25:32 | . That means that the sequence in channel that is permeable to potassium may |
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| 25:38 | very similar 95%. Uh the same in another potassium channel, but you'll |
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| 25:44 | different subtypes of these potassium channels. interesting thing is a lot of what |
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| 25:50 | you see in the sequences of these is preserved across species. So there |
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| 25:56 | equivalents of voltage gated potassium channels, gated sodium channels, calcium channels, |
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| 26:02 | name it that we find in primitive such as fruit flies. And that |
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| 26:09 | an important uh point in trying to first of all what the voltage gated |
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| 26:15 | channel is responsible for what happens if are mutations in that voltage gated potassium |
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| 26:23 | . And the first mutation that was was discovered in fruit flies. |
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| 26:28 | fruit flies a great model because you , you have big room and you |
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| 26:33 | house maybe 200 rodents in one big , but you have one little test |
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| 26:40 | and you have a couple of dozen fruit flies in there and they will |
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| 26:46 | uh they have short lifespan but they reproduce a lot. And so you |
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| 26:50 | grow colonies, you can do genetic . It's a great tool to do |
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| 26:56 | analysis and see how it affects but also the behavior of these |
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| 27:02 | And so what was noticed is that was a mutation that was discovered in |
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| 27:10 | gated potassium channels. And the way was discovered is that uh uh one |
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| 27:15 | noticed when they're studying these fruit flies a certain mutation, those fruit flies |
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| 27:21 | the fruit flies to shake and they're shaker potassium channels because the mutation in |
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| 27:26 | channels cause the flies to shake. you would say, OK, big |
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| 27:32 | , we discovered how flies shake. does that mean to humanity? And |
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| 27:38 | turns out because we have the conserved acid sequences. And then we have |
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| 27:45 | voltage gated potassium channels and other channels human brains. It turns out that |
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| 27:52 | shaking behavior that you observe in a fly is an equivalent of a person |
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| 27:58 | a seizure, epileptic seizure. And , it was discovered that you have |
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| 28:04 | in voltage gated potassium channels, it bring about seizures and that those channels |
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| 28:10 | sequence similarities and some physiological similarities with with the humans with the human |
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| 28:17 | So this is this is what it is that now you have a |
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| 28:21 | you have a system to reproduce these to target different parts of the |
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| 28:26 | to mutate them to find out which of the channel are important. |
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| 28:31 | Because you know how in almost everything is built, there are pieces that |
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| 28:36 | can take apart like big pieces of wall. And there's still like sort |
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| 28:41 | a the wall base is standing there you can pull the brick at the |
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| 28:45 | the base or like a corner and whole wall collapses, right. So |
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| 28:50 | really depends, you can have these and you know, as the |
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| 28:54 | some of them that are really, important in opening and closing of the |
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| 28:58 | are, are not as important. just sort of over there to support |
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| 29:02 | structure and we can learn a lot these primitive systems. And in your |
