Neuronal synapses (chemical) | Human anatomy and physiology | Health & Medicine | Khan Academy

I think we have a decent idea of how signal is transmitted a longer neuron we saw that you know a couple of dendrites maybe that one and that one and that one might get excited or triggered and when we say it gets triggered we’re saying that some type of channel gets open that’s probably the trigger that channel allows ions to be released into the cell or actually there’s situations where ions can be released out of the cell would be inhibitory but let’s take the case where ions are released into the cells in an electro tonic fashion it changes the charge or the voltage gradient across the membrane and if the combined effects of the change in the voltage gradient is just enough at the axon hillock to meet that threshold then the sodium channels over here will open up so sodium floods in and then you know we have the situation where because the chart the voltage becomes very positive potassium channels open up to change things again but by the time we went very positive then that electro tonically affects the next the next sodium pump and then we have this situation where that will allow sodium ions to flood in and then the signal keeps getting transmitted now the next natural question is what happens at the neuron to neuron junctions we said that we said that this dendrite gets triggered or gets excited and in most cases it’s getting triggered or excited by another neuron it could be something else and over here when this axon fires it should be exciting another either another cell it could be a muscle cell or in probably most cases of the human body it’s exciting it’s exciting another neuron and so how does it do that so this this is the terminal end of the axon there could be the dendrite of another neuron right here so this is another neuron right there I’m not drawing it that well but I think you get the other idea this is another neuron with its own acts on its own cell and so this would trigger the dendrite right there so the question is how does that happen how does the signal go from one neurons axon to the next neurons dendrite it actually always doesn’t have to go from axon to dendrite but that’s the most typical you could actually go from axon to axon dendrite to dendrite axon to soma but let’s just focus on axon to dendrite because that’s the most traditional way that neurons transmit information from one to the other so let’s zoom in let’s zoom in right here this little box right there let’s zoom in at the base the terminal end of this axon and let’s and then let’s zoom in on this whole area and then we’ll also zoom in we’re also going to get the dendrite of this next neuron and I’m going to rotate it actually I don’t even have to rotate it so to do that let me draw the terminal end so let’s say the terminal end looks something like this I’ve zoomed in big-time it looks something like that this is the terminal end of the neuron this is inside this inside the neuron and then the next dendrite let me draw it right here the next dendrite the dendrite of the next neuron we put it right there so we’ve really zoomed in so this is dendrite dendrite of next neuron this is inside the first neuron so we have this action potential that keeps traveling along eventually you know for maybe right over here and if you can zoom in which would be over here the action potential makes the electrical potential or the the voltage potential across this membrane just positive enough to trigger this sodium this sodium channel so then this so did so I’m I’m really you know maybe I’m getting really actually maybe I’m really close this channel is this one right here so then it allows a flood of sodium to enter the cell it allows a flood of sodium and then the whole thing happens you have potassium that can then take it out but by the time this comes in this positive charge it can truck it can trigger another channel and it could trigger another sodium channel if there’s other sodium channels further down but near the end of the axon there are actually calcium channels calcium channels I’ll do that in pink so this is a calcium channel this is a calcium channel that is traditionally closed so this is a calcium ion channel cow GM has a +2 charge calcium ion channel it tends to be closed but it’s also voltage-gated when when the voltage gets high enough it’s very similar to a sodium gated or a sodium voltage-gated channel is that if if it comes positive enough near the gate it will open up and when it opens up it allows calcium ions to flood into the cell so the calcium ions there positive to charge to flood into the cells now you’re saying hey Sal why are calcium ions flooding into the cells these have positive charge I just thought you said that the cell was becoming positive because of all of the sodium flowing in why would this calcium want to flow in and the reason why it