welcome to the biological basis of behavior series of videos we will start with chapter 2 structure and function of neurons and for this chapter I have five separate videos that I will present to you now I'm going to present these as five separate videos with questions embedded so that it's sort of an interactive process these questions when you answer them will send an email to me telling me how you're doing so you will get five separate videos if if that's what I end up using it for or one single long video that I will post on YouTube and connect to the learning management system this long video has the benefit of allowing you to simply click on speed and increase the speed by two times or one and a half time listening to the lecture pretty fast and getting the same amount of information but in a shorter period of time if you're for example reviewing the length lecture I like that capability because I've discovered that it is much easier to listen to long lectures on topics that I might have been exposed to at a faster rate stopping what I need to and then running it faster getting everything in half at that time you can do that on the full length video that will include the five components and for the five separate videos that one will be interactive and but unfortunately it won't allow you to speed up the video that being said let me tell you about five topics we're gonna cover in these five videos the first one is the structure and function of neurons and glial cells the second one is on membrane potentials diffusion and electrostatic force the third one is on action potential and rate law the fourth one is on neurotransmission and communication between neurons and the fifth one is about postsynaptic potentials the objective overall objective for this chapter is for you to be able to identify the parts of the neurons and the overall process of communication within neurons and between neurons so they'll act of action potential excitatory postsynaptic potential inhibitory postsynaptic potential how these aspects of communication within neuron happen and then how two neurons communicate with each other through the process of neurotransmission I would like to I'd like you to by the end of this class to be able to describe the measurement of action potential and explain the dynamic equilibrium of ions that's responsible they're responsible for membrane potential I'd like to I'd like you to also be able to describe the role of the ion channels in action potentials specifically the sodium and potassium ions and explain the all-or-none law and rate law of action potentials I'd like to have you be able to describe the structure of a synapse and the release of neurotransmitters and the activation of postsynaptic potentials and how that happens finally by the end of the class you'll be able to describe postsynaptic potentials in terms of ionic movement that caused them the process that terminates them either through reuptake or enzymatic breakdown and the integration of postsynaptic potential as the process of computational neural computation with that being said let's start with our first lecture which is the structure and function of neurons and glia start out with the most common way of drawing a neuron is as follows and let me use my pen for a little while as you can see the the neuron is the entry point for a neuron or in a way the ears the structure that is listening out for input from other neurons is the dendrite and there are hundreds of thousands of dendrites are on any single neuron these dendrites are in a structure sometimes called an arboreal structure like a tree so there is something called dendritic arborization as in dendrites creating a tree structure these are typically in neurons called inter neurons and we're going to talk about the different types of neurons so dendrites in a nutshell are basically the listening device of the neuron they are the input area for the neuron they are connected to the soma the body of a neuron the soma is where the actual computational process and the decision-making around whether to fire or not to fire an action potential takes place in the soma also there are quite a few structures that are very important for the molecular mechanism of whether it's ion exchange whether it's creation of receptors whether it is the creation of neurotransmitters and then that transport of those neurotransmitters down the axon to the terminal buttons practically every aspect of neuronal information and is is in the soma so obviously in the soma you will find the nucleus the nucleus is where the chromosome exists and this is where protein production to allow for every single function of neuron takes place so the soma as as you can tell has a huge set of functions but in terms of signal transmission the soma acts as the place where the integration signals whether the inhibitory or excitatory and whether they're modulatory this is where a lot of the integration of information takes place at at a very specific area the junction between the soma and the axon an area called the axon hillock have to excuse my mouse writing and the axon hillock is where the actual final common pathway and final decision is made to trigger an action potential or not and when you do trigger an action potential the action potential goes down the axon the axon is the structure that transmits the signal down to the terminal bucket and if the dendrites are the ears the input of a neuron terminal buttons are the mouth of the neuron the actual area output area of the neuron in the terminal buttons you're gonna find the neurotransmitter vesicles that then open up and release neurotransmitter between each terminal button and dendrite is an area called the synapse the area in between the terminal button of one neuron and a dendrite of another neuron that in a nutshell is the structure of in your own simplistic fashion this is the structure it's often drawn in this way with the cell body being more on the left and the neurotransmission going towards the right now in addition to this to exit that this is an image of once again this is an image of two neurons are actually three neurons communicating with each other as you can see this is the axon of a neuron synapsing on so this would be maybe neuron number one and this is neuron number two let's call this neuron number four and this neuron number three the neuron number one is actually in relation to neuron number two is the presynaptic neuron and and the presynaptic is just its before the synapse it it is the neuron that is providing the terminal buttons which will release the neurotransmitter which then will signal down to the postsynaptic neuron neuron number two however neuron number two itself is obviously presynaptic to neuron number three because it is providing the terminal buttons that then synapse on the dendrites of neuro number three so it's very important to keep in mind that we often call things presynaptic and postsynaptic and we that is a relative term because some presynaptic neurons are actually postsynaptic to others so this is a very important point to keep in mind when you hear the term presynaptic and postsynaptic another thing in this figure that is highlighted is the myelin sheath myelin sheath is a covering off the axons that's very important it has mainly two functions one is to speed out the speed speed up the transmission of signals down the axon some axons and our bodies are several feet long in fact in certain animals like a blue whale a an accent could be hundreds of feet long so unmyelinated axons tend to be slow in their transmission and therefore evolution has found ways to to create [Music] myelinated to create a mechanism that would speed up neural transmission and this is the process of myelination and we're going to talk about the details of myelination in a little while to turn that the pen off every now and then to move on so now there are several types of neurons inside the nervous system and I'm gonna mention only a few right now and as we progress in our class we're gonna learn about quite a few but the broad general types of neurons right now are multipolar neurons and these are the most common neurons excuse me multipolar neurons are the most common neurons in the central nervous system these are neurons that are extremely complex they have one axon but many many dendrites and the axon is also very complex it it sends many terminal buttons to the that multiple neurons tend to have extreme arborization making making it a very important neuron in terms of receiving huge amounts of information consolidating it making a decision whether to fire or not and then sending information to hundreds of thousands of other neurons one thing about multiple in neurons as I said is that they are the most common neurons in the brain and they're mostly in the brain inside the brain on the other hand bipolar neurons and unipolar neurons are neurons that tend to be outside the brain outside the central nervous system - in the peripheral nervous system and we're going to differentiate some of those those tend to be sensory and motor neurons and there are relatively simple neurons in the sense that they often have a section that's basically the dendrite a single dendrite and here