Transcript for:
Glial Cells - Structure and Function

all right ninja nerds in this video today we're going to be talking about glial cells we're going to talk about their structure and we're going to talk about their function also before you guys will continue this video please hit that like button comment down in the comment section and please subscribe subscribe all right engineers let's go ahead and get into it all right engineer so we got to talk about glial cells now glial cells is it's important to know what they are so you got to know that there's nervous tissue when we talk about neural tissue or nervous tissue it's made up of two cellular components neurons and glial cells okay that's the first thing so that's the first thing i want you know the second thing i want you to know is that you can find glial cells in the central nervous system which is consisting of your brain and spinal cord and the peripheral nervous system which is all the nerves right the somatic motor sensory autonomic nerves so you have glial cells that are basically associated with neurons in your nervous system now it's important to know what are those glial cells what do they look like and what do they do but not just list them we got to know how they do these things and why they do these things that's the purpose of learning right so the first one we got to talk about is the big daddy this one does so much that it's worth a pretty decent discussion on them and this is your astrocytes now astrocytes are these really amazing glial cells and they are only found in the central nervous system so that's really important they're found in the central nervous system which is your brain and spinal cord okay now they have multiple multiple functions okay what are those functions the first one is really important and it's actually going to be a part of your blood brain barrier right so the blood-brain barrier you should ask what is the blood-brain barrier made up of what does it do and how do the astrocytes play a role in that that's the questions you should have so what we're going to do here is you have a kind of like a a transverse section of the brain right and we're looking in the skull at the brain tissue and you see all these blood vessels that are kind of surrounding that brain tissue what we're going to do is we're going to take a slice of this blood vessel supplying this nervous tissue and zoom in on it but the question i have for you is what is nervous tissue made up of glial cells and neurons so here's our blood vessel that we're zooming in on and here's the nervous tissue that we're zooming in on the first thing that you guys i asked you what should we think about when we talk about blood-brain barrier what is it made up of it's actually three layers the first layer is this kind of like maroon color layer which is basically a part of the blood vessel and this first layer here is actually your endothelial cells very important you need to know the order of things here endothelial cells that's these cells here the next thing that's also important that you have to know is you see these little blue structures between the endothelial cells these are proteins called type junctions so there's lots of tight junctions that are present in this blood-brain barrier lots of them you need them why because they basically control what type of things if we had a molecule here it's going to be very difficult for it to pass between the cell so it controls the permeability across that membrane that's one big thing the second component is this green tissue here just situated outside of that see this green tissue here that green tissue is called the basal lamina it's called your basal lamina and the basal lamina is just basically connective tissue that's all it is it's just a little bit of protein the next part is the big part and this is your astrocytes but particularly what part of the astrocyte well here's the nucleus or the cell body of it and all these like little squid arms coming out to it is called the foot processes so this is called your foot processes of your astrocytes okay so that's the first question that we've answered when we talk about blood-brain barriers zooming in on this three layers endothelial cells with lots of tight junctions basal lamina foot processes of astrocytes there is other cells that kind of squiggle around in here not really that important but they're called parry sites in case you do want to know that you might see that sometimes in textbooks all right good we know what the blood-brain barrier is actually made up of now the second question is what does it do it's pretty straightforward it controls what molecules leave the blood and go into the nervous tissue that's all it does for example think about this think about it pretty simple what kind of things would be able to pass easily just completely diffuse over into the nervous tissue things that are lipid soluble or like respiratory gases right so co2 which is a byproduct of neurons that could just diffuse right over right because it could just move right over oxygen same thing it could diffuse from the blood and go straight to the neural tissues right what else could move across you know anything that's lipid soluble so drugs certain vitamins things like that that could easily pass right across this membrane because it's lipid it can passively diffuse other things like water things like sodium things like chloride right these kinds of substances it may be a lot more difficult for them to move across they need specific types of proteins if you will little shuttles to take and kind of shuttle them across so i might need little proteins present here to shuttle these molecules across into the neural tissues so that's important okay the next thing is what i really want you to understand let's say here we have a protein big old protein this protein we don't want it to be able to just diffuse across why do we not want this to diffuse across if we have proteins that come over here into the vicinity where the neural tissue is maybe it will undesirably stimulate or inhibit the neural tissues that could lead to undesired neural activity so we do not want these proteins to get across this actual blood brain barrier because they could lead to unnecessary processes okay that's important so what have we established we've established that the blood-brain barrier is made up of these components it's a barrier that