all right so we're going to start this video by talking about neuron structure so we're going to look at a single neuron and kind of the parts of that neuron and some of their functions okay so let's start kind of on the left side of the neuron and work our way towards the right okay and so our first structure is going to be these kind of thready structures right look like hair kind of coming off of the cell body of the neuron these are called dendrites okay and so dendrites are important becaus
e they receive information okay all right so they receive impulses okay so whether it's from you know sensory receptors or from other neurons they're going to receive impulses so impulses would travel in through these dendrites right towards that cell body so they're going to be receiving information okay uh and so the information then travels into the cell body so it's going to be this part here right also called the soma so the soma or the cell body all right so that's where the nucleus is oka
y so within the cell body you have the nucleus as i mentioned right so that would be this big purple structure you also have nissl bodies within the cell body so that's going to be these kind of whoops these kind of little blue structures here all right and so the nissan bodies um are going to be important for making proteins right so the nissan bodies are collections of um rough er all right so this is going to be rough er and proteins are going to be very very important for a neuron because yo
u know they contain lots of ion channels lots of sodium potassium pumps lots of receptors for neurotransmitters all of those things are made out of protein so neurons need a lot of rough er to make all of those proteins and that's why it has these dense collections of rough er called nissl bodies okay and then um neurons also have neurofilaments all right we can't really see them in this picture but these are going to be kind of collections of microtubules okay so that's part of the cytoskeleton
of the cell the microtubules and they're really important in neurons because they act one of their functions is that they can act kind of like railroad tracks right through the cell so you make your proteins in that cell body in the soma of the neuron and you send them all the way down these neurofilaments they can go all the way to the end of the axon all right so they transport throughout the neuron okay so you would make proteins or even neurotransmitters here in the soma and then you need t
o get them down to the end of the axon down here in order to send them out right if we're looking at a motor neuron right that acetylcholine needs to be in this axon terminal and so it would get there on these railroad tracks right that are the neurofilaments right so they would just be kind of like little threads that run all the way down the axon okay all right so that's the soma all right so then if we leave the soma all right information leaving the soma would next reach the axon hillock all
right so the axon hillock is kind of a triangular region here right just at the base of the neuron okay here i'll actually kind of just draw that all right so that is the axon hillock and we're going to talk about when we go through neuron physiology which is how an action potential is kind of received by the dendrites of the neuron and then sent out the axon of the neuron we'll talk about how exactly that axon hillock functions but it's general function we'll just write now is that it determin
es right if an action potential will be sent all right potential will be sent so it's kind of like the traffic cop of the neuron right so it decides yes you can go no you can't go for action potentials all right so will it be sent or won't it that's the function of the axon hillock and again we'll talk about how it does that a little later right now we're just mainly going over the structure of the neuron okay so then we have the axon right so from the axon hillock the axon extends all right so
i'm not going to point to it because it's this entire structure here all right we need to label some other smaller things within it but that entire extension from the soma is the axon all right and so it's that yellow all right line running through here all of that is axon okay and so we learned in the last video that the axon is often covered by these little blue beads all right that is the myelin sheath okay so these this is myelin sheath all right here all these little blue guys okay so this
looks like it would be a neuron that would be found in the peripheral nervous system because each of those little myelin beads is separate from each other so those would be made by schwann cells and so let's talk about myelin in a little more detail okay so if we zoom in on one and kind of cut it in cross-section like this to look at it this specifically is myelin made by a schwann cell okay so i'll just make a note of that up here this is a schwann cell and i know that because i can see the nuc
leus here right so i know this is just one cell its nucleus is there as part of it okay and so what this cell does all right the schwann cell to make myelin sheath is it will take its cytoplasm right it'll extend it out so if we have all right like a little axon here we're looking at in cross-section so we're looking at it straight on all right this schwann cell will take its plasma membrane all right and it will send it out and it will start wrapping it around that axon and it'll just wrap it o