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| 29:10 | , there is a really good section called uh Pathway of Discovery. And |
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| 29:16 | talks about individuals that have discovered different . And one of the really cool |
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| 29:24 | is about Roderick mckinnon, who in thou 2003 won a Nobel Prize uh |
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| 29:32 | studying the structure of the potassium channels also studying neurological disorders, inherited neurological |
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| 29:42 | by studying the structure of the voltage potassium channel. So why is Roderick |
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| 29:49 | story so interesting. It's because this of the potassium channel that you're seeing |
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| 29:56 | , it's something that he was the one to describe and this is the |
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| 30:00 | crossing through the channel. So this the channel looted. This story is |
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| 30:05 | interesting and that's something you should keep mind as you're embarking on your future |
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| 30:10 | , professional careers, professional development, and these phd S nursing degrees or |
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| 30:17 | degrees, whatever you may pursue after that you have to have a passion |
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| 30:23 | something that you're not doing it just make money. Ok? If you |
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| 30:28 | school of business, they'll say what's with that. That's why we came |
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| 30:32 | school here. But typically you make on good things. Of course, |
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| 30:36 | bad things that people make a lot money on too. But our society |
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| 30:42 | to improve things. Strives for the part toward progress. Once in a |
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| 30:47 | , we step back and, you , we destroy things and oh, |
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| 30:51 | have to fix it. You now that's really important. Why am |
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| 30:56 | talking about this? And potassium Roderick mckinnon, he was an MD |
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| 31:04 | he had a successful medical practice, medical practice career and I wanna say |
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| 31:10 | one of the top universities don't quote , but I believe it's Harvard. |
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| 31:15 | his passion was as he was a doctor and he was understanding the physiology |
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| 31:21 | the body and the brain. His became, I want to solve the |
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| 31:29 | of the potassium channel. So he driven by the quest. And as |
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| 31:35 | medical doctor practicing medicine, he cannot into it. He cannot really do |
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| 31:41 | studies because guess what? He has work with fruit flies. So maybe |
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| 31:46 | he can make a collaboration with somebody that institution, maybe it can work |
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| 31:50 | . But he decides I'm gonna do myself. So he leaves his MD |
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| 31:57 | and gets a researcher position at another , another institution. And as a |
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| 32:04 | at another university, he's studying this channel. He's using fruit flies. |
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| 32:13 | . So shaker flies, fruit it's a primitive system. He's doing |
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| 32:21 | is called sight directed muta Genesis. is doing electrophysiology. He is using |
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| 32:45 | which is essentially neuro pharmacology. He's using all of these techniques to |
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| 32:59 | the structure of this channel in the and the nineties. And why does |
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| 33:04 | have to do that? Because you visualize, you cannot just look under |
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| 33:09 | microscope, you cannot look under electron and see the structure of the |
|
| 33:16 | We don't have that kind of resolution just visualize the channels under microscopes. |
|
| 33:23 | what he has to do is he to mutate different parts of this |
|