it's shown as an axon and that's wrong from your book a tree is this is still the dendrite but it is acting like a single pole here and the axon is on the other side of the cell body and it tends to be very simple in the sense that it's a one-to-one structure so you have a you typically have a receptor receptor might respond to physical stimuli might respond to photons might respond to sound or movement of the cilia that is the result of a mechanical effect of sound on on the fluid inside the ear all of these sensory all of that sensor information then triggers a postsynaptic potential in the dendrite and notice the word postsynaptic so it is it is the postsynaptic dendrite and that then gets sent down there is not much of a decision making there is potentially some modulation that might say look don't don't trigger an action potential and but there isn't much of a and integration of information from multiple sources let's take an example of this imagine you're sitting here and you flick a hair on on your skin just touching it very lightly that triggered an action potential that then reached the somatosensory cortex in your brain this process actually was triggered by a receptor that was connected to potential - most likely a a bipolar or unit unipolar neuron in which the decision was very simple that you know trigger or not trigger based on a single touch however obviously your your skin is not continuously feeling the stuff on it and so it has to be inhibited some way and so there is some modulation of these neurons but the majority of the function is basically to trigger or not trigger so it's relatively simple so the the fact that this neuron has two poles makes it look like a bipolar bipolar antenna bipolar neuron that that's what it's called a bipolar neuron a unipolar neuron sir tends to be the same structure with dendrites connected to the axon directly with the cell body the soma being on the periphery not even part of the connection so as you can tell the decision-making is very limited here it's just you know you trigger a sensory activity and it results in a release of neurotransmitter here that then connects to another neuron that reaches the somatosensory cortex so this in a nutshell these are in a nutshell the three types of neurons we're interested in and keep those in mind because I might be asking you questions about that at some point this is actually a picture of a fast spiking inter neuron and you can see how our bure eyes it is these are all dendritic spines of the neuron thousands and thousands of dendritic spines on these dendrites this is the cell body and as I say here this is a part of this striatum the basal ganglia and it suffice fast-spiking inter neuron again it shows incredibly dense local acts on collateral fields so in addition to extreme arborization the axons themselves and split into these terminal buttons and and and these arbors that are very dense allowing you to see the potential complexity of the process of decision making it has to be this neuron has to receive as is likely to receive hundreds of thousands of messages and send out hundreds of thousands of messages that are the result of a single action potential that gives you a good example of what happens inside a a an interneuron and you can think of of the fact that there are millions of billions of these neurons inside the brain all interconnected in complex ways talking to each other reflecting reality that's the outside world and then making decisions about movement within the outside world world now let's talk about glial cells so we've covered the neurons and basically we've covered the three structures and we're interested in just for the sake of this chapter the bipolar unipolar and inter neurons we've covered the basic structure of the neuron itself the components that then write the soma acts on the terminal buttons now let's talk about Liam and I'm gonna focus on four types of glia the first type is a strobe Liam and or astrocytes actually I apologize the term as astrocytes and astrocytes are a type of glia that have mainly four functions the one of the more important functions of astrocytes is that they connect to the capillaries in the brain the blood vessels in the brain in order to extract new nutrition the fact that neurons are actually very sensitive to blood so any seepage of blood into the the actual matter of the brain that what we call the parenchyma any seepage of blood into areas where neurons exist will kill the neurons blood is neuro toxic that fact has resulted in an involution mechanism that protects it protects the brain from direct contact with blood unlike your muscle or your skin if you prick your yourself you see blood seeping that's because actually blood is being exchanged between the vascular system the capillaries the blood vessels in your body and the muscles and the skin and this exchange is usually through junctions just like this that are much bigger and open they are open junctions that allow for exchange of blood blood is then blood exchanges and in fact affects those areas directly in the brain these junctions are our clothes they're called tight junctions there there are a special type of junctions where they are actually tight there they don't allow for exchange of their blood what happens at these tight junctions is that the astrocyte so acts like a little sucker connecting to the tight junctions and taking the nutritional material and then transferring it in fact going through a cycle a metabolic cycle transferring it transferring what could be used by neurons to neurons so then the one of the main functions of astrocyte is that they are part of what we call the blood-brain barrier so these tight junctions and astrocyte this these these this composes the blood-brain barrier that's number one number two the astrocytes provide support for the whole that for neurons so in a way this process of holding two neurons to provide the nutrition and to provide other chemicals that allow for the functioning of neurons also provides a supportive mechanism to keep neurons in place so that's the second function the third function is that astrocytes remove debris from the area around so let's say there is an inner on dyes for for whatever reason axons break off and astrocytes then get rid of the debris from the area so we've talked about several of these Astro CITIC functions and I would like to just highlight where they are in your book the Astro site description is on page 36 we've talked about the physical support the production of certain chemicals that help neurons for example astrocyte take take back they are part of the reuptake mechanism of glutamate as it breaks down so glutamate is one of the major neurotransmitters in your brain astrocytes pulled back the glutamine recycle it and create the neurotransmitter again to be sent to to the neurons and so they predict produce some chemicals that neurons need to fulfill their function and they are also involved in the transmission of nourishment so these are the major functions of astrocytes they're an extremely important glial cell in the brain another very important and also extremely famous glial cell is the only good on dendera site so the early good under site is a real cell that inside the brain and inside actually the central nervous system acts as the source of the myelin that we talked about a few minutes ago or in the previous video and it's basically a the oligodendrocytes has processes these are the processes that wrap around axons and what you have to notice here is the fact that the different that a single oligodendrocyte is wrapping around in this case just three acts and this is just an example it could be hundreds a single oligodendrocyte actually wraps around several axons and provide these this myelin sheath provide creates the myelin sheath around axons and one thing to notice also that the myelin sheath is not just a sheath that just a continuous sheath in fact in between the different myelin wrappings is there are areas like this called nodes off from the air and we're gonna focus on them later on in this lecture nodes are from being our nodes that allow for the regeneration of signals down an axon so once again a single neuron might actually be a single axon excuse me might actually be wrapped up by several or multiple oligodendrocyte and a single oligodendrocyte is probably is likely to also wrap around multiple axons so it's a very complex system between oligodendrocytes and axons of neurons now that being said the oligodendrocyte is actually located inside the brain and the spinal cord on the other hand in the peripheral nervous system what you find is another type of cell that acts like an oligodendrocyte this cell is called one cell so um this figure from your book shows you in oligodendrocytes and shows you how it wraps around a single multiple axons from different neurons and that a single Oregon under sight might wrap around multiple axons and a single axon will have multiple Oregon under sites now in the peripheral nervous system it's a much simpler process but much simpler system it is basically a single Schwann cell wrapping around a single axon so it's a one-to-one relationship this is what happens in the peripheral nervous system