it controls very selective movement to and from across this membrane and one of the big reasons is we don't want things like proteins that could get into the neural tissue and cause undesired neural activities okay third thing what in the heck did the astrocytes contribute to this you would probably quickly say oh they're just a part of the barrier yes but that's actually mildly what they're involved in what they're really actually doing is they a secrete they secrete like little molecules little molecules like growth factors and what these growth factors do is is they stimulate these endothelial cells you know what these endothelial cells do as a result of that they make more tight junctions if i make more and more tight junctions what happens to the permeability across this blood-brain barrier it becomes even more selective so why is that important if you damage these astrocytes what happens to the tight junction production it decreases if the tight junction production decreases what happens to this actual blood-brain barrier now it becomes more permeable things can easily diffuse across it and that becomes disastrous that's important why i want you to know that the next thing that we have to talk about is that there is areas in the body where the blood-brain barrier is actually broken and that's important too what are those areas and why are those areas important let's take a look over here the next thing i want you to know is that where is the blood-brain barrier so there's areas in the brain where the blood-brain barrier is broken if you will in other words it doesn't completely exist there and there's a reason why what is the blood-brain barrier's job to control the movement of molecules from the blood to the nervous tissues right that's pretty much what it does what if you actually want things to be able to cross the blood and go into the nervous tissue easily why would that be important think about it see this little area here this blue area present within the medulla very interesting area it's called the area postrima area postrima the area postrima is cool because blood that's coming to this area the blood that's coming and supplying like this area here it's actually carrying maybe molecules that are a little bit nasty in our body and maybe it's things like ketones maybe it's certain drugs and what the area postrema does is it samples that stuff from the blood maybe it's toxins and it signals an area in our midbrain called the chemo trigger zone you know what the chemo trigger zone does it triggers vomiting so whenever there's certain toxins or drugs or certain things in the body that we want to get rid of that area posterior can sense that because the blood-brain barrier is broken there and it can tell our body to vomit it out and get rid of that toxin that's pretty cool and very interesting the next area is up here okay this is kind of just around your hypothalamus so just around the vicinity of the hypothalamus there are these other special structures here called osmoreceptors there are some pretty intense names of these sons of guns i am definitely not going to write them out because one they are too long and the other one is i don't know if i can spell them that they're called the subfurnicular organ and the organum vasculosum of lamina terminalis anyway regardless of all that they sample the blood and why is that important what if the blood is really salty what if the blood is really sugary what if the blood is really watered down we need to take into consideration all of that why because they can sense that and then tell us to drink more water if we need more water drink less water if we need less water or signal our pituitary gland you know the posterior pituitary makes adh signal adh production so that we can go ahead and actually maybe increase our water reabsorption and pee less all of those things come into our water balance so that's really cool the last area is between the hypothalamus and the pituitary we're just going to denote this as the hypothalamic pituitary axis the hpa the hypothalamic pituitary axis what happens is between the hypothalamus and the pituitary gland what is the hypothalamus doing it's secreting little hormones things like crh and gr grhh and gnrh all of these different releasing factors that have to circulate through the blood and get to the anterior pituitary so we need a connection between that for hormones to flow between the hypothalamus and the other areas of the tissue of the brain which is the anterior pituitary also we need to be able to sample things around the area of the hypothalamus so again you're sampling different things from the body so these are important areas that i need you to remember the blood-brain barrier is broken and these three areas there is other ones but i think that these are the most important ones for you to remember okay that covers the function of astrocytes with respect to the blood-brain barrier now let's talk about the second function which is how it acts as a potassium buffer you know with the neurons right here we have a neuron you know whenever a neuron is generating an action potential right they're sending positive charges down the axon because sodium is flushing into the cell during repolarization what happens voltage data potassium channels open and potassium starts exiting the cell right so during the repolarization phase of the action potential potassium is like really leaving this axon and really accumulating out here in the extracellular space right same thing when the cell is at rest right when the cell is at rest resting membrane potential you have what's called potassium leaky channels and potassium will be leaking out of these actual neurons at rest right and a lot of this potassium can sit out here into this extracellular space now our neurons have a way to push some of that potassium back in what is the way that they do that well there's little proteins present here throughout the neuron called sodium potassium atpases and what they do is they pump two potassium into this neuron and then they pump out three sodium ions so some of the potassium that sits out here we can try to pump it back in but nonetheless over time there's still just going to be a little bit potassium that's remaining out here that those sodium potassium atpases won't be able to push enough back in we don't want all the potassium sitting out here why think about this think