| 33:28 | To understand that if he mutated this of amino acid, the channel is |
|
| 33:33 | closed and to determine that the channel almost always closed, he has to |
|
| 33:39 | electro physiology because that's gonna help him the currents. If the channel is |
|
| 33:43 | , there's no flux of potassium, no potassium currents. You're using the |
|
| 33:49 | because it's an easy system. He's the toxins because the toxins that are |
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| 33:56 | in nature and scorpions, insects and and snakes and venoms. These toxins |
|
| 34:05 | very powerful and the way that they in our brains and our bodies is |
|
| 34:10 | lot of times they are blockers, or blockers for, for channels. |
|
| 34:16 | for example, a scorpion toxin is a blocker toxin envisioned directly in the |
|
| 34:22 | structure of days by probing the channel a toxin of no structure. So |
|
| 34:27 | has to use all of these techniques order to deduce and calculate the three |
|
| 34:35 | structure of this protein. That's a of work but he doesn't still is |
|
| 34:44 | satisfied and why he's not satisfied. in the nineties, there's this technique |
|
| 34:53 | starts emerging and becoming more common in universities. It's called X ray Mr |
|
| 35:04 | Gray. By the way, these great matching questions for, for the |
|
| 35:11 | . So he wants to do X crystallography. So he now leaves this |
|
| 35:17 | as an MD and a researcher, leaves this university. He opens a |
|
| 35:22 | lab in another university and he is X ray crystal. So you |
|
| 35:28 | the first time when he was leaving MD, people are like, what |
|
| 35:31 | you doing man? You know, just got here. You like I |
|
| 35:35 | , it's like I have this I have this quest. I'm gonna |
|
| 35:37 | this question. Then it's like you've so much work. What are you |
|
| 35:42 | moving to them? Because I need money, more start up funds to |
|
| 35:45 | X ray crystallography and mind you doing ray crystallography, it's like completely different |
|
| 35:52 | . This is in the department of . So he now is an X |
|
| 35:58 | crystallographer. And the way that X crystallography works is extremely complicated signs because |
|
| 36:05 | you have to do is you have take that single protein isolate it and |
|
| 36:11 | it inside a crystal. That's why crystallography. You trap it inside the |
|
| 36:16 | , that one suspended protein is trapped the crystal and then you pass X |
|
| 36:22 | through that crystal and as you pass X rays through that crystal, you |
|
| 36:28 | have a photo, you develop a image or digital image, digital photo |
|
| 36:35 | of that structure. So for doing of this work and using all of |
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| 36:42 | techniques, he's doing that because he's the quest to visualize now, not |
|
| 36:47 | to solve the structure, but because heard that there is a technique that |
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| 36:51 | you visualize these. I'm gonna go pursue that other technique. Now, |
|
| 36:56 | the channel, publish it, tell whole world and collect the Nobel Prize |
|
| 37:01 | 2003. It's a really inspirational story your degree is a degree is a |
|
| 37:11 | , your, your, your, toolbox that you have here that you're |
|
| 37:15 | here, just the tools of answering call and you must have a |
|
| 37:23 | And if you don't find it and for it, sometimes searching for a |
|
| 37:27 | to find it is more important than a degree without having that goal or |
|
| 37:33 | having that passion to, to do . OK. So Roderick mckinnon's example |
|
| 37:39 | fantastic. Now, last year I the news, it's called A I |
|
| 37:48 | on already known sequences and the three structures that have been derived through decades |
|
| 37:55 | work by an army of graduate postdocs and professors. We now have |
|
| 38:03 | tools that can solve channel structures within minutes or so. And that's because |
|
| 38:10 | are certain interactions and sizes of the assets, the charges the distances, |
|
| 38:15 | spatial arrangements that is somewhat limited. it almost uh it, I'm not |
|
| 38:21 | that it is obsolete, but it makes all of the especially visualization techniques |
|
| 38:28 | X ray crystallography pretty obsolete. Uh that you still want to have a |
|
| 38:33 | proof, not just a calculation, ? So it's not completely obsolete. |
|
| 38:38 | yeah, I don't know if I answer it but it's a what? |
|
| 38:44 | , I can't recall the name but was maybe a year and a half |
|
| 38:48 | . It was pretty big in the because, you know, like if |
|
| 38:52 | were in biochemistry and you're doing this your life and build your career all |
|
| 38:56 | a sudden you're like, probably have emotions. You're extremely happy because more |
|
| 39:00 | are salted. You're like, it's like cutting grass with scissors and |
|
| 39:06 | realizing that there is, you robot, cutting your grass. |
|
| 39:12 | yeah, yeah. But you you then you have to chase the |
|
| 39:16 | things and I'm sure they've given his . He's probably on the cusp of |
|
| 39:21 | really interesting, really new, you , unless he's uh about retirement age |