wants to flow in is because the cell also just like it it pumps out sodium and pumps in potassium the cell also has calcium ion pumps and the mechanism is nearly identical to what I showed you on the sodium potassium pump but it just deals with calcium so you literally have these proteins that are sitting across the membrane you know this is a phospholipid layer membrane so maybe I should just draw it maybe I’ll draw two layers here just so you realize that it’s a bilayer membrane let me draw it like that it makes it look a little bit more realistic although the whole thing is not very realistic and this is also going to be by lipid membrane get the idea but let me just do it just to make the point clear so there are also these calcium ion pumps that are there are also subsets of ATP ASE’s which they really did just like the sodium potassium pump you give them one ATP one ATP and a calcium will bond someplace else a calcium will bond someplace else and it will it’ll pull apart the phosphate from the ATP and that’ll be enough energy to change the confirmation of this protein and it’ll push the calcium out or essentially it what was the the calcium will bond and then it’ll open up so the calcium can only exit the cell so that’s what the it’s just like the sodium potassium pumps but just it’s good to know then the resting state you have a high concentration of calcium ions here it’s all driven by ATP a much higher concentration on the outside then you have on the inside it’s driven by those ion pumps so once you have this action potential instead of triggering another sodium gate it starts triggering calcium gates and these calcium ions flood into the terminal end of this axon now these these calcium ions they bond to other proteins and before I go to those other proteins we have to keep in mind what’s going on near this Junction right here and I’ve used the word synapse already actually maybe I haven’t the place where these two where this axon is meaning with this dendrite this is the synapse this is the synapse this is the synapse or you can kind of view it as that the touching point or the communication point or the connection point and this neuron right here this is called the presynaptic neuron let me write that down it’s good to have a little terminology under our belt pre synaptic presynaptic neuron this is the postsynaptic neuron post synaptic neuron and the space between the two neurons between this axon and this dendrite this is called the synaptic cleft synaptic cleft I’ll just write it as ass in synaptic cleft and it’s a really small space in the terms of so what we’re going to deal with in this video is a chemical synapse in general when people talk about synapses they’re talking about chemical synapses there are also are electrical synapses but I won’t go into detail on those this is kind of the one the most traditional one that people talk about so your synaptic cleft and can in chemical synapses is about 20 nanometers which is really small if you think about the average the average width of a cell is about well depending on your view at 10 to 100 microns that’s 10 a micron is 10 to the minus 6 this is 20 times 10 to the minus 9 meter so this is a very small distance and it makes sense because look how big the cells look next to this small distance so it’s a very small distance and you have on this on this the the the presynaptic neuron near the terminal end you have these vesicles remember what vesicles were these are just membrane background these are just membrane brown things inside of the cell so you have these vesicles they also have their phospho by lipids by lipid layers the little membrane so you have these vesicles so these are just you can kind of view them as containers I’ll just draw I’ll draw one more I’ll draw one more just like that and they can train near these molecules called neurotransmitters and I’ll draw the neurotransmitters in orem all that I already use that I’ll do it in green so they have these molecules called neurotransmitters in them you’ve probably heard the word before in fact a lot of a lot of drugs that you know people use for depression or or other other things related to our mental state they they they affect neurotransmitters and I won’t go into detail there but they contain these neurotransmitters and when the calcium gets the calcium channels they are voltage-gated when it comes a little more positive they open calcium floods in and what the calcium does is it bonds to these proteins that have docked these vesicles so these little vesicles they’re docked to the synaptic they’re docked to the presynaptic membrane or to this axon terminal membrane right there and these proteins are actually called snare proteins it’s an acronym but it’s also a good word because they’ve literally snared the vesicles to this membrane so that’s what these proteins are and when these calcium ions flood in they bond to these