about it really simply if you have a cell and we know with cells potassium is really high in the cell and really low outside the cell and that establishes this concentration gradient that moves potassium out of the cell well if because you don't remove any of this excess potassium what happens the potassium concentration will start rising out here if you don't get rid of this excess potassium now this concentration grating where potassium wants to go from inside the cell to outside the cell is going to be decreased because there's not as much of a gradient here now so now less potassium will leave this neuron and why is that a problem well if less potassium let's just say for a second less potassium left this neuron why would that become a problem well then if less potassium left potassium is positively charged you'd have more positive charge inside of this neuron and initially initially that would increase the excitability now we're not going to get into this i might contradict myself and the reason why is it's it's a little bit more complicated initially because of all these positive charges it increases excitability the neuron but eventually it'll actually decrease excitability and this is due to the voltage-gated sodium channels whenever it's positive for too long they become inactivated but regardless that's important so how do we prevent this increased excitability initially and then eventually a decreased excitability well guess what these astrocytes are here to for the rescue they have little channels on them and what they do is they take this potassium up and they can store some of this potassium inside of them you know what else is really cool if maybe too much potassium is getting stored inside of this astrocyte guess what it can do it has little gap junctions and it can actually connect with another astrocyte and push some of these potassium ions into another astrocyte to make sure that not too much potassium is sitting in this one astrocyte now if the potassium levels are maybe depleted a little bit guess what the astrocyte can do it can push some of that potassium out if it needs to but the main job is that it's mopping up this excess potassium so that we don't affect the action potentials of these neurons initially you'll have increased excitability but because those voltage-gated sodium channels become locked in inactivation from prolonged positive charges their excitability decreases that's why we need them all right let's move on to the next function you got let's say here we have a neuron and here we have a neuron let's say there are two different types of neurons let's just label this neuron here as a glutaminergic neuron what the heck does that mean that means that it produces and makes glutamate now this other neuron here let's call this one a gaborgic neuron okay gaborgic neuron that means that it produces gaba all right cool this neuron let's say that it's going to be releasing some glutamate right let's say that there's an action potential right so you have an action potential positive charges from the sodium ions are rushing down triggers calcium influx into this neuron causes the cell membrane of these vesicles to fuse with the actual cell membrane here and then once it fuses it starts to release out what the neurotransmitters what is this neurotransmitter here again this is called glutamate now that glutamate can then do what it can then move across the synaptic cleft here and then bind on to little ligand-gated ion channels right there's little pockets for it to bind onto and trigger ion influx right positive ions generally well after it works on this neuron here what happens we don't want it to stay there because it's just going to keep stimulating this neuron so how do we prevent that well eventually what happens is this glutamate after it binds with this receptor it'll disassociate and then there's little transport protein here let's call this a glutamate re-uptake protein that's we're gonna we're gonna denote is that glutamate re-uptake protein and what happens is this is going to take this glutamate back in to this glutaminergic neuron and we're just going to recycle it we're going to put him back into these vesicles and just keep reusing him problem is though these glutamate receptor reuptake proteins can get saturated after a while and they can't take up as much glutamate and so guess what will happen glutamate will just start accumulating and sitting in the synapse we can't let that happen it's going to keep stimulating this neuron so how do we prevent that these good old astrocytes they got little proteins here they got little special transporters that can take this glutamate up any of the excess glutamate that's sitting in this synapse it can take up into its cell then what it does is it takes the glutamate and does something really cool that these other cells can't do it converts that glutamate into what's called a glutamine there's an enzyme called glutamine synthetase and it catalyzes this step here and makes glutamine now here's what's cool okay that glutamine can then be transported out of this astrocyte and then guess what there's a glutamine transporter present on this neuron we're going to take that glutamine transport it back in let's actually right here glutamine it's going to take that glutamine that it's synthesized and put it back into this neuron guess what inside that glutamine inside of this actual neuron here guess what it can do it can get converted via an enzyme okay there's an enzyme here called glutamines and it'll take and convert glutamine back into glutamate and guess what we can do with that glutamate we can incorporate it back into these vesicles and reuse it whenever this protein is just too saturated with too much excess glutamate so that's what our astrocytes can do now what else can they do we've got to remember gaba is actually a precur it's actually developed from glute glutamate as well so on the other situation where the same thing could happen let's say that also from this you're releasing lots of gaba right and that gaba sitting into the synapses and you need to make more of it right same thing here guess what we can take this glutamine push it into this gaborgic neuron then what we're going to do is we're going to take that glutamine guess what we're going to do we're going to use an enzyme called glutaminase convert that into glutamate then what we're going to do is