ver and over and over like a jelly roll all right until it doesn't have any left okay and so then you end up with a little bubble kind of on top like this with that nucleus in it okay so we would have axon here in the middle all right and then the swan schwann cell is wrapped around it over and over and so you can see that here the yellow axon in the middle and then the many many layers of plasma membrane of that schwann cell and then the outermost layer has the nucleus and most of the cytoplasm
in it okay that's how myelin is formed all right so if we're talking about oligodendrocytes it would be exactly the same except for this outer layer with the nucleus and you know most of the cytoplasm it wouldn't have that all right and so by doing this adding these many many layers of plasma membrane right so it's really just layers of plasma membrane it's gonna insulate the axon okay and so think of like uh you know with your wiring in your house right those wires on the inside have that kind
of bare copper wire and then they're surrounded by that kind of plastic maybe latex you know i don't know what it's made of the insulation right it's usually either blue or white or something like that or black um so that's kind of what this myelin is so it's the insulation around the axon and so if these axons were bare all right without any myelin those electrical signals can easily kind of you know dissipate out and remember they're bundled together in nerves and tracts so they could be touc
hing each other they could short-circuit each other so it helps a lot with that all right and then it also helps with what we'll talk about in a minute speeding up the action potential all right so it insulates it and speeds up the action potential all right we're going to abbreviate it ap for action potential okay and then the last thing to point out with this myelin sheath is that outer layer of the myelin that contains the nucleus and a large you know kind of bit of cytoplasm so you can see k
ind of has this thicker layer here on the outside of cytoplasm and then we have the nucleus you know in here that is called the neurolima okay so the neurolemma is the outer layer of the myelin that has the nucleus and most of the cytoplasm okay all right and so when you have a myelinated axon like we're looking at here we have our little beads of myelin in between those beads of myelin we have these little gaps all right so there are little gaps in between and those are called nodes of ranvier
all right discovered by a nice french guy all right so nodes of ranvier all right so that's there we have like all these little gaps right it's the gaps between the mile and beats okay and so the way that an action potential would move through and we'll look at this with our physiology right exactly what's going on but basically it jumps right from one node to the next node and so that's why it can move so much faster instead of let's say running right step by step all the way through now i can
just hop right from one to the other and so it's much faster all right so the action potential this is called saltatory conduction right saltatory referring to jumping all right so the action potential jumps all right we'll kind of put that in parentheses we'll we'll go into that more in our physiology part right so it jumps from one node right of ranvier to the next okay so for now just remember saltatory means it's jumping from node to node and then we'll talk about how that happens all right
so then the action potential will travel all the way down the axon until we reach the end where we have these little axon terminals which you guys are familiar with right from our neuromuscular junction so they would form that little bulb right that sits on top of the muscle cell okay so this is where neurotransmitters are going to be released okay so neurotransmitters are released here okay all right so that is the general structure kind of the anatomy of a single neuron all right so let's go t
o a few more topics relating to kind of the structure of neurons all right so with neurons we can classify them and based on their kind of arrangement of axons dendrites and cell body so we can have first of all a multipolar neuron that's what we've been looking at all right and so this is just the names come from how many branches are coming off of the soma all right so if you have a bunch we call it a multi-polar neuron all right so this is going to be many branches okay and so you can see com
ing off of it there's a bunch of dendrites all right so these are all branches coming off and you have the axon coming off so that's our mini branches for a multipolar neuron right we see this most often with our motor neurons all right this is actually the most common type of neuron based on its shape right we would see many many multipolar neurons okay and then we have our bipolar neurons all right so bipolar meaning there's two branches okay so we would have one branch here and here so we wou
ld have our dendrites all right coming in this direction information reaches the soma and then it's sent out this direction through the axon all right that's our bipolar neuron we'll see this in some of our sensory systems right so in related to smell and vision we'll see some bipolar neurons when we talk about those all right and then lastly we have a unipolar neuron so we have our soma here there's just one branch coming off of it okay and so with this you would have your dendrites right comin