and it has a very interesting clinical implication the clinical implication is that inside the brain recovery of axons if there is axonal shearing or some sort of injury to an axon the axon axons could technically regenerate and try to find the target they initially initially were connected to inside the brain that's a very difficult process since there are multiple oligodendrocytes that an axon needs to navigate so if there is shearing of axons and you have these all of these oligodendrocytes still alive and well and an axon has to find its way back to its target it won't necessarily know which which which alligator which wrapping to go under there is really in a way no scaffold to allow it to reach its target in an easy direct way now there is recovery in axonal injury there's recovery in the myelin there is recovery in the axonal injury but it is very very difficult in the brain to sort of recreate what you initially have however nerves in your body we call you know we call neurons in the brain and and bunch of axons in the brain either tracts or fascicular or comma sure's however are in the peripheral nervous system we call them nerves so nerves are only in your body neurons and tracks and Fisichella etc are in your brain so in inside the body nerves actually have a very nice scaffold ready for for use if there's an injury so if an accident gets injured wants to find its way back home it can use the Schwann cell to help it go back this is why we find that nerve regeneration and nerve recovery is much easier than neuronal recovery in the brain with that being said that's one of the clinical implications of having Schwann cells are supposed to myelin sheath the result of oligodendrocytes now interesting another interesting clinical fact that you should be aware of is there are two diseases famous for impacting the myelin sheath one disease is specific to the central nervous system and it's as you probably can guess multiple sclerosis and that if affects the myelination inside the central nervous system on the other hand because of the different type of cell that that creates the myelin in the peripheral nervous system another disease it's an autoimmune disease affects the p NS the peripheral nervous system that disease you some of you might know if I give you a hint swine flu and the inoculation of swine flu it was discovered that when people were inoculated against the swine flu in many many years ago in the 50s people there was a little spike of what is called the disease called guillain-barre a and that is a disease that Allah that makes people become paralyzed it actually is also one of the rare effects of Zika Zika could cause be embryo so Geum Bray is the result of a reaction in the peripheral nervous system and demyelination and the peripheral nervous system while multiple sclerosis is in in the central nervous system and the reason they're different is because we're talking about different cells that will have different proteins that the immune system will attack in different ways so that's another implication of after having these two different types of neurons so we've talked about now we've talked about three types of cells right so we've talked about astrocytes oligodendrocytes and schwann cells and unfortunately I don't have a picture for the fourth fourth type of so but in reality it's a very simple very interesting cell called the microglia so microglia are are obviously the smallest layer because they're micro and they perform the function of other cells that we have in our body called phagocytes phagocytes come the word comes from Fei Jia which is swallowing and phagocytes and the word sight which is cell so these cells actually eat up other cells invading cells like bacteria or viruses these cells when they encounter an invading body will actually just wrap around that sort of and swallow it and digest it this process is called phagocytosis and this process is is done by a specific cell inside the brain called microglia okay so these are the four types of cell that cells the glial cells that I would like you to be aware of and I want I want you to know their functions let's move on and talk a little bit about here here by the way and on your slides that I've put in different places you're gonna find a lot of definitions I've removed some of these definitions but just to remind you know Joe from the air are these naked portions that are part of the myelinated axons between adjacent on the oligodendrocytes our senses may already oligodendrocytes and schwann cells microglia are let's smallest glial cell and they act as phagocytes and schwann cells I said are in the peripheral nervous system the astrocytes create the blood-brain barrier which is a semi permeable barrier in the brain that prevents the blood from seeping into the parenchyma there is an area a very interesting area inside the brain called area post armor and area postera excuse me it just jumped area post armor is a region of the medulla where the blood-brain barrier is weak and it is it evolved in order to allow in order to allow organisms to detect potential poisons in the blood pretty quickly and to initiate the process of vomiting right away you don't want to totally not be able to detect certain chemicals inside your bloodstream that could cause you to to die and therefore evolution has selected for organisms that have the ability to detect that and that's the area of posture of the brain now that we've covered this section of my lecture let's move to the next section we've covered up to now the basic structure of neurons we've covered big liam so we've covered the two videos I talked about now we're gonna cover membrane potentials diffusion and electrostatic forces this is a relatively intense description of what goes on inside the neuron to create what we call the membrane potential so bear with me and you might not want to go at two times the speed in the beginning because this is gonna be a little bit heavy let me first talk about what a potential is if you think back to your old physics classes you've got two types of energy potential and kinetic energy potential energy is the energy that is stored somewhere because of a differential kinetic energy is the energy that is taking place that is active now so if I hold my pen like this there is it has potential energy because it's above a certain point it's being attracted by gravitational forces towards the center of the earth if I actually just let it go it will fold down towards the center of the earth because it has that potential energy something will stop it obviously the earth will stop it and it can go all the way to the center because that energy that allows it to then dig into the center is it needs much more energy but it will fall towards the center of the earth if I leave it and obviously if I hold it like this I can't hold it more than an hour because I'm actually expending energy to hold it in this position this process of expansion of energy is what creates potential energy in it its natural state is to actually fall this this process also is mimicked in many things artificial as well as natural items a battery for example is in in a in a nutshell a process that holds ions in different positions against each other with a membrane in between them and those ions really want to move in the opposite direction but they're not allowed because of the membrane and therefore if you give them another route and that's when you create a circuit that route allows them to then jump from the negative side the positive side next side has more electrons that negative will you know allow the electrons then to fly towards the positive side and because there is the potential energy and you just have to open the route for it and it will find its way just like if I'm holding this here if I just let it go it will fall it has potential energy well inside your body there is potential energy and the potential energy is in the form of electrostatic and and diffusion well it's the result of electrostatic and diffusion forces so let's talk a little bit about the actual state the natural state of the membrane and its potential neurons are have have a negative charge and competin a more negative charge inside compared to the outside this is the result of a fantastic balance across a membrane so this is the neural membrane the neural membrane is made out of a by lipid layer that is semipermeable so things are gonna go in and out if you let them just stick around so this is a semipermeable membrane in a way it's it's not open but it has channels on it that open up and close but let's leave the channels out of the picture for now let's focus more on what happens inside the cell that causes it to have a potential well first of all the cell itself has organic compounds inside of it these organic compounds are large compounds and they tend to ionize with a negative charge that is inside a negative charges are cold and ions said the two terms you might want to remember our cations and anions cations I I remember it I think the book suggests this if you think about the you know cartoonish I it's a I that's how you remember cations are positive and ions are negative now uh these organic anions are so large that they cannot escape