we're going to take that glutamate and use another enzyme called glutamate decarboxylase and we're going to take off a structure here and turn this into gaba and then from here we can put that into these vesicles where we need more gaba so astrocytes are important in removing any of the excess neurotransmitters and helping in the synthesis of what two neurotransmitters gaba and glutamate okay that's important all right the next function of these actual astrocytes is they control glycogen and glucose metabolism the next thing that's really important here is that glucose is a very important fuel for neurons right so if we look here here's our neuron right this is our neuron and then this is our astrocyte now what happens is glucose is really important inside of these neurons you know why because glucose can be broken down into pyruvate you guys know all this and then eventually can go into acetyl coa go through the krebs cycle all that jazz and then eventually make atp we know this process but let's say right that the neurons maybe they're not getting enough glucose maybe there's not enough oxygen whatever the reason is there's a decline in atp production okay astrocytes will be able to sense this decrease in atp and guess what they they they help out they're little uh they're little awesome little guys here and so what happens is there's a special protein here remember how we said that the blood-brain barrier the astrocytes are part of that well there's a protein remember how we said how water and and and glu and sodium and chloride and all those things have to be they're select permeability we need proteins to transport them same thing i'm going to represent glucose as this g in order for me to move glucose from the blood into the nervous tissues i need a transporter to move them across and that's actually called a glut transporter and if you really want to know which type it's called glute one now once we take this glucose inside guess what we can do with it well we can first off maybe store it as glycogen because you know that that's called glycogenesis right or we could take that glucose convert it into pyruvate and then eventually take this down to acetyl coa go to the krebs cycle and make some atp for itself right here's what's really cool though in situations like this where there's decreased atp inside of the neurons these astrocytes take and convert their pyruvate into lactate right so let's say here right there's some situation where there's a decreased atp in the neuron right glucose can convert into glycogen whenever it's taking up in the astrocytes exercise can take up glucose converted into glycogen if there's decreased atp what can happen is the astrocytes can break down the glycogen into glucose called glycogenolysis then glucose will get broken down into pyruvate astrocytes also have the ability to convert pyruvate into lactate then what they can do is they have special transporters that push the lactate out of this actual cell the astrocytes and guess what they do they have transporters on the neuron that pump lactate in and then guess what we can do with that lactate we can convert that lactate into pyruvate pyruvate can then go down make acetyl coa go through the krebs cycle and do what make lots of atp so let's review this real quick because i know it's a lot of stuff neurons have decreased atp astrocytes sense that astrocytes have the ability to make glycogen whenever there's low atp they can break their glycogen down into glucose glucose can then get converted into pyruvate pyruvate to lactate there's transporters on the astrocytes they pump the lactate out lactate gets taken up by the neurons lactate can get converted back into pyruvate and then pyruvate can go down and make atp for the neuron so that's that's that's the cool part they can act as a glycogen reserve whenever neurons need the fuel that's cool all right the other important thing just to add on there is glucose transporter because i only talked about how glucose is getting into these astrocytes it is important to realize that there is glut transporters present on these neurons so just in the same way this glucose can also pass this blood-brain barrier and also get into neurons how they're special transporter here called a glut 3. so when we talk about glut transporters on the blood-brain barrier it's one if it's on the neurons it's three all right cool the last thing that i want to say before we move on to the satellite cells which is kind of like the pns counterpart of the astrocytes is astrocytes have another function here the exact mechanism i couldn't find in any textbooks and i don't think they truly know but they also have seen that astrocytes can increase these synapses within neurons so increasing interaction between neurons can play a role in a lot of different aspects that means a bunch of different things so again another additional thing to add on but without an underlying mechanism is that astrocytes also can increase the function of synapses between neurons all right the last thing i want to mention here to finish up here is that we also have satellite cells now the important thing to remember is satellite cells are like the astrocytes if you will of the peripheral nervous system that's basically the easiest way to remember them instead of having to go through all the mechanisms for that the satellite cells just remember that they perform most of these functions with the exception of like the blood-brain barrier they pretty much do all the functions of the astrocytes just in the peripheral nervous system so again they control nutrient metabolism they control glucose i'm sorry neurotransmitter regulation they control nutrient control potassium mopping all of that stuff the only thing that i want to add on to that is that we only really find them in two places is what i really want you to know one is if you look here we have our spinal cord right spinal cords are part of the central nervous system they're only found in the pns so you have to talk about nerves here you know when you talk about a nerve there's this nerve in actually the dorsal root it's called your dorsal root ganglion right and a dorsal root ganglion is just a group of cell bodies outside of the dorsal in the dorsal root right and what happens is these satellite cells they love to surround the cell body of this dorsal root ganglion and so they again are controlling the the nutrient