proteins they attach to these proteins and they change the conformation of the proteins just enough that these proteins bring these vesicles closer to the membrane they bring these vesicles closer to the membrane and also kind of pull apart the two membranes so that the membranes merge let me zoom in of that just to make it clear what’s going on so after they’ve bonded with this is kind of before the calcium comes in bonds to those snare proteins then the snare protein will bring the vesicle ultra close to the presynaptic membrane so that’s the vesicle and then the presynaptic membrane will look like this the presynaptic membrane will look like this and then you have your snare proteins and I’m not obviously drawing it exactly how it looks in the cell but it’ll give you the idea of what’s going on your snare proteins have pulled have essentially pulled the things together and have pulled them apart so that these two membranes merge and then the main side effect the reason why all of this is happening is this allows those neurotransmitters to be dumped into the synaptic cleft so those things that those neurotransmitters that were inside of our vesicle then get dumped into the synaptic cleft so they get dumped into the synaptic cleft this process right here is called exocytosis it’s exiting the the cytoplasm you could say of the the presynaptic neuron but these neurotransmitters and you’ve probably heard the specific names of many of these sirata nin dopamine epinephrine which is also adrenaline but that’s also a hormone but it also acts as a neurotransmitter norepinephrine also a both a hormone and a neurotransmitter so these are things that words that you’ve probably heard before but anyway these enter into the synaptic cleft and then they bond they bond to parts they bond on the surface of the membrane of the postsynaptic neuron or this dendrite so they’ll bond let’s say they bond I was doing yellow so let’s say they bond here they bond here and they bond here so they bond on special proteins on this membrane surface but the main effect of that is that it that will trigger ion channels so let’s say that this neuron is is exciting this dendrite so when when these neurotransmitters bond on this membrane maybe sodium channels open up so maybe that will cause a sodium channel to open up so instead of being voltage-gated its neurotransmitter gated so this will cause a sodium channel to open up and then sodium will flow in and then just like we said before that’s the quit you know if we go to the original one that’s like this getting excited will become a little bit positive and then if it’s enough positive it’ll electro tonically increase the potential at this point on the axon hillock and then we’ll have another we’ll have another neuron in this case this neuron being stimulated so that’s essentially how it’s happens it actually could be inhibitory you could imagine if if if this instead of instead of triggering a sodium ion channel if it triggered a patek if it triggered a potassium ion channel if it triggered a potassium ion channel potassium ions gradient concentration gradient will make it want to go outside of the cell so positive things are going to leave the cell if it’s potassium remember I used triangles for potassium and so if positive things leave the cell then the if you go further down the neuron it’ll become less positive and so it’ll be even harder for things to for the action potential to start up because it’ll hat it’ll need even more positive someplace else to make the threshold gradient I hope I’m not confusing when I say that so this connection the way I first described it it’s exciting when this guy gets excited from an action potential calcium floods in it makes these vesicles dump their their contents into the synaptic cleft and then that will make other sodium gates open up maybe sodium gates and then that will stimulate this neuron but if it makes potassium gates open up then it will inhibit it and that’s how frankly these synapses work and there’s there’s you now have about to say there’s millions of synopses no but that being correct there’s trillions of synapses the best estimate of the number of synapses in our cerebral cortex is 100 to 500 trillion trillion synapses synapses just in the in the cerebral cortex and the reason why we can have so many is that one neuron can actually form many many many many synapses I mean you can imagine if this original drawing a cell you might have a synapse here synapse here synapse there you could have hundreds or thousands of synapse even into one neuron or going out of one neuron this might be a synapse with one neuron another one and another one another one so you have many many many many many connections and so synapses are really what give us the complexity of you know what would probably make us tick in terms of our human mind and all of that but anyway hopefully found this useful