the inside of the cell making the cell more negative because they are negative and similarly there are chloride ions inside the cell and they tend to also be negative adding to the negativity of the inside however that's also balanced by the fact that there are potassium ions inside the cell and these ions are just more concentrated inside so in this image bigger boxes boxes like this indicate that there is more concentration of potassium on the inside compared to the outside and smaller obviously smaller is the opposite also pay close attention to the arrows the arrows mean something so let's talk so you've seen now potassium chloride is more concentrated on the outside than it is on the inside and sodium is concentrated on the outside compared to the inside well as I said this is a semipermeable membrane and there are two pressures on these ions one pressure is the force of diffusion the force of diffusion you've experienced on a daily basis it's the force that a porous vacuum man in a nutshell it's not true but this is how I would like you to remember it remember if you put a single drop of fragrant fragrance in a room what will happen it'll within minutes diffuse all over Rome and you can smell it at the extreme end why is there such a pressure for molecules to diffuse because because of the fact that molecules want to be equally distributed across anything that is open obviously if you put a membrane if there's a glass a glass door between one room and another the the molecules won't go into the other room except if you open the door then they will rush out and you can smell it in the other room another example of the force of diffusion is when you drop a little bit of sugar and your tea even without stirring it the sugar will diffuse equally in a cup of tea and within minutes you probably can taste the sugar everywhere you stir it because that makes it faster and diffuses faster it allows for more interactivity with more molecules making it spread equally faster so this is the force of diffusion and the force of diffusion has is dependent on a gradient of concentration when there is more of a an IR on one side the force is gonna for the forces gonna is gonna push the ion to the inside when there is higher on the inside it's gonna push it to the outside so potassium wants to go to the outside chloride and sodium ions want to go to the inside there's another force that balances this at a certain extent which is the electrostatic force or electrostatic pressure the electrostatic pressure is the result of a fundamental principle in electricity and magnetism that positive similar charges tend to repel and opposite charges tend to attract positive and positive repel negative and negative repel positive and negative attract so there is always an electrostatic pressure for different ions to either stay in this case for example Dasia ms is positive and the outside is positive so that electrostatic pressure actually is to send it in and you can see the electrostatic pressure here is represented as a as a an arrow to the inside while the diffusion force because of the gradient of concentration is gonna force the potassium to go to the outside the opposite is true about chloride chloride is less concentrated on the inside so the the force of diffusion is pushing it to the inside while the electrostatic pressure is pushing it to the outside because of the fact that there is positive on the outside and it's negative so by by the force of attraction of positive to negative it would want to stay outside or at least or even leave the cell to go outside because there is also repulsion between negative and negative what is really interesting is you what you're gonna observe here is sodium has two pressures against it so sodium here is actually wants to go in into the cell because of the force of diffusion and also wants to go into the cell because of electrostatic pressure so both forces are working on so now if you leave the system alone and because of the semi permeability of this membrane these ions will then balance across the membrane and you're not gonna have a charge very soon this battery will be depleted if you think of it as a battery or a capacitor if you want it will be depleted because things will move naturally just like if you leave a battery for a long time it actually will just die because even with the best membrane if there is any sort of permeability it's gonna cross and it's gonna it's gonna then balance out these ions will then cancel each other out and the charge the potential will go away now what keeps the potential inside them or inside the neuron and it's a it's a really important question what keeps it is a very active process called the sodium potassium pump the sodium potassium pump is principally a pump that is continuously working to keep 3 sodium on ions outside the cell for each 2 potassium ions into the cell so it's continuously pumping sodium and potassium and this pumping is at the rate of 3 sodium to 2 potassium inside 3 sodium outside to 2 potassium inside this is an active pump it takes a lot of energy part of the metabolic energy used because you your brain actually uses 20% of the energy that your body produces although it's much smaller than 20% of your body yet its uses 20% also it has to continuously get that energy you can't just you know unlike other parts of your body like your muscles on your stomach when there is a lung in the usage of the energy it just stops but your brain is continuously using energy even when you're sleeping there is a continuous need for energy well the sodium potassium pump is one deplete or of the energy and it's it's basically an active pump which keeps sodium out of the cell and keeps toss him inside the cell in order to keep the potential going okay so fundamentally what you see here is a membrane that has a potential because of a balance that that to start out with there is slight imbalance and because of the anions and and the and the potassium and sodium pumped causing that involves creating a potential with caveat and the important point to observe that the the actual ion that really insists and has the urge to go into the cell being sodium the sodium is one of the most important oils in terms of its role in creating action potentials and graded potentials inside the brain now with that being said let's go on and add another interesting component to the membrane so the membrane obviously are as I said before they are a lipid barrier at semi permeable but there are many channels on these membranes and these channels are specialized channels they're basically proteins that change in shape if they are closed or closed or open and they allow specific ions to enter there are put potassium channels specialized potassium channels specialized sodium channels specialized chloride channels etc that beings calcium channels so these are in the big four ions now that being said channels differ some channels are activated and this is a really important point are they're they're activated by a change in the membrane potential so if the if the membrane potential here as I said usually the incident always the inside is negative except when you have some sort of action professional and the outside is positive if there is any change the membrane potential whether there's a flipping of the membrane potential making the outside negative and the inside positive or even a slight potential change some of these channels will open up they these channels are called the voltage-gated channels channels that respond to changes and the voltage of the membrane but there are other types of channels these are ligand gated meaning neurotransmitter gated and they have a receptor site where neurotransmitters would bind to and open them so you've got these two types voltage-gated and receptor or neurotransmitter gated or ligand gated the final type is actually well not final there many types but for our purposes our our third type is the voltage and ligand gated so that type of channel only opens when you have a change in voltage and you have a neurotransmitter that's trying to open the channel so you have to tap the two conditions met in order to open these channels more complex channels because you can tell so with that being said how how does all of that then result in what we call an action potential so now we move to our third hour our fourth actually I've covered structure of neurons and glia the membrane potential and now with the third action potentials and the rate law so one of the things I would like to start with is to describe what happens in the process of signaling the action potential as I said there are two types of channels voltage-gated and ligand gated so in the action potential we are going to focus specifically on the voltage-gated channels and there are two types of voltage-gated channels that we're going to focus on and these are the sodium and the potassium channels now the membrane potential as I said is negative on the inside so we call that a potential of my it's actually about minus 70 millivolts okay remember your AAA battery is 1.5 volts it could be minus or positive you know everything is actually positive or negative depending where you're measuring it from right so we're thinking