the neurotransmitters the potassium all the things that are diffusing across and it's modulating that for these dorsal root ganglions the other thing is it also is surrounding the cell bodies of your autonomic ganglia your autonomic angular that's for what structures your sympathetic nervous system and your parasympathetic nervous system so it's also surrounding the cell bodies of your autonomic ganglia if you guys can think about this what is the name of the ganglia for the sympathetic that's in front of the uh vertebra or in front of the actual kind of spinal cord we call that pre-vertebral what are the ones on the side para vertebral those satellite cells are surrounding those ganglia what about the ganglia for the parasympathetic nervous system those are close to the target organ those are called terminal ganglia so you see how we're kind of mixing in a bunch of different things so satellite cells astrocytes of the pns everything we talked about situated where dorsal root ganglion and autonomic ganglia boom roasted let's move on all right ninja so now let's talk about the oligodendrocytes versus the schwann cells pretty straightforward stuff what they do just they do it a little bit differently in different areas of the body okay so the oligodendrocytes they pretty much what they do is they put around myelin cheese which is these lipid fatty sheaths and we'll talk about what that does a little bit later but what i want you to know is these oligodendrocytes they put these lipid protein sheaths on the axons of neurons in the central nervous system so when we talk about oligodendrocytes what do they do they myelinate axons in the central nervous system now we got to add in one more thing because this is where it's really cool technically cranial nerve two the optic nerve is a nerve that is technically considered to be a part of the central nervous system so oligodendrocytes myelinate axons in the central nervous system and cranial nerve to the optic nerve now the schwann cells on the other hand what they do is they myelinate axons in the peripheral nervous system right now when we talk about that what are we actually saying we're talking about the spinal nerves right so all your spinal nerves as well as cranial nerve three all the way till 12. so that's important to remember so we're myelinating axons in the peripheral nervous system for the schwann cells myelinating axons in the central nervous system and cranial nerve two for the oligodendrocytes next thing that's also important to remember i look at dendrocytes if you look at them one oligodendrocyte myelinates how many axons multiple axons that's going to come into play a little bit later so the next thing i want you to know is that oligodendrocytes myelinate multiple axons how many one oligodendrocyte can myelinate up to 30 to 60 axons holy crap where schwann cells one schwann cell can myelinate segments of one neuron or one axon so schwann cells myelinate one axon and sometimes multiple schwann cells for one axon that's very very important okay the next thing that i want you to understand here is that whenever there is damage to the oligodendrocytes and it causes this demyelination of the axons in the central nervous system that cannot be regenerated so whenever there is damage to the oligodendrocytes they cannot whenever there's damage okay let's say let's put it like this oligodendrocyte damage there's no ability for regeneration okay whereas when you talk about schwann cells if there's damage to the swans the schwann cells if they're damaged they do have the ability to regenerate that is very important so the next thing that's important of why we should know this is that when you know whenever you demyelinate basically you remove the myelin sheath around the axons and the central nervous system you know what that's called so demyelination of the axons and the central nervous system can lead to a disease called multiple sclerosis that's important to remember whereas if you demyelinate the axons in the peripheral nervous system what's that called that's usually referred to as gion syndrome that's important to remember all right beautiful so now we know what oligodendrocytes do we know what the schwann cells do we know how they for the most part do it differently the next thing that i want you to know is something a little bit special about the structure of the schwann cells myelinating these axons so i want you to take for example we're going to be looking at this we're going to take a section right here and we're going to look on end at the nerve and the schwann cell surrounding it here we're going to have the cell body right and what happens is the cell body gives off kind of like these little like extensions if you will that tries to come around and like swaddle this nerve so it's trying to just kind of swaddle that nerve and this state if we just looked at it like this this is technically not myelinated because it's not completely covering the axon so this in this state it's actually not myelinated unmyelinated if you will okay so unmyelinated all right but then guess what happens what these schwann cells do they're really cool what they do is is they take these little swaddling arms and when they come together they start kind of wrapping around and wrapping around and twisting around the axon to where they make multiple kind of like concentric layers of their like little swaddling arms around that nerve this in this state it's myelinated so that's really cool but here's what we got to actually expand on a little bit more if you really look at it there's a special name and i want to make sure that we kind of like outline this the way the schwann cells do that they have let's say that we kind of do it like this here this part here i'm going to kind of i'm going to like show this kind of like like lines here okay and then everything else we're just going to do these solid lines all of these solid lines inside of these kind of like dotted lines here these kind of like lines here this right here from this here to this here this is called your myelin sheath but then this layer right here outside of the myelin sheath which is formed by the actual swan cell schwann cell this is referred to as the neurolemma which is basically the cell membrane of the actual schwann cell the neurolima is why schwann cells have the ability to regenerate whenever you damage this actual like entire structure