g in that information would go up to the soma right it would kind of process it send it back down and that would go out through the axon okay not as clear a division between axon and dendrite right but one side would be the dendrites the other side would be the axon okay we'll see these are very common with so this means we have one branch right unite means one branch and we see these with sensory neurons all right uh and so that would be relating to you know like touch and that sort of thing ou
t in the peripheral nervous system so those neurons are going to be unipolar neurons and when we talk about the spinal cord we'll see those okay so those are our neurons kind of classified based on their shape all right so now we're going to start touching on a little bit of physiology so we're going to talk about resting membrane potential and action potentials all right and then in the next video we'll go through how a neuron actually takes on that action potential potential and sends it down
its axon okay so let's just talk in general about action potentials first though okay so to understand action potentials and we briefly talked about them uh in our muscle physiology chapter we first have to really understand resting membrane potential okay and so resting membrane potential right remember the membrane is always slightly polarized all right meaning that the charge on the inside of the cell versus the outside of the cell is not the same okay so if we have so intracellular fluid so
inside of the cell here extracellular fluid that's the outside of the cell here all right the inside of the cell is going to be slightly negative relative to the outside of the cell so it's going to have a slight negative charge all right like that okay and so typically resting membrane potential right this negative is about minus 70 millivolts all right that's a typical for a neuron a typical resting membrane potential okay so it's 70 millivolts more negative on the inside than it is on the out
side okay so let's talk about why it is negative on the inside versus the outside okay and so this first has a little something to do with our ion concentrations all right so this is just a reminder kind of a recap right remember sodium is going to be in really high concentration outside of that cell right outside of the plasma membrane and potassium is going to be in really high concentration on the inside of that cell and that is from that sodium potassium pump okay so we're going to have we'l
l put our sodium all right so we'll do uh we could just put a bunch of little sodiums out here all right we'll put a bunch of potassiums out here or in here since we're inside the cell okay so that's one thing all right so that would make sense why if you have these positive charges on the outside this outside would be positive however you're probably looking at this and saying well there's you know positive charges from potassium on the inside too so wouldn't that make the inside also positive
which you're correct but there's other things that are making the inside negative okay so this difference in charge across the membrane membrane the inside being negative compared to the outside all right let's talk about why that's true okay so one thing contributing to it is the fact that there are negative proteins that kind of just hang out we'll just draw them as like little blobs right just on the inside right of the membrane like this all right so these proteins have little negative charg
es right they're negatively little negative proteins okay and they hang out just inside of the membrane okay so that's going to help to make that membrane more negative on the inside so that's one thing okay and then there are also these little leak channels is what they're called right so i'm gonna draw one in here okay hold on there we go all right so these are little leak channels and so they allow for the slow movement of potassium all right out of the cell okay so these leak channels are mo
stly for potassium all right and so they allow for a constant because they're always open very slow leak of potassium okay and remember you have that sodium potassium pump working constantly to always put potassium back so that's why it stays high on the inside but there is always this kind of little slow leak of potassium going out so you have this very slow kind of trickle of positive charges constantly leaving the cell that will also make the inside of the cell membrane more negative okay all
right so let's see so positive charges leaving all right that's gonna make uh the inside whoops running out of room all right negative okay so those positive charges of the potassium leaving are going to make the inside negative that paired with our negative proteins right or why the inside is negative relative to the outside okay and so what we say is that these neurons have a an gradient all right and so we know that a chemical gradient right just means that something is going from high to lo
w concentration all right so chemical gradient is high to low concentration okay so let's talk let's kind of think about this in terms of sodium right so with sodium it has an electrochemical gradient with this neuron all right so we have a high concentration of sodium on the outside right meaning it wants to go from this high concentration into the cell where there's less of it okay that's the chemical part of the gradient it also has an electrical gradient right and so with electrical gradient