it this this is a minus 70 with - being you know more inside more negativity inside or more anions inside right so that's the resting membrane minus 70 now the resting membrane thoughts it's not always my certainty but there's a specific point a specific threshold that if you hit that threshold in terms of its fluctuation now will result in a cascade of events that opens a lot of channels so I want you to imagine I want you to imagine a membrane here I'm going to draw a membrane on this side that has channels that are distant from each other and imagine that these channels are voltage-gated let's say you've and I'm simplifying everything I'm gonna talk about a single ion let's say let's talk about a bunch of sodium ions coming in resulting in a bit of depolarize depolarization meaning you know because the sodium is coming in things are going from minus 70 to like - 69 to minus 68 because you're adding positive to the inside you're actually depolarizing you you're making it move towards zero so one thing to remember is depolarization is equivalent to moving towards zero hyper-polarization is moving away from zero that those terms are all relative and just keep in mind depolarization moving towards zero meaning not losing polarization if you use that term that would help depolarizing is losing polarization hyperpolarizing is becoming more polarized hyper well here we go we've got some sodium coming and the sodium comes in okay gray sodium goes around and it dissipates it sort of hits some other ions maybe cancelling each other out they become a non charged molecule and nothing happens at this at this at this channel now imagine instead that these channels are much more concentrated they're next to each other and they're much more concentrated together now what do you think would happen if let's say sodium comes in here and our bunch of sodium come in here into this ion and this excuse me into this channel what will happen is the membrane potential next to this channel will change triggering an opening off the channel next door and this process tends to cascade because all of these channels are next to each other well there is there are areas inside the neuron specifically at the axon hillock and in the nodes of ranvier where the channels are really next to each other and any triggering that reaches a certain threshold will cascade into a process that is non stoppable it's an all-or-none you know if you trigger at a very low rate you know with with let's say five ions that might not reach the potatoe threshold it will just die but if you trigger it at the higher threshold if you reach the threshold of of the action potential or excitation threshold then you trigger an an a process a cascade that is non-stop about it just gonna happen all through all all the way down the axon this is called again the threshold of excitation and it happens because there is a concentration of a type of a type of channels in a certain area specifically the axon hillock that I highlighted in an earlier segment of this video or of the series of videos so if you look at this you get what you see is sodium channels will if when this potential with it when this threshold is reached sodium channels will just begin to open up and they will open up very yes they will open up very quickly and they will cascade into this process of opening up that's the first step it's the opening up of the sodium channels and this is number one here okay well if it will actually number zero here number one here it's the opening of the sodium channels at a certain voltage another channel that responds but responds at a different voltage opens up and that's the sodium channel excuse me that's the potassium channel as I mentioned sodium is primed to get into the cell remember the membrane potential and remember that two arrows of the sodium the two arrows indicated that the sodium is really one of my students likes this word jonesing to get into the cell he uses it we study drugs so he likes them the turn jonesing them the sodium is jonesing to go into the cell now that process then triggers a cascade of sodium channels opening up and as they open up the membrane potentials keeps increasing to the point that tre trigger openings of sodium channels now remember the balance of six you keep saying sodium potassium channels remember the balance of the potassium channels they that the two potassium potassium wanted to go out because of the force of the force of diffusion they're much more concentrated on the inside so sodium was begin to leave the cell naturally now at some point you hit a non non polarized state and then you start polarizing in the opposite direction well at a certain point sodium channels react in a funny way they say look we're done we're gonna close up and we're gonna become refractory we're not gonna respond to any impulses even if you try to trigger us to open we're not gonna respond so now they've become refractory and closed so that's the state number three where they are simply closed and refractory now at the same time potassium is still leaving the cell right so it's still leaving what is happening here well potassium is leaving and it's a positive ion it's leaving the cell therefore making the inside of the cell more and more negative it's gonna reach zero and it's just gonna keep leaving while potassium well sodium is not doing anything it's not coming in there's nothing to balance the potassium and potassium keeps leaving keeps leaving at some point potassium channels close and then sodium channels reset they say okay we're ready to respond again but still there is a little bit of a dip you see that dip at the bottom here that basically shows that there is a hyper polarization for part of a millisecond where potassium is actually there's extra potassium outside the cell which will diffuse away and what you get now is back to the membrane potential of minus 70 so this act this act takes place at as I said the area of the axon hillock and it's a process that goes down the axon this process is called the action potential and it either happens or does not happen there is no graded component here there's no gray it's it's black and white either happens it's triggered or it's not triggered and there are a few interesting tidbits here that are relevant to psychologists one of them is the fact that the threshold effect excitation like this threshold of excitation is not constant it actually could change medications could change it hormonal states could change it so certain individuals and in fact the best model of this is an animal model with mice mice have a very interesting way of having sex they exhibit lordosis which is basically the mouse will arch its back and a female Mouse will arch his back and allow the male to allow the male Mouse to penetrate her this process called lordosis happens only at a certain period of the animals monthly cycle the female animals monthly cycle during estrus during that period there is a hormonal change that affects the neurons in a specific area of the brain that makes the threshold of excitation lower and therefore more likely to be triggered by visual stimuli by smell actually I take it back by smell so this process as you can tell happens only under certain conditions in this case hormonal conditions and however the same processes could be happening in humans for example certain aspects of their physiology changes throughout their their development their physiology changes throughout their throughout the day which makes them more or less susceptible to certain medications and that's another clinical implication that is worth keeping in mind is that this threshold at excitation is dependent on factors such as hormonal conditions and other industries including medications okay that in a nutshell is what the action potential is all about now the action potential travels as I said down the axon and there is a really interesting interesting process that it travels in so because most axons as I said they are myelinated older more primitive axles are unmyelinated imagine this is an online light acts on here are the channels and the channels probably are next to each other so that triggering one will open up and allow the triggering of another because you've changed the membrane potential right so this is this is a cascade that takes place that allows the signal of sodium or the depolarization to go down all the way you know so you're moved to the terminal button so what you have here are you know the positive is turning to negative and the negative is starting to positive so let's say here where things are still in the normal state it's positive and inside is negative but here where things are opening things are actually negative on the outside and positive on the inside right and so this changes as it progresses this this will become a negative and this will become positive this will become negative negative and this will become positive beautiful cascade and sodium is just cascading down obviously potassium also is opening up so there's a lot of activity however this activity is actually a funny sort of activity because the I the channels are mechanical it's a mechanical activity it's a it's well electro electro mechanical you've got an