from a condition like guillain-barre syndrome the these actual schwann cells that neurolemma allows for them to be able to regenerate reform and help to remyelinate axons after they've been damaged that's very important all right cool we've established that understanding now we talked about what these glial cells do right how they form myelin we need to know what the heck myelin is and why it's important to know that all right so what does myelin what is it and what does it do i guess that's the important thing for us to ask right so we know the oligodendrocytes from the schwann cells make myelin what does it do what is it made up of myelin is basically a combination of a bunch of lipids and some proteins that's really what it is and all it's designed to do is act as a good insulator and increase action potentials down the axons of a neuron that's all it really does so if you were to take into comparison looking at this neuron which is myelinated in this neuron which is not myelinated which one will have increasing action potentials obviously it's going to be the one that's myelinated and this one will have slower action potentials there's a specific name of the type of action potentials upon which these neurons send their information down and this one here with the myelinated axons is called saltatory conduction and we'll talk about what the heck that means in a second whereas those who are not myelinated this is called continuous conduction okay so it's called continuous conduction all right beautiful so we understand the basics of myelin now let's get into a little bit more detail about how it increases the action potentials and what the heck is the saltatory conduction all right so if we really zoom in here okay on this portion let's say of the actual neuron really zooming in on the axon this is the view we're looking at right here these are basically our schwann cells if you will with the myelin sheaths around them around the axon now in between there's little concentrated areas located in between each myelin sheath right so here we have like a little space here and here we'll have a lace little space here where there would be another uh schwann cell with myelin sheath here these little spaces here are called the nodes of ranvier so what is this little space here called if we were to kind of highlight it this space here is actually called the nodes of ranvier and what's important about these nodes of ranvier is that each of these nodes of ransomware are highly concentrated with voltage-gated ion channels what kind sodium and potassium so let's say that at this point here this this area of the axon has been stimulated it's reached its threshold potential or voltage and these voltage-gated sodium channels are activated if they're activated what happens is they open and allow for sodium ions to do what to flow into this actual axon then what happens is this sodium ions will do what so whenever all these sodium ions rush into the axon right generally what they would do is these positive charges would go by to the next voltage-gated sodium channel stimulate it and have more sodium ions to flow in it would just be a propagation right but what happens is those voltage-gated sodium channels are absent they're not in the vicinity of where the myelin sheath is so because of that these ions can't actually let go and stimulate other channels for more ions to flow in instead they have to move down the axon to the next area the next node of ranvier if you will where there's more concentrated salted voltage-gated sodium channels stimulate them and they open sodium will flow in at this area at this node and all these positive ions again there's no voltage-gated sodium channels where the myelin sheaths are so they'll have to move down really fast until they encounter the next voltage-gated sodium channel activated and then the ions will come in so this type of way if you were looking at it like this you would see depolarization at this node and then you would have this period of where you wouldn't see kind of any action potential where the myelin sheath is then another depolarization wouldn't see it because of the myelin sheath and then another depolarization so it looks like the action potential is like skipping from each node of ranvier to the next and that is what we call saltatory conduction now the question that i would always want to know is how does these myelin sheaths basically help these ions to move faster there's two reasons we're going to write them down in a second i just want you to trust me for right now one is that there is no permeability at this point where the myelin sheath is there's no voltage-gated sodium channels or potassium channels here so because of that no ions can actually come in at these points so now the ions have to move along the axon the second reason is a little bit more physics incorporated to really kind of dumb it down this also has the ability to act as a has what's called a decreased membrane capacitance so myelin sheaths decrease membrane capacitance what does that mean it means that there's less negative charge on the cell membrane now why is that important let's imagine here that there was a lot of negative charges if positive ions had to flow this way down the axon to get to the next area of voltage-gated sodium channels and they had this little like negative charges over here what made that one like what they kind of want to do they may want to come over here and interact with these negative charges and that might slow down the flow of positive charges down the axons guess what myelin sheets do they allow for less negative charge because they have decreased membrane capacitance right and if there's less negative charge there what happens now less of these positive ions are going to want to interact with the negative charges and most of them are going to want to now do what only move down the axon okay so that's the reasons why it's due to membrane capacitance and membrane resistance basically they have decreased permeability at the level of the myelin sheaths and less negative charges that's the whole point the same concept exists here for the voltage-gated potassium channels we're not going to power through in the same detail but the same concept exists you hit a voltage here what happens potassium moves out if the potassium moves out negative charges is there any permeability at this point where the myelin