we we had gone like I said through quite a bit of detail uh if you find this amount of detail overwhelming and I understand that it is a lot of detail what I would do is I would just I would just make myself a giant sequence of events and that usually helps you know organize your thoughts i think most people once they can organize the information they feel a lot better about it it's not so intimidating it's not so like anxietyridden um but there this would be a great sequence of events opportunity because everything here happens absolutely sequential in a healthy um individual there are there's really no room for artistic interpretation here thing one leads to thing two which leads to thing three so I think that's a really good way to see the big picture if you're feeling extravagant I might recommend that you start with actions at the motor implates and see what happens when the necotinic acetylcholine receptors open you get a little bit of sodium in you need a lot of sodium in where does that go i think that would be useful um so anyway just wanted to sort of mention that see if there are any questions from this on Tuesday that you might have thought of questions all right well seeing none we're going to get into some new stuff um we're going to talk about what happens once uh we get all of this calcium from the SR once it finds its target which troponent uh C and how are we actually going to have actin and measin interaction so we're going even further um as far as cellular details this is still kind of big picture i think you need all of this though to understand really appreciate sliding filament theory which is how actin interacts how they work in tandem to cause muscle contraction that's where we're going so no questions on this so far so good okay [Applause] it's so hard being tied in one place very strange i have an actual microphone down here but it's the kind you have to hold and it looks like you should sing and trust me nobody wants that absolutely not last time I tried singing I actually scared all of my livestock so that's all I need to know if it's so bad your livestock can't stand it then you have a problem so that's all I need to know um so we're going to be looking at excitation contraction coupling part two and as I mentioned I think um you know sort of breaking this up into cellular stuff like we looked at on the last page and then breaking it up into more um sort of molecular stuff honestly and where are the ions and what are they doing that might be helpful as well so and also it's a matter of time before I knock into this and knock the iPad off and it will all be captured and posted on YouTube um so a couple things when we get started here um we're going to see that a lot of what we're looking at on this particular drawing is going to require ATP but we use ATP in places that are maybe not what you'd expect so this is a softball but where is ATP produced in a muscle cell what's the obvious answer it' be the same as any other cell where's ATP produced mitochondria where else it's not the only place what about glycolysis can we produce ATP via glycolysis we can where does that occur cycloplasm and then there's uh one other area happens in the cycoplasm and I haven't talked about this much but I'm curious to know if if any of you have taken a class that talks about um a way we can produce ATP really without having to break down um you know ATP and get the phosphate and then phosphorolate something else i'm looking for a compound that exists in skeletal muscle that can donate a phosphate and create ATP creatine creatine phosphate yeah good job so the creatine phosphate system that is a way that we could also make a little bit of ATP in the skeletal muscle um you're not going to get much and it doesn't last very long but it's just a little extra boost so creatine phosphate system and that uh pretty much hits the highlight so when we talk about you know glycolysis and mitochondria we're talking about or together that would be all part of aic respiration we took a good look at that at the beginning of the semester it's really similar in skeletal muscle cells so if you need a a refresher on that it might be a good idea to look at that um creatine phosphate this is just a reaction where we can take a phosphate and put it on ADP to create ATP but we don't have to break down ATP in the process okay next one uh this says "What is the relationship between blood flow oxygen delivery and ATP production?" Kind of another softball another way of looking at this is can you produce more ATP with oxygen present you can so that's the obvious answer blood delivers oxygen and oxygen boosts ATP production that's pretty straightforward but what happens when skeletal muscle cells don't have adequate access to blood and oxygen and there are skeletal muscle cells they're contracting too much you only have five liters of blood you can't peruse all cells equally is there something else they could switch to instead of aerobic respiration we would call that anorobic respiration so some cells are absolutely some skeletal muscle cells are really great at handling a low blood flow low oxygen delivery environment so the relationship again is um more blood more oxygen more ATP production but some cells can do anorobic glycolysis and they're really good at it they're really kind of set up for anorobic glycolysis type two muscle cells the fast twitch they're really good at that now they fatigue quickly because if you don't have access to adequate amounts of blood and oxygen and you're trying to you know sort of solve your ATP need with glycolysis it just doesn't last very long but it's something on the other hand you have slow twitch fibers and they can stay contracted or be used for long periods of time they don't fatigue very easily or quickly compared to fast twitch and that has to do with the fact that blood is very um or these slow twitch fibers are very well perfused they have a really dense vascule and so that delivers a lot of oxygen to them consistently and so they don't have to um turn to other ATP producing methods that might not have such a payoff so I will speak more to this on this particular piece of paper but just some things I wanted to remind you of before we go any further so we're going to get into um how we're actually going to make a sarcimeir contract or get it to contract and we're going to do two things on this piece of paper we're going to start with a sequence of events leading to crossbridge cycling and then I'm going to talk about what happens in the sarcimeir during or as a result of crossbridge cycling so we're going to do per usual some sort of background type things over here and then sort of see how it works so give me just a minute to rearrange all this before it totally collapses on itself okay so crossbridge cycling just as a reminder in case you're studying this piece of paper differently or in you know not sequentially from the last bit of notes that we looked at i wanted to make sure that you have sort of the the primer um for what we're going to talk about mostly here on the same piece of paper i think it's important to remember these things are linked so sort of as a review or a summary of what we talked about on the past um in the past lecture on the past piece of paper we talked about muscle contraction so muscle cells shortening um that is going to be triggered by calcium binding to traropponin C tropponin C part of that traropponin complex and that's what reveals the meosin binding sites those mice and binding sites are on actin i'm going to draw this out a little bit bigger it was pretty cramped in the last drawing and I'm not sure how incredibly clear it was so I'll give you a