electrical change in the membrane and then the protein will turn around protein that composed that that channel will will twist in a certain angle allowing the aisles to slip in okay now this act is mechanical to a certain extent it's triggered by electrical now that act of mechanical movement takes some time unlike just pure diffusion diffusion tends to be faster than opening channels just the act of sodium swimming down comfortably and fluid is an actually faster process than sodium being entered into the cell through opening and opening off the channels well that's how the actual process of speeding up the signal in myelinated axons takes place so here here it goes it's a bit complex so give it a let's give it a shot okay so you've got you've got the axon you've got Reedy concentrated channels here next to each other that open up at some point okay allowing sodium to come in allowing then potassium to go out changing the membrane now it's interesting is sodium and potassium will start will start just diffusing under this area of the myelin now this area of the myelin does not have channels so no channels here no channels sodium doesn't get doesn't enter here under this the myelin it just simply diffuses and you can see they've drawn the arrows thicker and then thinner because in process of diffusion you know these these ions will hit other ions and they might cancel out these ions will hit other molecules and we'll just be impeded in their progress down the axon so they they diffuse but they degrade under the myelin we call this decremental condition a conduction meaning that they degrade under the myelin however fortunately evolution obviously favored organisms in which the myelin is just at the right spacing where by the time the sodium reaches this point it will trigger another opening it's not so weak as not to trigger an opening but it's right at the action potential threshold that will actually trigger another opening of sodium channels and potassium channels and the soul this whole process of an action potential gets repeated at the node so at each node there is a process of reenergized or recreating the initial action potential that happened at the axon hillock okay so although we call it all or none it is still all all or none it's gonna either go down or not go down but it's going down at nodes of ranvier so in a way we call this saltatory in a way the signal is jumping between nodes and it's really jumping from under right so it's jumping like this you know there's degradation and it jumps to the next node and at this node of ranvier what will happen is there is a recreation of the action potential and it keeps jumping so there is an action potential that's read regenerated at the node of ranvier at every point in the process of transmission of the action potential down a myelinated axon once again the trick is that actual diffusion takes less time than opening all of these channels sequentially in that cascade in an unmerited neuron there are online leases may not invalidate unmyelinated axons there are unlike unmyelinated axons they are more primitive they evolved earlier but they still exist in our nervous system forward know another question that is of significance is well if if the signals are an hour an all-or-none meaning they are digital signals like well turn on turn off how does all of this variety of intensity gets could get coded because you know if you remember from if you're a vinyl collector or you're into analog versus digital if your hi-fi guy or girl the big thing is when you when you're looking at analog information the information is given fully in the full spectrum meaning you get the intensity from not nothing to the maximum intensity in terms of signal and that's represented let's say on a LP if you remember LPS that's represented by how deep a groove is the deeper a groove on an on an LP the maybe the higher the amplitude or the higher the frequency something about the and the depth is continuous so this is not just an a 1 a 0 but it's actually a continuous number so you can represent the whole set of potential intensities by you know picking one level the group will go at right so if it's here it might be a half the the intensity as if it goes all the way well neurons don't allow you to have that full range because they are really on/off switches right at least the part of the axon so how does intensity get represented how do you see brighter light or weaker light and the answer to this is that the action potential rate how many do you have per second or a millisecond well second is a more appropriate time frame because it's an action potential is about one millisecond how many can you have by the way certain action potentials like in the ear they require much shorter periods and that's why they actually are a different type of action depending more on tasking than sodium but that's a little digression but you know action potentials have different lengths and most typically is the one in one millisecond way bottom line you've got this is that the presentation of intensity is the result of the actual rate of action how many do you get per second and that represents intensity so we've talked about the intensity process and we've talked about these here are some just reviews a review of some of the concepts let me just mention a few so ion channels mentioned ion channels that they typically are specialized proteins that permit specific ions to enter or leave the cells and then you've got the voltage dependent and the ligand dependent we mentioned those and all or not the olenin is the principle that once an action potential is triggered in an axon its propagated without any decrement I mean there are decrements under under the myelin but as a whole it does not decrement it's an old or non process you know you've got the same voltage at the end as you get in the beginning they decrement under the myelin sheath but it's not detrimental as a whole rate law is the principle that the variation in the intensity of a stimulus or other information being transmitted is represented by variations in the rate at which axons fire basically we're talking about the frequency of firing that's what represents intensity and we talked about saltatory conduction the conduction of action potentials and the myelinated where it happens to regenerate at each node of ranvier and that's why it's called saltatory it's sort of jumping like somersaults the same route jumping from one node to another that that concept becomes it's it's a little bit wrong because it's not really jumping it's just being recreated because there's enough sodium to read trigger it at each node of ranvier so that that in a nutshell is the section on action potentials and how they work now we're gonna move into the concept of neurotransmission up to now we've talked about the structure of a neuron and structure of glee as how they work the membrane potential because that's the building block of the action potential now we're gonna talk about what happens and all of that is within a single neuron so the next thing we're gonna tackle is how do neurons communicate with each other so so in the beginning we're communicating communicating within a single neuron now we're gonna talk about communication across neurons and that process has several components so the first component that is important to this is to understand the structure of the synapse and the process of the release of neurotransmitter the activation of receptors and the process of postsynaptic potentials and how they work and then the termination of postsynaptic potential and the effect of postsynaptic potentials in terms of integration this is going to be divided into two videos if for the purposes of as I said having questions integrated into the videos but as a whole you can see the whole thing in in a bit okay so let's talk a little bit about the synapse we've talked about presynaptic and postsynaptic and I hope you remember that can somebody can you tell me what this is this is a terminal button and it's clearly than a presynaptic neuron here this is the postsynaptic neuron postsynaptic and this is presynaptic so this is a very enlarged cartoonish view of that you can see we've got the axon reaching the terminal buttons these are microtubules microtubules are the structure that allows that the movement of vesicles and other material up and down the axon there is anterograde and retrograde movement of material and this is aided by microtubule it's actually aided by two proteins called by niacin and kind niacin and the process goes up and down when these neurotransmitters that are transported from the soma to the turbo button reach the turbo button they just basically sit around waiting for an action potential and action potential reaches the terminal button and results in a cascade the cascade makes the synaptic vesicles dock on the membrane opening a pore allowing the neurotransmitters to leave so what we're going to talk about is the details of that process but just to orient you to this this is the synaptic cleft this is the area between the two sets and abscess excuse me between the synapse and the dendrite or what we call the postsynaptic density so on the dendrite there is a density area called