hey everyone welcome to professor long flexures in anatomy and physiology quick you know disclaimer we're in the coronavirus shutdown if you watch the name my videos before you know this so these videos are being done rather rapidly very crudely with the minimum of electronic wizardry I'm at home during this shutdown trying to deliver some instruction on my face-to-face classes in an online format so these are old-school lectures like I would be doing in class I'm trying to break them up into smaller chunks because it's very hard to watch long videos occasionally they get up to about 30 or 40 minutes I apologize I'm trying to keep them about 15 to 25 minutes but it's very hard to do um and also have kids and dogs in the background so if you hear beeps and noises and barking I'm sorry but I have no options other than to do this way that I'm doing it so this video is number 5 in this series of my nervous system videos for my biology 24:01 class this video is intended for students in my human anatomy and physiology one or 24:01 class its nervous system lecture 5 we've covered the outline of the nervous system how the nervous system functions from a ferret to even you know send CNS pns and all the subdivisions we've talked about the anatomy of neurons and their morphology we've talked about ion channels and the establishment of resting membrane potential and we've talked about action potentials we are not done with action potentials they're very complicated as you saw from the last lecture and I need to go over a couple of other things with you so now when I look at a multipolar neuron right and this drawing should be familiar to you from my last drawing I have the axon coming down here and ends up in a bunch of Tilo tendría and synaptic knocks as you guys know there's synaptic vesicles thousands of them microscopic vesicles but they exist filled with chemicals called neurotransmitters so I'll put a whole bunch of little neurotransmitter in here now as you guys learn the action potential will travel down the axon stimulate some voltage-gated calcium channels which would be sitting here in the cell membrane and there are thousands of these per you know micron but I'm only drawing one when the action potential arrives the calcium channels open the concentration of calcium is much higher outside the cell and inside it will rush in causing the synaptic vesicle proteins and their membrane to fuse with proteins in the membrane of the neuron and as these neurotransmitters are these synaptic vesicles are fusing with the cell membrane then they would start to release neurotransmitter if and only if the calcium enters the neuron which by the way gives us some interesting pharmacology that we can talk about and if we have time we may do so as that neurotransmitter enters the synaptic cleft it'll travel through the synaptic cleft or across it and on to the next neuron which will have receptors for these neurotransmitters those receptors are connected to chemically gated sodium or chemically gated potassium channels that can then hyperpolarize or depolarize this neuron once you understand how synapse works you know how all synapses work there's also an enzyme that lives in the synaptic cleft that can break down the neurotransmitter ending the signal now what causes that release is the arrival of an action potential here stimulating those voltage-gated sodium channels we talked about how action potentials are we talked about how they are propagated or rush down the axon and because of the the location I'm just going to draw the voltage-gated sodium and potassium channels on here I'm not going to do all the chemically gated stuff we've done that before but because starting at the axon hillock I have an abundance of these voltage-gated sodium channels distributed at a certain distance down the cell membrane they run all the way down the axon which is why once we start the action potential and one patch of membrane we can't stop it once it starts the next channel gets stimulated then the next one and then the next one we call that action potential propagation and we have our voltage-gated potassium channels and extending here allowing for repolarization and all that good stuff okay you should have a pretty strong understanding of how action potentials occur in the knurl we're going to look at several concepts that are in the notes I've essentially covered if you're in my class we've covered everything from about page 65 66 we skipped page 67 to some degree we get everything on page 68 there's the steps of the action potential and we're going to talk about a bunch of stuff that's on page 69 so essentially we're gonna do page 67 on page 69 possibly page 70 if I can get that far I'd like to finish these lectures of this one so now as you guys know there are neurotransmitters being released by other neurons on the soma and on the dendrites and at the axon hillock so let's say that every one of these neurons that I've used black for here is going to be excitatory and I have them everywhere I have all these neurons synapsing releasing neurotransmitter onto this neuron and they're all going to be excitatory they're all gonna cause this cell to depolarize I also have some inhibitory neurons synapsing and their neurotransmitter might inhibit the cell from firing how does the neuron know when to fire the action potential how does sort of a thinking at the neuronal level occur well this neurons going to be getting mixed signals if several of these positive neurons release neurotransmitter to open chemically gated sodium channels that would start to bring us closer to threshold if simultaneously