  1. excitatory conduction is caused by an influx of Sodium of which yields a depolarization of the membrane (Na comes in and if enough come in, it causes an action potential)… inhibitory conduction is caused by an increase in chloride or potassium permeability (chloride and potassium influxes cause hyperpolarization- where instead of the action potential becoming more positive, it becomes less positive and no action potential can happen). Hope that helps.

  2. K+ influx is inhibitory (same w/ Cl- influx) … which channels are stimulated to open in the post-synaptic membrane depends on the neurotransmitter being released from the pre-synaptic membrane from those vesicles in the pre-synaptic axon terminal… It depends on the NTs and also the receptors on the post-synaptic membrane that those NTs are binding to. Hope that helps.

  3. Sodium channel open up at hill-lock and sodium ions enters, and the voltage become very positive then you said something like potassium channel open up to change thing again. What happened when potassium open up? Why is there a change?
    After each schwann cells, the sodium channel will open and another action potential will occur again or action potential only occur at hill-lock? If action potential only happens at hill-lock, when and where does the potassium channel open and cause the graph going negative? 

    (Sorry, i am trying to understand the whole procedure by linking the graph of action potential to its theory.)

  4. So, when the calcium ions are entering the presynaptic neuron they are following more their chemical gradient that the electrical one?

  5. @Nicole Geissinger It would actually be K+ efflux that hyper-polarizes the neuron. You are right that Cl- influx would hyper-polarize it as well, because an influx of negative charges has that effect. But you are saying that K+ influx, which means bringing in more positive charges, would hyper-polarize a negative membrane, which is simply not true. There is more K+ (positive charge) inside of the cell than outside of the cell, so when the voltage-gated potassium channels do open, K+ (positive charge) would leave the cell, causing it to become more negative. So, for an IPSP, the membrane would become more negative, which is caused by K+ efflux.

  6. If at the post synaptic neuron K+ is released, causing an inhibitory action, what happens to the neurotransmitter that was released from the pre synaptic neuron? Does it just sit in the synapse?

  7. what you are doing sir is just amazing! I cant put into words how much I am thankful to your videos. You are such an inspiration. Love you …I hope people like you become eternal

  8. is this number 2 or not. I got confused on my way here, so, i'm probably out of order in the videos…

  9. What I want to know is: How on earth did anyone find out how this works in the first place?? Thank you for the lessons though

  10. Thank you for you amazing videos. I am also a teacher. What format/program are you using for these videos? Connie
    Does anyone else know?

  11. the fact that he keeps saying "actually maybe/ actually probably" is really annoying. other than that, great vid! hopefully gnna ace my exam thanks to you 🙂 

  12. Where can I get an general information about principles of operation in neuron system?
    Also I am interested in functional model of neuron system.
    I found in internet a lot of information about chemical processes in neurons and synapses, but not much about how it works together.

  13. That's the reason people think, the neurons react to a number of chemicals in the brain to counteract with various emotions you can list in your mindset. Generally speaking the neurotransmitters play a vital role of gathering information every picosecond depending on how strong the intellectual capacity is in terms of using the functions of the brain.

  14. I thought there were receptors that allow the neurotransmitters into the next neuron, not really channels. And he said "I think" when he drew it. I need a little clarity on that.

  15. Thank you so much for posting this video!! Helped me BEYOND what I expected from it, and now I will be even more prepared for my exam in Anatomy. Your explanations and drawings are more than detailed and excellent. Thank you profusely from a very grateful college undergrad. 🙂

  16. Thank you! I am so happy that I foud your video! In scool I am writing a text about the brain, and hawe had a werry hard time to find some information that I undrestand.Our scool teatcher dosent know so much about it just the basic facts. The only bad thing about the video is that i dident know some of the words, but thats probrebly becouse I hawent english as my first langush, good that gogle translate exist thoe. 😉

  17. Great concised presentation!
    Many thanks!

    Where does magnesium come in?
    How does nearby astrocyte play a role with calcium?

  18. what is the video between this one and the anatomy of neurons?? you should number the videos when you continue a drawing or topic :/

  19. At 14:05 you mention that if the neurotransmitter initiates the opening of a potassium channel, the potassium inside the cell will want to rush OUT of the neuron. A little before this you mention that if the neurotransmitter initiates the opening of the sodium channel, the sodium ions will flow IN the neuron. Isn't this a bit contradictory since both ions are positive and will flow in the same direction depending on membrane potential?


  20. But the movment of ions does not seem like data used for remembering an action or doing something, and a nucleus normally is the factory for cell reproduction – not like a node holding digital information. Do you have any ideas for consciousness. As currently it all seems to be driving Electromagnetic fields.

  21. sir i hv been watching ur channel in all three subjects since i joined 11th . all are great sir but can u plzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz make chap wise videos of 1- 2 hrs its too difficult to fin all topic of a chap all together

  22. This is a great explanation thanks. Im wondering what decides weather the next neuron will be inhibited or exited? I now know that if the neurotransmitters bind to potassium pumps it will be inhibitory but what makes them bind to the paticular ion gates? It cant be random. Is there some kind of magnetism towards the ion gates? And what causes that? Thanks hopefully my question makes sense

  23. My lecture at uni was 4 times as long and it was nowhere near as coherent and informative as yours. And I go to supposedly the one of the best unis in Australia.

  24. Ca+2 intake is bcoz of Na+ intake results in depolarization and then Ca+2 channels are sensitive to depolarization thats why Ca+2 comes to Inside…
    Source-Dr.Najeeb Lacture..

  25. Why calcium ions adhere to these balls with neurotransmitters? Is there some negative charge on the balls?

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