the postsynaptic density this is the synaptic cleft these are this is the presynaptic membrane the postsynaptic membrane and what I'd like you to notice is that this is a one distance now as some some synapses are so close that that it it's an electrical synapse so there the process of nerve transmission happens directly through electrical impulses as opposed to the neurochemical process that we're going to be talking about these are the terminal buttons and the synaptic vesicles that are transferred from this so on with that introduction quick introduction to the to the overall structure of the synapse and what's around it let's move to the process of communication I'm gonna skip these and talk about what happens at the level of synapse so at an action potential that action potential reaches the membrane of the terminal button obviously the membrane will depolarize so things will flip from minus to positive and positive to negative on the outside well that ends up opening a cluster of proteins in this presynaptic membrane that excuse me that ends up opening calcium channels allowing calcium to come in this calcium the calcium man meets a cascade that makes them the vesicles that contain the neurotransmitters such as this dock unto the membrane okay and what that what the calcium is doing is it its opening it's helping the process of opening the fusion core so this these are the fusion pores these are cluster of proteins and the presynaptic membrane sitting there the these pores that when when the vesicle ducts onto those pores the fusion pores with the help of calcium open up now this process of opening up after after the docking of the synaptic vesicle and the open opening up of their synaptic pores neurotransmitter is released into the synaptic cleft and these molecules just begin to leave the synaptic cleft and travel to the postsynaptic membrane as you can see what happens to this a nap to the best to the vesicle is that it becomes part of the membrane and in fact that's not the only way that the vesicle ends up it could end up as part of the membrane or it could end up as follows you have and you have hiss and stay which is what we just talked about the process of excuse me a merge and recycle is the process we were talking about where the the the vesicle merges with the membrane the fusion pores open the membrane then just takes the actual vesicle the vesicle becomes part of the membrane and then later on buds of the membrane are are pinched and recycled into the cycle a cytoplasm creating the vessel recreating the vesicles so this is called the merge and recycle process there is the kissin stay where the the vesicle actually does fuse but then closes up again and stays around for the next action potential to allow it to open and release more of the neurotransmitter and then there is the kiss and leave where the vesicle actually releases its neurotransmitter and then leaves and gets recycled filled up with a neurotransmitter again and recycled through the process again so you've got the kiss and stay kiss and leave and merge and recycle these are all processes to maintain the vesicles and to reuse them because you don't want to be transporting a lot of this up and and down up down the axon so it's easier to keep some of this material and recreate it inside the terminal button now we've covered the process of the process of the release of the neurotransmitter now what happens on the other end on the postsynaptic potential excuse me on the postsynaptic neuron and on the postsynaptic neurons what you were gonna see are let me set the pen excuse me what you're gonna see our channels these channels are again special channels unlike the sodium and potassium these are ligand gated these are not Caselli voltage-gated some of them are voltage and Leiden gated but you know we're going to talk about ionotropic channels that are basically ligand gated this is an example of ionotropic and this channel receives a neurotransmitter at a binding site as you can see here when it see when it receives the bite of the nurse husband at the binding site it opens up and allows a specific ion to enter into the cell now this is often a misconception typically I see it in undergraduates but sometimes people forget and some people think that neurotransmitters actually enter the postsynaptic neuron and they do not they have nothing to do in terms of their entry into the postsynaptic neuron their main function is just as a key in a lock they open the door and they stay outside they get we either broken down in the synaptic cleft or taken up back into the presynaptic neuron or into an astrocyte that's another function remember so either they go into the presynaptic neuron or into an astrocyte where they get recycled so neurotransmitters are either broken down which is the rarer form the more primitive form but most neurotransmitters are recycled and they never enter the postsynaptic neuron what enter what what enters into the postsynaptic neurons are ions and the ions depend on the type of channel you're opening so different neurotransmitters will open different ion channels and change the membrane potential accordingly so let's talk about some of these receptors and their subtypes okay we've talked about the ion tropic which is the simplest type of receptor where the actual binding site is on the channel itself and opens that channel there is not much metabolic activity taking place here it's a very simple process the neurotransmitter itself changes the membrane potential excuse me that neurotransmitter itself changes this and the protein structure making it and twist so that it allows the ions to seep into or Russian - sometimes like sodium would rush into the cell well there are other types of receptor receptors and they're more complex and they take more time but do more functions so iron iron tropic receptors tend to open up and activate or inhibit the cell they tend to be either excitatory or inhibitory and not much other than that you know they trigger an on/off switch on the other hand there are receptors that that are called metabotropic metabotropic oops excuse my horrible handwriting the meta Tropic type of channels and and and are basically they use energy because they are more complex they require several steps and therefore use energy and therefore use they have a meta meta Baalak cost to them that's why they're called metabotropic so what happens here is a neurotransmitter actually it binds with a receptor that's separate and apart from the channel itself what this binding results in is a activation of a protein called AG protein this G protein then activates the ste protein then cleaves out an alpha subunit this alpha subunit then goes to the actual channel and at the channel is the process of opening so as you can see there is the role of the G protein which is activated by the neurotransmitter the neurotransmitter itself is called the first messenger the G protein is just the g-protein alpha subunit Cleaves off it activates the receptor site the binding site and an opening the channel okay so I ins now entered the channel another type more complex metabotropic involve a second messenger I talked about the neurotransmitter as being the first messenger but other other types of systems metabotropic receptors actually have a second messenger messenger involved they they go through the three steps we talked about the g-protein the alpha subunit the alpha subunit then connects to something in this case we've got the molecule of the neurotransmitter binding to the receptor the receptor activates the g-protein the alpha subunit Cleaves off and activates an enzyme so this is the new thing so you've got the role of an enzyme which then creates what we call a second messenger these are things like an eye a nose I I honest atolls these second messengers then open up the channel but they do another thing they go down to the nucleus of the neuron will go up to the nucleus of the neuron and activate a process potentially a process that produces new proteins that might have long-term effects see and this is bad by the way especially calcium type or calcium type channels where there is an activity that goes beyond just the opening and closing and triggering an on and off action potential but something that is long-term such as maybe memory for example that would be a good example of why you would have such a structure because you're not just responding to stimulus response in the environment but you want to somehow have a long-term learning of what has happened in the environment and so you need to activate proteins that then will result in changes and maybe the structure of the neuron structure of the dendrite that then will stick around for let's say days months years etc these are the metabotropic receptors in a nutshell we're gonna go through some meta petrovic receptors later on but I would like to now revisit what goes on and this is sort of related to the balance right we've seen something similar to this in the balance of ions well the balance of ions is taking place in on both presynaptic and postsynaptic but the balance of ions is what causes the act of membrane potentials here what you're seeing is postsynaptic potentials so now we're gonna talk about