these red neurons are releasing inhibitory neurotransmitters that might cause the neuron to depolarize I'm sorry to hyperpolarized so let's say I'm looking at a neuron and I'm sitting here at resting membrane potential minus 70 millivolts as you guys know our threshold is minus 60 millivolts and then we would fire our action potential if every time one of these cells causes it to start to get excited what if more cells fire in an inventory neurotransmitter I would not get to threshold but then other neurons are firing like crazy and then some of these inhibitory neurons are firing like crazy so I'm gonna be moving around up and down getting excited no wait yes no yes no the only time I fire an action potential is if I receive so much more stimulus to reach threshold than I do have inhibitory signals I hope that makes sense the example I like to think of is imagine if you were you know at the store walking through the mall with a group of friends and let's say you see your sister's boyfriend at the mall not that big a deal except for you see him handing a gift bag to another girl they just came out of some perfumery or I don't know Victoria's Secret or something they're giving a bag to another girl they lean over and kiss her and say hey call me later I want to see you I hope to see you tonight what do you do well you pick up your cell phone you snap a picture and you start oh my god can you see this you want to tell your friend look this guy is cheating on you you tell your sister and what if you go oh my god I'm going to call my sister that rat whatever and start calling him names what if one of your friends says oh no no no no no no you know it's your sister's birthday this week that's his sister she flew in from out of town and he's buying a gift for your sister because it's all a big surprise so don't say anything you're gonna ruin the surprise he wanted his sister and everyone to be here because he's going to ask her to marry him ersal did so you've received one signal that says call her and say something but now you're getting confusing information saying no wait don't so you you don't message her then what if another friend's sending it she goes mm-hmm he doesn't have a sister that's a girl he's been seen BAM you might reach threshold and fire off so this neuron is receiving mixed signals all at one time fire no built fire fire don't fire how does it know when the overwhelming abundance of excitation brings it to threshold it's gonna fire and once we start the action potential at the axon hillock we call this summation adding all this up like the sum two plus two the sum of two plus two is four when we add everything up if at the axon hillock we're enough to reach threshold and I open the first voltage-gated sodium channel there's no stopping the action potential it will spread and spread and spread now we covered we covered the idea of the action potential now we're going to talk a little bit about summation and graded potentials are not strong enough individually to reach threshold the average graded potential somewhere around a half a mil evil but to go from here to here I need 10 millivolts so I would need a whole bunch of graded potentials to occur so how do we reach threshold there's two ways it's called a temporal summation tempo meaning over time and then there's spatial summation within a small space here's what happens in temporal summation imagine a particular synapse like this one firing and firing and firing and just keeps firing so much neurotransmitter over time multiple fires holding all those chemically gated sodium channels open because every time the removing neurotransmitter to break it down another one binds and holds the channel open so in temporal summation we're firing us one synapse or a small number of synapses over and over and over over a period of time holding those sodium channels open the chemically gated ones so that enough sodium rushes in to reach threshold so we're adding a whole bunch of single stimuli together to add up to be a 10 millivolt change in spatial summation imagine if in one particular small space I have a whole bunch of these excitatory neurotransmitters and lots of synapses in this small area and they all find it at once there would be so much neurotransmitter dumped in that so much sodium would rush into the cell that would reach threshold and in a small period of time so temporal summation as a single synapse or a small number firing over a long period of time spatial summation is a whole bunch of neurons firing neurotransmitter all at once stimulating enough channels to open to reach threshold facilitation is when we continually fire on a particular area so long so let's say my neurons are dressed and let's say you drink a cup of coffee and ate a whole bunch of chocolate which which has the ability in our body to stimulate certain synapses to open a certain number of sodium channels chemically gated sodium channels they excite us so let's say that I stimulate it I have just enough in my body that it doesn't reach the rush Holt but it holds enough sodium channels open that it raises my resting membrane potential so that I no longer need a 10 millivolt change I only need 5 millivolts for example now under normal conditions a 5 millivolt stimulus synapse is not going to cause me to reach thresholds but now because I've taken some kind of chemical that's gotten me closer to threshold smaller stimuli will cause me to fire off that's why people who drink energy drinks and lots of coffee and take all these you know stimulants 5-hour energy and all these bang and all this stuff they're wired and jumpy people who