s positive charge wants to move towards a negative charge all right actually let's just say is attracted to negative all right so that's the electrical part so okay not only does our sodium here want to go from high to low concentration but also has a positive charge and that positive charge wants to move towards that negative charge so it has an electrochemical gradient right so that's a really strong force telling sodium it wants to get into the cell okay okay so it's very important to underst
and for action potentials all right so one why is the resting membrane potential negative all right meaning the inside is more negative than the outside so one reason is that there's negatively charged proteins near the inside of the membrane the second reason is that there are those potassium leak channels that slowly let positive charges out of the cell okay and that's because potassium wants to move from high concentration towards low concentration okay so that's going to make the inside nega
tive all right and those create the charge right that are part of that electrochemical gradient all right now let's talk about action potentials okay so we went through this briefly um with our muscle physiology all right so we're going to give it much more detail now okay so the basics of the action potential that we talked about before is that what you have is you have this increase in voltage of the membrane all right so if here right we're at resting membrane potential here so this would be
minus 70. all right millivolts okay this uh membrane right would become less and less negative right so the charge would increase that is depolarization okay and so um it can get up to some positive voltages um let's just say maybe like plus 20. all right it depends on the neuron so maybe we'll just go up here all right this would be plus 20 okay and so this side here where it's increasing right is d polarization all right we're just going to look at this graph first and kind of the steps and th
en we'll talk about what's happening all right after depolarization that is always followed with repolarization so we bring back that membrane potential back down towards resting okay so you depolarize the cell that's you know the action potential right to a positive charge and then you repolarize it back down to normal and typically with most neurons there is a point where you dip below the resting membrane potential that is called hyper polarization all right hyperpolarization just means we've
gone below right more negative um resting than this resting membrane potential all right so below that minus 70 that's hyper polarization okay so right now we are just focusing on this part of the graph we'll talk about the other part in a little bit okay so we know we depolarize the cell it repolarizes and often it hyperpolarizes now let's talk about what's happening with the actual ions during those steps okay and so our neuron membrane has uh and we talked about these with our muscle right t
hey have voltage gated that's the v so v is for voltage voltage gated sodium channels okay so what happens is this membrane potential starts to slowly creep up right we're going to talk about this part this creeping up part in a little bit how that happens but it slowly starts to increase until it reaches threshold okay so this dotted line here this is threshold okay and so threshold is the voltage at which our sodium channels open okay so they open typically at about minus 55 millivolts okay so
we can label that on here so minus 55. all right and so that is called threshold okay so the point at which those sodium channels open is thresholds so we have a bunch of sodium right on the outside of our cell the cell is slowly creeping up it reaches threshold and what's going to happen is this little kind of door right this little hatch on our voltage-gated sodium channel is going to open all right at threshold and that means that sodium can now rush into the cell and that's exactly what it'
s going to do right down that electrochemical gradient okay so now we have a bunch of positive charges all right entering the cell and so and it's very very fast it's a lot of sodium very very quickly alright and so that is going to bring the charge of the membrane up all right and it brings it up all the way until it becomes positive right that's depolarization so now we have positive right on the inside negative on the outside okay so this part we just talked about is depolarization all right
so sodium rushing into the neuron all right causes depolarization and we can kind of add this here all right so sodium in is responsible for this part of our graph okay so now we know that it needs to repolarize so we need to get that membrane charge back down so the inside is positive now all right and so what happens is that at those positive charges right the positive potential all right these voltage-gated potassium channels are going to open all right so opens at the positive potential so t
hink about this as it opens at the peak of depolarization right right so the top of our graph there these potassium channels are going to open and so then all right that means we have this little little gate opening here and we have a bunch of potassium remember on the inside of the cell okay and so once that gate opens all of these little potassiums are going to rush out they're moving from high to low concentration and now they're also moving across that electrical gradient because the outside