|
| 39:25 | , but you can look him up your books. So this potassium |
|
| 39:30 | these concentrations that you have on the and the inside are very tightly |
|
| 39:35 | There's a certain dynamic range. And told you that there are fluctuations in |
|
| 39:38 | membrane potential because there are also minor of concentrations of ions inside and |
|
| 39:44 | But for the most part, they kept within a certain normal dynamic range |
|
| 39:49 | of activity. However, if they increase and in particular, if the |
|
| 39:55 | potassium concentration increases, it can have very profound effect on the membrane |
|
| 40:02 | And in this case, what is here is the regular all potassium concentration |
|
| 40:06 | your book says it's about five If you double that concentration from |
|
| 40:12 | where you have it about minus 70 minus 65 millivolts, you double it |
|
| 40:18 | it by five millimoles. You change membrane potential from about minus 65 to |
|
| 40:25 | minus 45. You're almost reaching the for action potential generation. So remember |
|
| 40:32 | talked about how these concentrations of this is extracellular concentration of potassium. |
|
| 40:39 | if extracellular concentration of potassium goes the membrane potential has a si significant |
|
| 40:48 | , you can run through this If you are interested, add more |
|
| 40:53 | on the outside and run through the calculation. And you will see how |
|
| 40:57 | affects the membrane potential. Ok, resting or active membrane potential. |
|
| 41:04 | the other thing to understand is that concentration will always linger around that five |
|
| 41:12 | value millimolar value that we talked And that's because it's very tightly |
|
| 41:18 | So the amount of potassium or amount ions in the brain and tightly |
|
| 41:22 | we already spoke about the blood brain . So there can be an increase |
|
| 41:28 | potassium concentration. Maybe you even eat or something got in your blood. |
|
| 41:32 | doesn't mean all of that is just to cross into the brain. So |
|
| 41:36 | have blood brain barrier that controls what and how much of certain things cross |
|
| 41:42 | the brain. And also if you , it's ostracizes, ostracizes will up |
|
| 41:49 | abnormally high concentrations of ions or neurotransmitters they will spatially buffer through their own |
|
| 41:58 | processes. And also they're interconnected with Astros. So they will be sending |
|
| 42:05 | uh to other Astros also basically spatially it, buffering it through the whole |
|
| 42:11 | through the whole complex in the brain complex. All right. And that |
|
| 42:18 | our lecture and ring me and So ring number and potential and we |
|
| 42:28 | now venture into the action potential. by the time we're finished with the |
|
| 42:42 | potential, which is gonna be next , you're gonna know more about the |
|
| 42:48 | potential that you probably wanted to But you'll also if some of you |
|
| 42:54 | studied ring memory potential, some of have studied the action potential and |
|
| 43:01 | human phi uh courses. It's different way that we talk about it. |
|
| 43:07 | really talk about it in depth, talk about it in relation to equilibrium |
|
| 43:11 | , we talk about it in relation the dynamics of the voltage gated sodium |
|
| 43:16 | potassium channels. And at greater greater with these questions on resting number and |
|
| 43:23 | physiology, neuromuscular junction action potentials. are very common questions and you |
|
| 43:30 | standardized advanced tests to uh across the , the GRE or or or MC |
|
| 43:37 | or some other professional health care related also. So action potential action potential |
|
| 43:45 | very fast. It conveys information over distances. If the resting number and |
|
| 43:51 | is about minus 65 the action potential its peak crosses over zero mill value |
|
| 43:56 | the number and potential beyond becomes So it really is reversal of charge |
|
| 44:02 | it's relative to extracellular stage. Action is a neural code. So we |
|
| 44:09 | about action put down Schulz's dialects that come in certain frequencies that they come |
|
| 44:16 | certain number. So action potential is a digital code. It's a one |
|
| 44:25 | a digital code and this is analog action potential is all or none. |
|
| 44:34 | talk about that and these are greater potentials are analog digital. How |
|
| 44:44 | the digital code work? Is Zero and 0.25 and 1.3. What |
|
| 44:53 | the digital code? Yeah, zeros ones. That's all you're using zeros |
|
| 45:02 | ones and all of these devices and that's digitized is being placed through zero |
|
| 45:09 | one code to be digitized. What your voice? Is that digital? |
|
| 45:20 | . What about music from iphone? that digital? Yes. OK. |
|
| 45:29 | is better. Live music, live . One more. One more. |
|
| 45:36 | completely different sound. Actually, you're used to digital sound that we only |
|
| 45:42 | human sound and then we hear an we're like, what's that? You |
|
| 45:46 | , it's the same instrument that you hearing on your phone maybe. But |
|