smoke a lot of cigarettes or dip a lot of snuff and take all these CNS stimulants they are actually facilitating their neurons to facilitate means to help they're helping them get closer to threshold now small stimuli cause them to fire off like crazy very irritable or jumpy people you know people who abuse certain drugs that way are that way as well so anyway we've covered everything that's on page 67 we've already done page 68 in the last lecture page 69 is going to talk about action potential propagation and velocity okay you guys can fill in the table up here in my class there's a table that wants you to compare and contrast graded potentials I did a little bit of that in the last lecture my greater potentials can vary in size actually are always the same magnitude or amplitude graded potentials can be excitatory inhibitory action potentials require you to always depolarize to threshold and a bunch of other things created potentials or local events action potentials spread all the way down the axon now you can look all that information up fill in that team for actual potential and propagation to propel to propagate means to travel down right or to force something to move so an action potential propagation there's two types of ways that the action potential will spread down the axon one is called continuous conduction okay and what we call continuous conduction this occurs in what we call unn myelinated axons and if you studied the lab videos so far and done lab and you know the difference between a myelinated axon and unmyelinated axon that'll make sense to you if not I'm going to explain in an unmodulated axon you don't know what myelin is yet and just hold on a second it an axon like this what happens is I have to stimulate this patch of membrane that patch of membrane will fire an action potential and all those ions will travel down to the next patch of membrane and then stimulate it and then the next one and then the next one and by the time I'm down here by the way that's the reset but I cannot skip any patch of membrane the voltage has to roll and open next voltage-gated channel and then the next one so we call that continuous conduction and in continuous conduction basically what we're saying is that every patch of memory has to be stimulated to fire an action potential without skipping any patches of membrane so I think of it this way if you were walking across a tile floor but I told you you had to step on every single tile then you can walk across the floor without skipping any okay that's continuous conduction we have to depolarize each consecutive patch of membrane without skipping any until we get to the end now that would be in an unrelated axon so what's myelin myelin is a fatty substance that also has this white appearance so we find it where there's white matter in the nervous system and it's found in some certain cells called glial cells I mentioned these at the very beginning of the nervous system lectures but there's a type of cell in the nervous system called neuroglia or some people say neuroglia or we do certain shorten it to glia glia means glue and these are the cells that kind of glue the nervous system together help the CNS or the neurons and then they're found in the p NS some of them there's different types of glial cells in the CNS and in the pianist and we'll get to those in a moment one of the ones we talked about in lab is a glial cell called the Schwann cell hey chuan Xue is filled with myelin and schwann cells can wrap around axons the way a paper towel wraps around a paper towel holder a roll there's they can wrap themselves around and around and become very thick and if I wrapped a Schwann cell around this part of the membrane very tightly then this patch of membrane would essentially be covered with the Schwann cell and any ions out here like sodium or potassium would not have access to the outside world it would almost be like if I wrapped a piece of saran wrap around your arm very tightly and I poured water on your arm the only parts that the water would have access to you were the exposed parts so this Schwann cell is going to Milan eight the axon it wraps this fatty substance it's filled with a fad called myelin and it wraps around the axon a bunch of times Schwann cells only cover a certain distance on an axial it turns out that there's a small gap between this and the next Schwann cell but then the next one cell will extend the same distance and cover the excellent here and then I have a small gap and another Schwann cell and a small gap and another Schwann cell and they will myelin eat they'll put the smile in and wrap the axon with this myelin sheath now the whole collection of fatty substance all the way down all of the Schwann cells down is referred to as a myelin sheath a sheath is a soft structure that would cover like the long blade of a sword if I put a sword on my hip and the sword holder the sheath the thing that I stick it in is long and hard like um wood or metal and we call that a scabbard but if it's a soft material like a leather holder then when I slide the sword in that puts a sheath over the length of the sword well since the myelin ran the entire length of the axon they called it a myelin sheath a little bit later on this guy lingsha Juan came in with a better microscope and said that's not just a big tube of fat those are cells filmed with that fat called myelin that are running down the axon wrapping around it so they call them Schwann cells after his discovery so the myelin sheath is all the myelination of the entire axon it is accomplished by individual cells called Schwann cells another guy decided