postsynaptic potentials potentials taking place at the post snaps typically at the dendrite and in the around the soma the membrane of the cell body those synaptic potentials are are not all or none action potentials action potentials are all or none they signal messages in a digital format yes or no active it activated or not activated now postsynaptic potentials are more analog they give you the full range of possibilities and they're two types one type of postsynaptic potential is called the excitatory postsynaptic potential and another type is called the inhibitory now if you think about it remember sodium sodium rushed into the cell in order to cause an action potential because as it's rushed into the cell early on like if you activate something that allows sodium to come in if you reach the threshold of excitation or a threshold of the action potential what'll happen is a cascade of sodium coming into the cell takes place well before you reach the threshold there is a balance and sodium is coming in it's not necessarily yet reaching the shore threshold it might you know the sodium might be coming in somewhere in the dendrite and that's on healing this is quite a distance away so a sodium comes in as sodium as channels open up like this these channels open up in the membrane at the dendrite the postsynaptic membrane and sodium comes in these sodium's will result in a movement towards depolarization and but the movement isn't triggering an action potential it's a still graded movement right it still isn't - sixty it's between minus 70 and minus 60 let's say it's excitatory because it's moving it towards an action potential so it's exciting the cell hopefully it'll trigger an action potential but it might not if it encounters other ions that will cancel the sodium like chloride well everything goes dead right you balance out there is no potential you don't trigger the cascade now sodium as I said is excitatory and because it's excitatory it's basically you know that it's depolarizing getting it closer to the action potential that's the word excitatory is that it's exciting it getting it closer to the action potential on the other hand potassium which by its nature remember it had these arrows wanting to leave well potassium when you open potassium channels what what happens is potassium typically leaves the neuron this process of leaving the neuron causes what we call a inhibitory because as you become more negative because potassium and what you're doing here is you're making the cell more negative on the inside by having potassium leave causing it to be more negative on the inside more positive on the outside and everything is relative and therefore you're actually causing what we call inhibitory postsynaptic potentials these again are graded they are not action potentials they're you know they're a full range and as they progress towards the axon hillock this potassium my Ted chloride and creating potassium chloride and cancel out so they're not necessarily gonna cause a full blast action potential they are still graded they're still in a way analog now chloride remember it wanted to go into the cell from that figure because it had there is much more chloride on the outside that on the inside also is inhibitory because it's gonna increase the negativity on the inside because it's coming in and it's negative so it's it's inhibitory so it's inhibitory postsynaptic potentials I PSPs and it's hyperpolarizing making it even harder for the cell to trigger an action potential because if you're at 70 and you want to go to minus 60 to trigger an action potential well you're now at minus 80 so you need minus twenty plus twenty millivolts to get to an action potential so you're actually telling the cell look please inhibit don't go chloride is actually what the channels gaba opens is a you know diazepam and all the benzos open chloride channels gaba chloride channels that that inhibit the cell sort of calming it down finally calcium tends to actually have long-term effects but calcium itself is obviously positive and going going in tends to be excitatory bottom line these potentials are taking place on the soma and the dendrites and they're building up they're building up either to trigger an action potential at the axon hillock so this is the axon hillock okay or inhibiting so if you're if you're making it more and more negative you're inhibiting if you're making it more and more positive you're exciting and the balance the calculation is what will the what will the decision be will there be more negative or more positive and will that happen closer to the axon hillock or further away from the axon hillock so quite a few decision-making takes quite a bit of decision-making takes place inside the soma depending on where the signal is coming from and how strong the signal how how fast the rate of the signal is the signal is really really letting sodium open up and and allowing sodium to come in at a very fast rate well that sodium even if it's at the long distance away from the axon hillock might reach the axon hillock and trigger the action potential if it doesn't hit inhibitory postsynaptic potential like chloride you know where it cancels out with it so you can see you can see is a computational process taking place and the computation is basically the fight between the excitatory postsynaptic potentials and the inhibitory postsynaptic potentials these are fighting against each other to build up to trigger an action potential if there is enough excite-ation and less inhibition or to inhibit the cell to tell it not to even trigger even when there is more sodium sent in it's sort of inhibiting it making it harder for it to be triggered this process is obviously graded it's not like an action place where it's a yes or no it's actually it build up and it's a fight between different potentials that are graded in nature and therefore it's an analog process if you want to think about it that way it's not as simple as yes or no it's a lot of computation that's based on analog principles based on distance from the axon hillock based on the amount based on the rate based on so many factors and based on modulation - so there are many other things modulating this process of excitation and inhibition so let's let's move to the next slide and and just touch base on this process right so you've you've got the release of the neurotransmitters and the thing is if the neurotransmitter is just sitting around in the synaptic cleft they will just keep triggering the opening of channels so you don't want that otherwise you know you might actually have too much of these I especially calcium causing something what we call excite to toxicity and that actually is something that happens and seizure and an alcohol related disorders excited toxicity too much of these ions going in could kill the neurons specifically calcium in the case of alcohol entry of calcium into the neuron could actually kill the neuron well so you need to get rid of this of these a neurotransmitter at some point you need to somehow figure out a way to get rid of them one way is to use these presynaptic transporters transporter molecules excuse me transporter channels that take up the neurotransmitter and recycle them so this is the process of free uptake using the transporters on the presynaptic membrane another way to recycle this is to actually have the astrocytes dig them up and recycle them and then send them back to the neuron so that's another way a third way is to deactivate them deactivating them so let's say this is acetylcholine which is often so sometimes as some Colleen is reuptake and sometimes it's deactivated by an enzyme called acetylcholinesterase and as you remember anything that ends with an S means it's an enzyme that breaks down in this case it breaks down acetylcholine to the acetate group plus the choline group and and it's flushed away you know it diffuses into the CSF and it's flushed away with the CSF into into the body and gotten rid of so this process of recycling is not very effective obviously you excuse me the process of breaking down using enzymes is not very effective it's better to recycle as everybody knows so that's another component of this process of neurotransmission and what goes on at the synapse this figure shows you what I was talking about in terms of epsps and ipsps so working against each other so if there is activation and red here represents excitatory postsynaptic potentials if there's enough activation taking place all over you know especially as it progresses towards the axon hillock if there is enough of the excitation to actually trigger an action potential you actually have that you have a message stand down on the other hand even if you have enough excitation if on the other hand you another neuron is telling this postsynaptic neuron to inhibit if it's opening if it's opening chloride channels or potassium channels that both are inhibitory ipsps what you're going to get is an inhibitory process that resolved in no triggering of the axon hillock so this is this is showing you that a balance like this will result of a non triggering of the axon hillock as opposed to a clear triggering of an axon hillock as the because of the over abundance of epsps