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Neuron Physiology Basics · Acedra
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Neuron Physiology Basics
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0:00
hello and welcome to another video here I'm gonna talk about the physiology of neuron so first I'm going to talk about the membrane potentials which is a differences in voltage between the inside and outside of a cell and there are basically three types of potentials and neuron can be in it can be in something we call resting potentials and the name doesn't come randomly it is basically
0:21
when the neuron is at rest meaning it's not doing any work and if you look at this chart a neuron is at this resting potential when the voltage inside the cell is at negative 70 millivolts then we also have something called graded potentials so when we receive a signal we call this stimuli and that stimuli is going to stimulate which is the changes in membrane potentials and we'll talk
0:44
more about this later it can be a small or a big change as you see here but as soon as we hit that threshold of negative 55 millivolts and that's gonna trigger the neuron is sending the signal further away and this process we call this an action potential and that's basically what this long line is going to represent I'm also going to talk about the mechanism of impulse conduction in nerve fibers we've got this slow continuous conduction in unmonitored nerve fibers and it's
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quite slow compared to the saltatory conduction in an myelinated nerve fiber so before I go detailed into this topics I want to go through the basic physiological rules to kind of get a better grasp of what's typically happens in a cell so if you feel like you're comfortable with the physiology of a cell you can just skip this part and go right over to make membrane potentials so there are a couple of things you need to keep in mind when it comes to the cell so let's imagine this
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is a cell where you got the cell membrane outside and the nucleus on the inside and when it comes to membrane potentials you need to keep in mind that there's a greater number of negative charges inside the cell than on the outside so if you look into the chemical composition of the intracellular fluid and the extracellular fluid you will see that we got a huge amount of sodium without of the cell band on the inside and that alone gives us a very huge difference in charge we've
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also got a lot of calcium and outside of the cell that on the inside but we have more potassium at the inside and this is a very important factor when it comes to a membrane potentials in the neuron because it helps a trigger that action potential like I'll get more into that later in this video but we also get a greater amount of calcium and bar carbonate outside of the cell than on the inside and you will also find more proteins within the cell and they're typically
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negative charged now in the late 1800s Walter Nance developed an equation especially for the membrane potentials that's the so called Nernst equation he used this formula to calculate the potentials of a charged ion across the membrane so here we can put in the concentration of a certain ion both outside and inside of a cell and we'll get the equilibrium potential of a certain ion in millivolts and this is very big because it helps us understand the membrane potentials even
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better so now what are the rules in transporting things through the membrane so there are two ways transportation through the membrane can happen either it requires energy or not so first we get the passive transport and since it's passive it does not require any source of energy because passive transport always goes towards the chemical gradient and there are a couple of ways this can happen so let's imagine this is the cell membrane oxygen and carbon dioxide
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can easily diffuse through and this will call simple diffusion and there are some criterias for this the molecules has to be small enough to fit through the phospholipid bilayer and they also need to be lipophilic as they dissolve in the lipid bilayer they also have to have no charge or also just get repelled we also have something we called faciliate diffusion it is exactly like simple diffusion except that it needs some help proteins because polar molecules or ions can get
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through that easily like nargis potassium iron right here for this to be able to go through we need the so called channel protein so that it can go through that channel we also have something called gated channels on cell membrane and they're gated so they're closed but they can be opened when a chemical like a neurotransmitter for example binds to the protein and that will cause the gate to open and ions will just flow in so gated channel can also be opened by a stimuli
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so if there's a difference in charge across the membrane it will also open so examples of gated channels would be voltage-gated channels like the sodium potassium pump we also have ligand or at chemically gated channels which will open when a substance binds to the channel protein like this example right here we got the information gated channels which will open the two attention on the Meccano sensory proteins and we also got temperature gated channels
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which will open due to a temperature increases or decreases so that's these now other types of passive transport would be an a filtration or the blue Meadows capsule of the nephron and the kidneys would be an example of that we also get an electro kinetic transport remember inside the cell is way more negative than on the outside and since we got more positive charge outside of the cell than on the inside the direction of the electrical gradient would be like this hence
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positive positive charged ions will diffuse in so that's electro kinetic transport then we got something as simple as osmosis where water travels towards the concentration gradient so that's passive transport now the other type of transport is active meaning it requires energy so let's just remove the passive transport so that we don't get confused so here we see the cell active transport is moving molecules against a concentration gradient and this process burns
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ATP and we got two types of active transport first one is primary active transport and a famous example of the primary active transport is the sodium potassium pump so if you add the same number in again again there's going to be more potassium inside the cell than on the outside and the opposite goes to sodium now I'll talk more about this later because the sodium potassium pump is very important for the membrane potentials but basically if we add the pump right here with the