is now negative so they're going to rush out right from high to low and towards that negative charge and those positive charges leaving the cell are going to flip that charge back now it's going to make the outside positive in the inside negative okay and so this step with potassium leaving is repolarization okay all right so this part of the graph here is from potassium going out of the neuron okay and so we see hyperpolarization simply because these potassium channels stay open for a little w
hile and so more potassium moves out than is needed to get to resting membrane potential so it kind of overshoots right the potassium leaving is a little over enthusiastic so we get this little kind of dip here right from that all right that's all hyperpolarization is okay the potassium leaving is a little too effective and it hyperpolarizes the cell okay so this is an action potential right and how it works so just to review we start at resting membrane potential right that membrane slowly star
ts to creep up from about 70 to minus 55 at minus 55 our sodium channels open okay when they open sodium brushes in that depolarizes the neuron all right at the peak of that action potential is where our potassium channels open all right and that is going to cause potassium to rush out repolarizing our neuron all right that repolarization will overshoot typically making it hyper polarized and then it kind of levels out with the sodium potassium pump putting everything kind of back to normal and
we come back to resting membrane potential okay so that's an action potential all right so now let's talk about these other two little kind of blips right on this graph okay and so they're extremely important even though they're you know just tiny little blips on the graph right they're important to understand and so these are called graded potentials and so graded potentials are just small changes in membrane potential all right so small changes in the charge of the membrane right that's what w
e mean by membrane potential right so small changes in the charge of the membrane okay so something to remember with these graded potentials is that they can be added together okay okay so they can be added together they can also go in either direction which we see here right they can make it more positive or they can make it more negative okay so we have two types right the positive or the negative type so the positive type they're called excitatory postsynaptic potential so excitatory just mea
ns that it is all right so this word is just means it's coming closer to threshold okay and the post-synaptic part we're going to talk about a little later that will make a little more sense right but it's excitatory meaning it's bringing it closer to threshold all right so it's a small positive change in the membrane potential okay it brings it closer to threshold but it doesn't reach threshold and so we can see that here all right so that is a little epsp excitatory post-synaptic potential oka
y it brings it more positive closer to threshold but it doesn't bring it one epsp will not reach threshold okay you can also have inhibitory postsynaptic potentials all right so inhibitory just meaning this taking it further whoops further from threshold all right so this is going away from threshold okay so these are going to be small negative changes in the membrane potential okay away from threshold all right so that would be here okay so negative change in the membrane potential this would b
e an i p s p okay and so remember these can be added together and so this will be really important when in the next video we're talking about how neurons receive information all right and that can lead to an action potential because what happens is you can add together a bunch of those graded potentials and that is what causes this slow increase all right so one epsp will not bring you to threshold but if you have a bunch together all right all added together that will slowly creep you up to thr
eshold okay so one cannot but multiple can all right and then they kind of are it's a balance between the epsps and the ipsps so you know the information right is all kind of added together and if you have more epsps that bring you closer to threshold you might get an action potential all right so we're going to talk about that in the next video when we look at neuron physiology alright so the last topic for this video is refractory period so we talked about refractory period with our muscles al
l right so this is just the time where another action potential cannot be sent all right and so um we have this with neurons just like we have it with muscle cells okay and so with the refractory period it can be absolute right so absolute means there's no action potential that can possibly be sent this is only during depolarization all right so very very short amount of time all right so during depolarization um because you can send a lot of action potentials very quickly back to back right but
you cannot send it during uh the time that the membrane is already depolarizing so that's the short absolute refractory period there is a relative refractory period during which you can send an action potential but the stimulus has to be strong enough for you to be able to send it okay so it's more difficult during the relative refractory period but you can send another action potential it's possible all right okay good so we're going to leave it there and in the next video we'll talk about neu
ron physiology all right so how do neurons receive information kind of integrate it all and then decide if they're going to send an action potential or not all right and then how that action potential travels and reaches those axon terminals
Comments