| 45:49 | you hear it in person, a semester around uh around 230 or so |
|
| 45:59 | university center there was a person practicing . Oh, all right. The |
|
| 46:08 | of music and that, and that spread throughout like it, it was |
|
| 46:14 | on the wind. I talk a about census in this class and I |
|
| 46:18 | music too. So uh what, I notice is something like this is |
|
| 46:23 | walking, walking, walking, walking then the earth and then they slow |
|
| 46:28 | and they're like, turn around and and they almost like get engaged with |
|
| 46:33 | they are because otherwise it's screen you know, airpods and you're not |
|
| 46:40 | like talking to people, you even families. Now when they gather |
|
| 46:44 | for dinner, it's like everybody puts phone in the basket, you |
|
| 46:48 | no phone for an hour. Can do that? So, a friend |
|
| 46:53 | mine was an NBA player here at Rockets uh about 10 years ago or |
|
| 46:58 | . And they did that for their , uh team dinner. It was |
|
| 47:03 | two hours. They put their phone the basket, they came back to |
|
| 47:07 | phones, it was emergency calls, police outside the door and it's like |
|
| 47:12 | happened to you. I didn't get text from you in 15 minutes. |
|
| 47:16 | is wrong. You know, we're used to this, this connectivity. |
|
| 47:20 | don't even know the way this world without this. Actually, you will |
|
| 47:25 | with it in your, in in your pacifier. And then, |
|
| 47:29 | know, so uh but so it's important to think about these things was |
|
| 47:36 | . What are you listening for? listening to a code that's been digitized |
|
| 47:40 | a voice. It's a real natural sound and so on. So it's |
|
| 47:44 | code action potential is also referred to spike, nerve impulse or electrical |
|
| 47:50 | The squid John Ao. This is great movie and now I just have |
|
| 47:55 | make sure that the sound works. it does this is how A G |
|
| 48:07 | works to upgrade your health routine by many quality ingredients into something. Let's |
|
| 48:20 | this oldie but Goldie. Mhm. is no better movie on the |
|
| 48:30 | Giant Diaw is so fun. The body plans and habits are so very |
|
| 48:40 | from those of humans that they might be aliens from another world. So |
|
| 48:45 | it's not surprising that it took a time for scientists to discover that there |
|
| 48:50 | fundamental similarities between the nervous systems of and vertebrates. Yet it was the |
|
| 49:00 | of a useful difference in their nervous , which enabled scientists to undertake research |
|
| 49:06 | has led to a growing understanding of mechanisms controlling our own nervous system. |
|
| 49:12 | breakthrough concerned the nerves that control the of the mantle muscles used in jet |
|
| 49:20 | . As this archive film shows by contracting its mantel muscles, even a |
|
| 49:26 | sized squid can eject a huge amount water with great force. In the |
|
| 49:36 | 19 thirties, the British zoologist, Jz Young was engaged in a study |
|
| 49:41 | the squid's anatomy. Young observed an of large tubular structures each as much |
|
| 49:48 | a millimeter in diameter in the squid's as these structures were never filled with |
|
| 49:54 | . They could not have been blood . From their similarity to surrounding nerve |
|
| 49:59 | . Young thought they must be single , giant axons that transmitted nerve impulses |
|
| 50:05 | a concentration of nervous tissue called the ganglion to the mantle muscles using |
|
| 50:17 | He stimulated the surrounding nerve fibers and that he could only produce large muscle |
|
| 50:22 | in the metal when the large tubular remained intact. So these were indeed |
|
| 50:34 | axons. Scientists quickly appreciated the significance Young's finding. For here at last |
|
| 50:43 | an axon large and robust enough to with the techniques available at the |
|
| 50:48 | And one that survived for several hours isolated from the nucleus, the intracellular |
|
| 50:59 | of the giant axon could be removed analyzed. Leading to the discovery that |
|
| 51:03 | ions were more concentrated outside the nerve and potassium ions more concentrated inside by |
|
| 51:13 | the empty axons with solutions of precisely chemical composition. Experimenters were able to |
|
| 51:18 | the mechanisms of iron transport across the . The giant axons are large enough |
|
| 51:29 | robust enough for fine electrodes to be through the cell membrane and into the |
|
| 51:41 | . In these early techniques, a glass tube was first inserted into the |
|
| 51:46 | and secured with thread. Then the was used to introduce a fine wire |
|
| 52:13 | from which the voltage between the inside the outside could be measured. But |
|
| 52:19 | formation of the nerve impulse was far rapid for detailed study with any of |
|
| 52:23 | electrical measuring devices of the late 19 . It wasn't until the 19 fifties |
|
| 52:31 | the wartime improvement of electronic equipment such the co dreya soos cope, that |
|
| 52:36 | progress was made. Scientists found that nerve impulse was transmitted as a characteristic |
|
| 52:45 | of electrical potential and that this all nothing action potential was generated mainly by |