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help of ATP the sodium potassium pump is gonna pump out three sodium ions outside of the cell and at the same time is gonna pump into sodium ions so we're basically moving stuff against the concentration gradients so this is basically the primary active transport directly caused by ATP so the second type is a secondary active transport so if you add the cell again and add sodium now after the sodium potassium pump has pumped out a lot of that sodium we're gonna have
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an extremely difference in concentration when it comes to sodium so it's not really enough going to diffuse in towards the concentration gradient slowly so if we add glucose for example now as sodium goes in glucose comes along goes in with it it could be glucose could be another charge molecules so that's the thing is the secondary active transport is the form of transporting down a concentration gradient because of the extremes between those which was generated by the primary
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active transport so it's still a form of active transport as it ultimately resolved from the use of ATP by the primary active transport so now that we understand a little bit more of what's going on we can finally go ahead and look the membrane potentials in neuron so here you see the basic types for nerve cell functionally we divided into three parts we got the dendrites which is a the receptive part which receives information from outer environments or inner
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environments meaning other nerve cells next the integrating part which is the cell body and at the beginning of the axon the axon hillock so the cell receives a signal and then it integrates it either to cause a response or not and last part is the transmitting part which is the axon because it transmits impulses from the cell body and so in this video we're going to look at potentials we
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got the resting potentials that's when the neuron is at rest it's not with at rest I'll show you why later and then we'll get the graded potentials based on the stimulus of that neuron and then we may or may not have an action potentials I'm gonna go through them in the following order and first we're gonna look at what a resting potential is and how the sodium potassium pump and how the permeability of different ions in the membrane contribute to the resting potential we'll then
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skip forward to the reaction potentials and the importance of voltage-gated channels and then we'll look at how a ligand gated channels contribute to either firing an action potential or not so what is potential potential is just a separation of charge but we can measure that voltage it's typically negative 70 millivolts in an typically human neuron and so when you're seeing pictures of an action potential like this on either side of that action potential we can
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see that the neuron is at rest now it doesn't mean that it doesn't require energy to do that so let's zoom in into the surface of a neuron to really understand what's going on now remember we're gonna have a higher amount of potassium inside the cell than on the outside I'm also gonna have many negatively charged proteins inside the cell as well and then on the outside we're gonna have salt we're gonna have a high amount of sodium ions and chloride ions on the outside now if
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you look at this there's still no potentials there are still no separation in charges and to really figure out how that works let's just remove a lot of it and only focus on the potassium itself because it's really the potassium and the permeability of the potassium that establish that number of potential and so remember I said that we've got channels on the cell membrane this is what we call ion channel or simply a potassium leak channel and it's a leak channel that allows
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potassium to move in and out and so if you think about where these potassium is going to go there's a chemical gradient from the inside to the outside in other words there's a higher concentration of potassium on the inside than on the outside and therefore naturally potassium is going to go towards it the concentration gradients but there's actually one more thing we need to think about and that is since we got all that positive charge on the outside of the cell these
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ions are gonna repel each other and make an electrical gradient to the inside so the more potassium that leaks through you can see that the electrical gradient is increasing and so actually what could happen is that potassium can start flowing back in now we got a chemical gradient outside and an electrical gradient inside but we've got an electrochemical gradient outside electrochemical gradient outside now why doesn't all the molecules flow out well that's because
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these are leak channels they don't really allow for many ions to go through they're fast now if we would add a sodium ion a channel as well we will see that ions are starting to move in and out of the cell through those ion channels but if we would let this go eventually we'll lose that potential because all that sodium will just come in so to solve this problem we need to add this sodium potassium pump or we cash in a little bit of ATP to move that sodium out three sodium
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ions for every two potassium ions in and so as all of this is moving the sodium potassium pumps gonna keep up making sure that the potassium stays inside the cell and the sodium outside and so that is our resting potential minus 70 millivolts is established because we will have that separation of those ions now it requires energy to do that and keep that in mind so what can resting potential eventually to action potential so here you see our ion channels
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again but we're going to have other types of channels here as well and those are the voltage-gated channels remember I mentioned them earlier we're going to have a sodium water channel and a potassium voltage-gated channel now these are gated meaning they need a certain amount of stimuli to get excited to open basically and one important word to remember here is excitability meaning the ability to respond to a stimuli so if you look at the sodium voltage-gated channel as
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an example there are two gates on the sodium voltage-gated channel one of them is called activation gate located on the outside of the cell and an inactivation gate on the inside so if we go back to this diagram again at the resting potential of negative 70 millivolts the activation gate is closed and the inactivation gate is open as you see right here and what's special with these gates is that they're both have their own properties activation gates for reason for