sheath is no is there any positive charges or negative charges at this point no so there's not much membrane capacitance and no permeability where can it only go down the axon gets to the next point stimulates these voltage-gated potassium channels what happens potassium ions leave if the potassium ions leave the cell what happens inside of the cell becomes negative again where the myelin sheaths are there any voltage-gated channels no is there a lot of charge here no so where is it going to move only down the axon so that's why conduction potentials move faster down the axon when myelin is there all right now what i want to do is recap this in particular terminology and then talk about a couple extra things all right so we talked about how myelin basically increases conduction velocity right but there's another aspect that i want to talk about when we talk about conduction velocity or the speed of action potentials down the axon there's two things that basically increase the conduction velocity one we've already discussed myelination the more myelinated the neurons are the faster the conduction velocity the second thing is diameter think about this it's pretty it's relatively straightforward when you think about diameter if you have some type of axon right that has a small diameter that means more resistance right and if you have more resistance to flow what does that mean for the actual flow of charge that means that there's going to be a decrease in flow in this case charge so if we have a large diameter that will decrease the resistance of charge flowing and that will actually increase the flow of charge down the axon so conduction velocity is dependent upon these two things that leads to the next concept which comes up there's different types of neurons that have different degrees of myelination you can remember them in the most simplest way right if you talk about the myelinated uh fibers myelinated neurons if you will there is type a and if you really wanted to get into the nitty gritty there's a alpha a beta a gamma and a delta all i really want you to know out of all of these is that these have the fastest conduction velocity so they are the most myelinated of the axons the second one is the b fibers okay the b fibers these ones are going to have a moderate conduction velocity okay and these ones are moderately myelinated and the final ones is your c fibers and your c fibers these are going to have pretty much no myelin or very little myelin so they're going to have very low conduction velocities so that is important and obviously there's various different types i don't want to cover that in this video maybe in future lectures we can cover all these different types of nerve fibers but again basic concept is that as you go down from a to c myelin decreases and so does the actual conduction velocity so that's an important thing to take away all right so now let's go ahead and finish up with the ependymal cells and microglia all right mr so let's move on to the epidural cells epinemal cells are these specialized like kind of like cuboidal cells located within the ventricles of our central nervous system so if we're taking a look here imagine here again we got that transverse section of the brain we're looking into that skull this is what you're going to kind of see right we see all these like blue structures here right you see like this is your lateral ventricle lateral ventricle and then here in between the thalami is your third ventricle what we're going to do here is is we're just going to take a little cut and here you have some blood vessels that are kind of supplying a little bit around that area what we're going to do is is we're going to take a cut right here and zoom in on it and take a look now first thing i need you to know what we're actually looking at here and what the ependymal cells are a part of is there a part of we're going to actually label this this is a part of what's called your blood csf barrier that is what they're a part of your blood csf barrier that begs the question then what the heck is the blood csf barrier made up of what does it do and how do the ependymal cells contribute to that that's what you should know all right the blood csf barrier what is it made up of it's made up of a couple layers first layer here kind of similar to the blood brain barrier but different first layer these are kind of like maroon cells these are endothelial cells here's the difference though do you see tight junctions between them no you do not so because of that these are different than the blood brain barrier and the blood brain buried the endothelial cells are really tightly connected the endothelial cells in uh in the actual blood csf barrier are actually fenestrated they're actually fenestrated so they're a little bit more permeable okay the second thing that you need to know is the green layer there that green layer there and right after the endothelial cells is called what that is called your basal lamina and again your basal lamina is just basically connective tissue that's all it is and then the last layer here is going to be these black cells here and these black cells are going to be your cuboidal cells but specifically which one the ependymal cells now here's where you've got to add on this additional thing if you look at the ependymal cells this one this one this one this one what's kind of situated between them lots of tight junctions so we have lots of tight junctions located between the epinemal cells whereas in the actual blood-brain barrier what did we have a lot of the tight junctions between the endothelial cells so important to remember this is the layer of the blood csf barrier and the tight junctions are in between the epinephral cells so now the most selective the most selectively permeable portion of the blood csf barrier is which area now the ependymal cells now we know what it's made up of now what does it do same thing as the blood-brain barrier if i want to move water across over here this is going to control it if i want to move oxygen across if i want to pick up co2 this controls it if i want to move glucose right we'll represent it like this if i want to move sodium if i want to move chloride any of these molecules i obviously need special transport proteins to move all of these molecules across okay that is really important now when all of these molecules like sodium chloride glucose water all