well there's these little gaps between the Schwann cells called a node of ranvier or I put should put nodes since I have multiple ones nodes of ranvier this guy's name was Ron VA they named him after him and they are exposed patches of membranes in between Schwann cells little tiny gaps between the Schwann cells so I'm gonna erase all of this we covered some of that in laughs now in myelinated axons we have a stuff that's called saltatory conduction this comes from the latin word salting which means to jump so saltatory conduction is a type of action potential conduction in which the action potential jumps from node of ranvier to node of ranvier and it actually jumps inside the cell and here's how it works if I have a patch of membrane I'm gonna magnify this little area right here and I have all these voltage-gated sodium channels in here and voltage-gated potassium channels thousands of them in every micron of memory and they're all sitting here interspersed like this and then I have my first chuan SEP wrapped around the cell membrane multiple times so tightly that no ions can access it and then I have a little piece of membrane exposed to you at a node of ranvier and I have all these voltage-gated sodium and potassium channels squeezed in here interspersed with each other and then I have another chuan step wrapped around and have another node of ranvier I'm not going to draw all the ion channels but you get the idea okay these ion channels would be there when I open my voltage-gated sodium channels each sodium ion carries a very tiny voltage so it takes literally hundreds of millions if not billions of these sodium ions rushing in here to reach threshold and to fire an action potential so if I have all these sodium ions every plus sign is a volt is a little sodium ion I'm sorry for the knocking of the steam I'm gonna fix it for the next video and this action potential has all these ions rushing in this direction due to the laws of diffusion they'll go from an area of high concentration to the lower concentration so right here let's say 30 millivolts on my action potential diagram all of those ions have to go somewhere and since I can't pump them out here some will get pumped out but some are going to diffuse underneath the Schwann cell and start appearing over here by the laws of diffusion they will move from areas of high concentration to lower concentration if enough of these ions make it to this point to just be a 10 millivolt change from minus 72 minus 60 I hope in the first voltage-gated sodium channel and I get an action potential here so the action potential comes in here jumps to this node of ranvier I get another action potential and enough ions to diffuse underneath here to hit the next node of ranvier and then a few and the next one so it looks like the action potential jumps from node of ranvier turn Oh drunk me that is one way that we can increase the rapidity or the velocity of the action potential how fast is the action potential traveling down the axon unmyelinated axons have to do continuous conduction the action potential just like stepping on tiles on the floor I have to step every step along the way and it takes a while to get to the end now imagine if someone laid a paper towel over every two or three tiles and I had to jump over three tiles then as I jump and jump and jump and jump I can jump across the floor of the room much more quickly than someone who has to step on every single tile so insult to Tory conduction the action potential jumps from node of ranvier to node of ranvier skipping patches of membrane that increases the velocity or the rate at which the action potential travels down the axon so unmyelinated axons have slower action potentials myelinated axons the action potential can travel more rapidly there's a second way that I can increase the velocity of an actual potential of our bachelor potential propagation if I just look at the diameter of two different axons I have a very large diameter axon here and a smaller diameter how many positively charged ions can flow through this at any one time and how many positively charged ions can flow through this one the larger the diameter that larger the voltage can travel down so it travels much more rapidly so neurons like pain fibers you know you put your hand on a hot stove and pull it away you want the fastest action potentials possible so the the most rapid or highest velocity action potential propagation that can occur occurs in large diameter axons that are myelinated and we get very rapid action potentials so you need to know that there are two ways to increase the velocity of an action potential one is axon diameter the other is myelination okay so I hope that covers everything on the bottom of page six 9 it's all written in the notes set for you and then we're gonna move on to page 70 but I'm gonna do this in a totally different lecture because this one's already venturing on 25 minutes we're going to talk about synaptic activity will talk about glial cells actually you know what let me skip and do the glial cells right now ok I want to talk about the different types of glia Schwann cells are a type of glial cell what's unique about Schwann cells is they're found in the p NS so I'm gonna erase on this and I'm gonna talk about a few different glial cells and you know what again this lecture has gotten already long I'm gonna skip this I'm gonna stop again I'll make them all this up as we go along so I'm gonna stop here we're at 25 minutes so thanks for watching hope you had as much fun as I did I hope you learned something and I'll see you on the flip side in the next lecture