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instance is sensitive to stimulus meaning it opens due stimuli applied in that evasion gate in other hand is not sensitive to stimuli but it closes when the membrane potential becomes more positive than negative 40 millivolts now let's go ahead and animate it a little bit and to see how it works let's say a stimuli is applied the first step is here at negative 70 millivolts at this point if
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we apply more stimulus to the cell the activation gate will open therefore we say that the cell is excitable then when the membrane potential gets close to the threshold the sodium gated channel is still in the same position remember because we haven't broken the threshold yet but since the membrane potential is close to the threshold will say that the cell is more excitable meaning less stimuli is required to open that activation gates so now as soon as it reaches the threshold
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of negative 55 millivolts the activation gate will open up and a lot of sodium ions will flow into the cell and this process of going towards a positive potential we call the depolarization and now since both gates are open the next stimulus the cell gets can't do anything to that voltage-gated channel because both gates are already open and so therefore we say that the cell is not excitable and it is already responding to with a previous stimulus then at the top here at
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plus 30 millivolts until the threshold level the potassium gate will open up and the inactivation gate of the sodium gated channel will close and that will lead to a huge out flocks of potassium ions and this process of going back to the resting potential this is called repolarization so since the inactivation gate is closed we say that the cell is still not excitable then it reaches the stage of hyperpolarization where the member potential is lower than the actual resting
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potential as you see right here in the graph and during this time the voltage-gated channels are gradually brought back to its initial States at the beginning the cell is not so very excitable but becomes more excitable as the ion leak channels established at resting potential again so that is the action potential so so far we've only looked at the segments of an axon how does the message move all over to the end of the axon what really happens is that we have
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an a depolarization of a segment in an axon what really happens is that we have a depolarization of a segment in axon which triggers depolarization on the next segment and then to the next one and so on so let's zoom in and look at these voltage-gated channels and then see what's going on so if we look at this should be a near the axon hillock at the beginning of the axon itself regard voltage-gated sodium and potassium channels and so when we hit that threshold those sodium gated
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channels are gonna open up and we're gonna have an influx of sodium ions and they're gonna change your voltage of the next sodium voltage-gated channel now why is it that they're only moving in this direction and not other not to the left well that's because to the left we've got that repolarization of segments where the outflux of potassium ions happening we're not even gonna be close to being excitable at that segment and so it's gonna travel all the way through that
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axon and so this is a similar that signal looks like as it moves down the axon but we have some really long accessory body we have the sky at eclair which goes from the base of the spinal cord all the way down to the big toe of each foot so this needs to go faster and so most of these neurons are myelinated which isolates that nerve fiber this process of leading that signal is called an impulse conduction and in an unmonitored in durafiber we call it a continuous
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conduction while in an myelinated nerve fiber we call it as saltatory conduction where the area of excitability lies inside the node of ranvier between the myelinated cells and since these myelinated nerve fibers are covered with myelin sheath impulses jump to the next node from here so impulses are isolated by the myelin which makes a signal faster and so the problem with non myelinated nerve fiber is that some current are lost on its way as the impulses are slower
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and there's no isolation now action potentials are followed the so called all-or-none law what that means is that an action potential will occur or not it doesn't depend on how much stimuli it gets it only depends on if you're able to hit that threshold or not we can get really close to it as possible without anything happening but once we hit that threshold an action potential will happen so so far we've looked at what happens in action potential and at the resting potential but
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what really happens here in between so when a signal is sent to another cell as you see right here what really happens between those two neurons so let's zoom into that to understand it better so the neuron that sends the signal is called a presynaptic neuron while the neuron that receives a signal is called the postsynaptic neuron this type of synapse is called a chemical synapse now I'm not going to go into much detail into this I'm just going to go through the basic
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so that you really understand what's going on so so far we looked at ion channels and voltage-gated channels but there's another very important channel when we're talking about graded potentials and these are they like and gated channels or simply chemically gated channels remember we mentioned those earlier as well so when an action potential reaches the end of an son it depolarizes those calcium voltage-gated channels which will open the way for a calcium
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to flow in and that calcium ion will dock to those to the surface of those vesicles containing neurotransmitters and it will allow these to connect to the surface of the presynaptic neuron and then release the neurotransmitters through exocytosis so these neurotransmitters will talk to the likeand gated channels and that will open them up and what does that do well it all depends on what kind of neurotransmitter it is and what kind of receptor it is some are going
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to be excitatory and if you look back at their graded potentials excitatory means that they're gonna move us towards the thresholds towards that all or non action potential now what's the easiest way to do that well that's by moving a positively charged ions in there's also some that's going to be inhibitory meaning if you move us away from that threshold and this is done by either let a chloride flow in or simply potassium flow out and so the all-or-nothing principle happens at
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the axon we either have an action potential or we don't well the graded potentials happens at the cell body so that was all I had for the physiology of a neuron for now and I hope that was helpful
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