|
| 52:51 | movements of sodium and potassium ions across nerve membrane. Research on the squid |
|
| 53:00 | Axon unraveled the mechanisms of the formation propagation of the nerve action potential. |
|
| 53:06 | understanding led directly to the development of that block action potential formation and so |
|
| 53:12 | as local anesthetics now used routinely as in dentistry and minor surgery name that |
|
| 53:22 | aesthetic lidocaine. Very good. So have lidocaine patches on the skin, |
|
| 53:31 | , they will put that on your for local anesthesia. Uh at the |
|
| 53:39 | office uh for minor things and this how it was discovered just, but |
|
| 53:44 | thing about it. So in 19 , you may find this giant |
|
| 53:49 | So it's not the giant squid that the ships, but it's a giant |
|
| 53:54 | . It's one millimeter, it's 1000 . So that's something that you can |
|
| 53:59 | with, you know, naked eye up with the tweezers and drop it |
|
| 54:06 | you can put electrodes in it, external transport which is really cool, |
|
| 54:10 | just put an electrode in there, it up, you know, put |
|
| 54:13 | dye in there. And you can for example, the speed the slow |
|
| 54:17 | fast external transport. But you need have really fast circuits to see the |
|
| 54:25 | action potential and that doesn't come into for another 20 years. So we |
|
| 54:29 | these fast oscilloscopes that uh that are of capturing action potentials because they're about |
|
| 54:36 | to few milliseconds in duration. You to have very, very fast |
|
| 54:40 | very fast displays. In order to it to capture. Once they |
|
| 54:45 | they understood that the ring number and phase action potential goes into rising phase |
|
| 54:51 | zero line in the overshoot and goes falling phase. It goes into the |
|
| 54:56 | when it goes below the ring number potential value. And during this portion |
|
| 55:00 | it gets rebuild to its number of value thrust with the help of a |
|
| 55:08 | pump that we talked about. So of the key properties and also key |
|
| 55:14 | that I will ask you to know the exam. So the key values |
|
| 55:18 | the equilibrium potential values for each Those are the four ions that we |
|
| 55:24 | . The resting membrane potential value. threshold value for action potential. Once |
|
| 55:30 | membrane potential reaches this number of potential , threshold value minus 45 or |
|
| 55:38 | it will produce this all or not during the actual rising phase of the |
|
| 55:46 | potential during this phase of the For show right here, after it |
|
| 55:57 | the threshold and before it goes back threshold, this area right here, |
|
| 56:07 | called the absolute refractory period. During period. A cell and membrane in |
|
| 56:16 | cell cannot produce another action potential. . And once it crosses through this |
|
| 56:27 | right here, which is crucial this right here, which is the threshold |
|
| 56:32 | . Once it crosses through there, now is in a relative refractory |
|
| 56:40 | That means that if the cell received strong enough stimulation, even if it's |
|
| 56:45 | polarized here. But the stimulus was enough, the cell could produce another |
|
| 56:49 | potential here and different subtypes of cells have different refractory, uh absolute versus |
|
| 56:58 | refractory periods and dictating how fast they produce action potentials or dictating the frequency |
|
| 57:06 | the action potentials that they can Some cells can be very fast and |
|
| 57:10 | fire up to 600 action potentials a , that's very, very fast and |
|
| 57:15 | cells will just fire two or few potentials a second. And that partly |
|
| 57:21 | on these absolute versus relative refractory periods some other um features and properties of |
|
| 57:28 | channels that we'll be talking about. . So these equilibrium potential values that |
|
| 57:34 | will ask you in the exam are the same table. So it's not |
|
| 57:38 | new, but I just added it . Uh You'll notice that throughout the |
|
| 57:43 | there are slides that have a information repeats. And that's because it's the |
|
| 57:51 | image, but we talk about it different ways. And also you'll notice |
|
| 57:56 | there are slides that are really good note taking. So for example, |
|
| 58:01 | you could summarize everything here. You put the N equation next to the |
|
| 58:06 | potential. You can put VM membrane , you can put gold. One |
|
| 58:11 | equation takes them to conquer the ability Y I and everything here. This |
|
| 58:16 | the membrane potential, this is this is the action potential as it |
|
| 58:22 | . And these dash lines are all of threshold value zero millivolt value and |
|
| 58:29 | the uh equilibrium potentials. So in for this action potential to take |
|
| 58:39 | the membrane has to be polarized to minus 45 millivolts value, it |
|
| 58:44 | So with opening of sodium channels, channels open up, there's a little |
|
| 58:50 | of depolarization that comes from the synapses comes from the synopsis. If the |
|
| 58:55 | and potential reaches this minus 45 millivolt is gonna open voltage gated sodium |