of that stuff is pushed over through all of these cell layers and goes into this little space what is this space here called this here is called a ventricle so we're going to call this portion here a ventricle which ventricle could it be what depends on which area of the brain we're in could be the lateral ventricle could be the third ventricle could be the fourth ventricle we're just zooming in on that area and so now if all of this stuff comes over here water maybe a little bit of oxygen maybe there's some glucose maybe there's some sodium some chloride stuff like that maybe there's even small amino acids present over here all of that stuff sitting in this ventricles now made what type of fluid cerebral spinal fluid so this is now what we refer to as csf cerebrospinal fluid so the ependymal cells are part of the blood csf barrier this is the component what do they do they control the movement of ions across this barrier and what do they make cerebral spinal fluid guess what else they do you see how they have like these little uh kind of like cilia if you will you know cilia is important because it kind of like uh has motion it can move and beat things and so any of the cerebral spinal fluid that's actually made here guess what these epinemal cells do they beat and move the cerebral spinal fluid so when someone says zach what does the epinymal cells do you say it's a part of the blood csf barrier helps to secrete make cerebral spinal fluid as well as circulate the cerebrospinal fluid boom roasted let's move on to microglia all right so the last uh glial cell that i want to cover is microglia now these are really cool but it's very interesting how they actually come to be so you know within your bone marrow right you have your actual uh your red bone marrow which is kind of present within flat bones red bone marrow well in the red bone marrow let's just say it's right here within this bone they make white blood cells a particular type of white blood cell and this type of white blood cell is called a monocyte now this monocyte when it's made by the bone marrow guess where it migrates to your central nervous system in the central nervous system it actually kind of differentiates into a special type of white blood cell and this is called a micro glia cell now microglia their function is basically they act as an immune system cell in the nervous system that's basically what they are so what happens is let's say that there's a pathogen for some situation there's a pathogen and this pathogen right is causing damage to this neuron so this neuron is now damaged what happens is from this neuron damage there may be cellular debris that's released out here there may be cytokines that are released out to this area what that does is those cytokines are going to do what they're going to stimulate this microglia and it's going to become activated when this microglia becomes activated and is informed that there's some type of injury of the surrounding nerve tissue maybe due to a pathogen this sucker goes ham and starts releasing nitric oxide it starts releasing free radicals okay it starts also releasing different types of really destructive molecules okay as well as cytokines maybe other different types of cytokines to activate other microglia cells so to release nitric oxide and different types of reactive oxygen species free radicals and cytokines this nitric oxide reactive oxygen species free radicals what it's going to try to do what it wants to do is destroy the pathogen that's causing this nerve damage however these things aren't like little kind of like they can detect exactly where the injury is sometimes there's a byproduct of that which it can also damage the neural tissue and that can sometimes become a problem if there's lots of uh inflammation in the area you want to know why you know if there's lots of this actual reactive oxygen species and stuff like that released what does uh surrounds the axon of a neuron that actually increases their conduction velocity do you guys know we just talked about it right what is it myelin guess what can happen whenever you have lots of inflammation and lots of microglia cells releasing lots of these damaging molecules guess what it can do it can actually damage the myelin and cause demyelination of axons that's important to remember now the other thing that these microglia cells can do is that let's say there is a pathogen in the area not only can it try to destroy that pathogen via releasing these destructive molecules it can also go and phagocytose that so then the other thing that can happen here is that it can undergo a phagocytosis process and if it phagocytoses that actual pathogen then look at this here's going to be our microglia it's phagocytose that pathogen over there took it in after it's destroyed it then guess what it can do it can make a protein that can express the piece of that actual destructive pathogen on its cell membrane and when it presents that on its cell membrane it presents in what's called an mhc2 molecule and after it presents a piece of that pathogen on this mhc2 molecules guess what can happen t cells can cross your blood-brain barrier and when these t cells cross the blood-brain barrier they can come to this area and when they come to this area guess what happens they interact with this and when they interact here and they become activated they start to release lots of cytokines and these cytokines may activate the microglia more to release more dangerous molecules or cause more inflammatory reaction to happen in this area the whole goal though is to break down and get rid of any cellular debris any pathogen that's causing this damage and the microglia cells can do that by these molecules amplifying the immune response or phagocytosing them and presenting them to t cells okay that is the microglia why am i stressing on all this you know whenever somebody has hiv hiv for some reason loves to attack these cells and that can lead to a lot of problems where these cells can become hyperactive release a lot of these molecules destructive molecules that can demyelinate axons and leads to a lot of encephalitis so it's important to remember that okay so that covers microglia all right engineers so in this video today we talk about glial cells we talk about their structure and function i hope it made sense i hope you guys liked it alright ninjas as always until next time [Music] [Music] you