|
| 59:03 | So there's a depolarization that's coming in cell from synaptic inputs from a stimulus |
|
| 59:10 | depolarizing the cell. OK. Electrical , chemical stimulus or it can even |
|
| 59:17 | mechanical stimulus or it can be a of light and photoreceptors. OK. |
|
| 59:24 | cause and effect that cause depolarization or polarization. So here you have |
|
| 59:30 | If these synaptic inputs depolarize the cell the threshold value, it will open |
|
| 59:36 | voltage gated sodium channels. So that that voltage gated sodium channels are closed |
|
| 59:43 | for the most part closed to the numbering potential. And you need to |
|
| 59:48 | it to minus 45 the number. in order to open the voltage gated |
|
| 59:52 | channels, once you open it there's more depolarization, more sodium coming |
|
| 59:57 | , more depolarization, more sodium coming , more polarization, more sodium coming |
|
| 60:02 | . And what sodium is doing because sodium is responsible for the rising phase |
|
| 60:09 | the action potential. It is driving overall number and potential. It's driving |
|
| 60:15 | toward its well uh uh to its potential value. So equilibrium potential value |
|
| 60:26 | sodium is right here. And so membrane tries to reach the equilibrium potential |
|
| 60:32 | for sodium but it does not reach value because notice that we talked about |
|
| 60:39 | force which is BM minus E What does. That mean that means |
|
| 60:45 | the difference between membrane potential and liberal for given ion is important as a |
|
| 60:53 | force for that. In this situation , it's minus 45 millivolts. This |
|
| 61:02 | the number in potential. That's when channels start opening up voltage gated sodium |
|
| 61:09 | . It's driving this membrane potential at level. Here. This is the |
|
| 61:14 | potential for sodium and this is the potential. This is a huge driving |
|
| 61:19 | right here for sodium. But as membrane potential goes up and up and |
|
| 61:25 | to these values, the driving force is the difference between VM and equilibrium |
|
| 61:32 | for sodium. The driving force for has shrunk. Everybody sees that |
|
| 61:38 | The number and potential is huge driving here it's a small driving force. |
|
| 61:44 | if we look at that uh right make an annotation for the driving |
|
| 61:54 | It's the difference OK. It's the between the equilibrium potential, the difference |
|
| 62:03 | the equilibrium potential and the VM. here is a huge driving force for |
|
| 62:12 | . And here the driving force for is small. OK. So the |
|
| 62:19 | the driving force, the more of drive there is for that sodium to |
|
| 62:23 | to try to reach equilibrium potential for . Mm. So at the same |
|
| 62:32 | , if we were to look at driving force here between the peak to |
|
| 62:37 | potential for potassium, now you're seeing at the peak of the action |
|
| 62:43 | there's a huge driving force for potassium . OK. So when the action |
|
| 62:51 | at its peak, now they have huge driving force for potassium. The |
|
| 62:56 | phase is taken over by potassium E . So this is sodium influx during |
|
| 63:01 | rising phase, this is potassium e during the falling phase during the falling |
|
| 63:07 | . Potassium because all of the potassium are open is dominating. It's trying |
|
| 63:12 | drag this number and potential to its equilibrium potential value year. And it |
|
| 63:18 | succeeded in doing so. But then gets rep polarized into the rustling number |
|
| 63:23 | potential state. With the help of , a TPNAK that uses energy in |
|
| 63:30 | to rebuild the separation of charge. second reason why the number of potential |
|
| 63:38 | , the wide action potential does not a colibri potential for sodium. One |
|
| 63:43 | is because you have the reduced driving . And the second reason is that |
|
| 63:48 | soon as sodium channels open, it inactivate and closed. And that's just |
|
| 63:54 | kinetics of the dynamics of voltage gated channels, they're open and they're |
|
| 63:58 | They're called transiently fast activated, transiently , very quick opening. So those |
|
| 64:05 | the two reasons the driving force And the sodium channels here at the |
|
| 64:10 | of the action potential, the sodium are now closed and the potassium channels |
|
| 64:15 | open. Potassium has huge driving force to drive the number of potential to |
|
| 64:20 | own equilibrium potential value and then the can repeat over again, right. |
|
| 64:25 | this kind of uh uh more more depolarization, more sodium is called |
|
| 64:30 | feedback loop. And if there was kinetics of the channel and there was |
|
| 64:34 | driving force and it would actually reach equilibrium potential. But because it had |
|
| 64:40 | driving force and the channels closed, follows the certain uh structure within the |
|
| 64:46 | potential shape and we'll keep talking about for two more lectures. So we're |
|
| 64:52 | end here today and I wish you good week. I will see everyone |
|
| 64:58 | on Thursday. I will be taking and engaging everybody so that we can |
|
| 65:04 | well in the |
|