chance to write that down and then I'm going to provide some more details i'm going to continue to fiddle with the sound system so I like I said I do want to talk a little bit more about actin a lot of this particular page is about measin and that's because meosin does a lot of the work but it wouldn't matter if we didn't have actin so that's why I'm going to spend a little bit of time right here talking about you know what is the tropponin complex in more detail what does actin really look like um and then what is this weird thing called tropomy can you guys hear me okay because I don't Is it is it good okay good um so I'm just going to sort of sketch out a little bit what we did on Tuesday and I want to introduce some more details so some of this will look familiar some of this could be new if you have questions let me know but I want to just talk about I'm just going to keep it isolated to actin and trommyosin so actin is made of little tiny molecules and each of these little circles that I'm drawing here is called um Gactin g is in the letter G-act so every little circle here is its own molecule and if we get a bunch of kind of stop it there if we get a bunch of the gactin hooked together so that's the molecule that's our protein when we put all these together we get um a string and this string is called so the whole string is called Fact so each circle G actin you put it together you get Factton you can think of F as sort of the filamentous um and then you have two F act strings put together so I mentioned on Tuesday that actin kind of looks like when you try to pull out a necklace from your jewelry box and it's all wrapped together and you spend 30 minutes on it before going forget it i didn't want it anyway um so you have actually two factin strands each of them made up of a lot of these Gactin molecules so Gactin is the molecule kind of the monomer I guess you could say and Factton would be the polymer now it probably sounds like a really nitty-gritty thing to to talk about you're like why are we talking about it and the reason I want to just mention this other than just trying to be as thorough as as possible is that you can elongate the size of this or the length by adding producing more gactin proteins and adding them to a certain end so as a person grows you can actually increase the size or length of this and that doesn't really make sense until you realize these are just monomers hooked together and of course if you needed more you would just create more monomers and hook them on to one of the ends so a little bit about that um the traroponin complex is the next thing I want to talk about and I want to talk about it in reference to what it does so we saw that there are what look like sort of upside down commas and these are each of these is a measin binding site so here's where the meosin heads uh will bind actin if if and only if um the site is revealed and there's a reason why we don't want these sites to just be revealed all the time and that's because measin loves actin like measin just loves acters of act on its wall it follows actton on I don't know anything about social media the Twitter is that so or whatever it is now some letter anyway it loves loves measin loves actin so we have to find out a way to make sure that measin and actin don't hook up when they're not supposed to and that is the job of tropomyiain so I'm going to draw another um protein here tropomyiain and when a muscle is relaxed because the neuron has not told it now is time to contract then our tropomyiain should be rotated or folded in such a way that it blocks these measin binding sites and it's a physical blockade that prevents your meosin heads from being able to access actin so this orange filaments protein is tropomyosin so at this point kind of have the protein built that is actin talked about its monomers what we call it together why we need tropomyosin now I want to talk about the tarponin complex in a little more detail um I was just trying to fit it in on the last drawing so you can situate it or like see where it lives where it's placed but again when we do that sometimes we lose the ability to provide clear details so what you don't see up here yet is something called the traroponin complex and I I will add this obviously but let's talk about what the traroponin complex is it's a series of of three proteins um three proteins make up the complex and they exist as a group you shouldn't find one without the others for example and so there's three um parts to these to this complex we have um traropponin C and its job is to bind two calcium ions uh one won't get the job done it takes two we have traroponin T and that binds um the complex so it it binds this protein complex or the tropponin complex to tropomyiain it is sort of um the handle that keeps it all together and then we have traroponin I and tarpon I has a strange role traroponin I I stands for inhibition or it's inhibitory and what it does is it prevents actin from being accessible if calcium's unavailable and you would think well that's the job of tronotonin C but it's sort of like we have some redundancy here so we can think of tarpon I as an inhibition um factor on actin making sure that we don't have meosin and actin interaction when we don't want it the reason why we have to be careful is not just because you would have you know extraneous muscle movement when you didn't want it but you you just waste a lot of ATP and so nature is always very clever about finding ways to prevent ATP wastage and every time we have actin and interact ATP will have to be burned or hydrayed and so we have sort of a redundancy to make sure that doesn't happen okay so I'm just going to do um a couple of tarpon complexes on this just to kind of help you understand the placement now that we see what this is and the traroponin complex is spaced very regularly i'm going to put traroponin C up here that's going to be the green one i usually use green for calcium um I'm going to put traroponin T down here i use a different color tropponin T it has to bind everything to tropomyiain so you can kind of draw it such that it intertwines or at least connects so red dot and then tropponin I sort of running out of contrasting colors but traroponin I also part of it and it also binds actin and it just keeps everything from interacting when it shouldn't so I have C traroponin C traroponent I and troponent T so these three proteins or in this case dots would be found at regularly spaced intervals and so that's really the especially troponent C you know this is our bridge between honestly the action potential happening and the mechanical events that are going to happen next so we took a good look at on Tuesday just how important calcium is so the amount of calcium in the cytoplasm really dictates the force of muscle contraction so just a little bit about that okay questions on actin or tropomyosin from Tuesday or today so far so good so everything that we're going to see going forward here on this piece of paper the timing allowing actin and meosin to interact timing is is simply controlled I mean it's not a simple process but in the essence it's controlled by the action potential at our neuromuscular junction and the spread or that action potential has to spread all along the saroma on the periphery has to go in and that's um you know the ability to go into a muscle cell is given to us by the T- tubules themselves made of sarlemma a good analogy of T-tubules if you're having a hard time kind of thinking about how that works you know there are holes in the membrane but it also has ECF it's kind of weird um imagine you had a flat piece of clay or Play-Doh i think Play-Doh is underrated we should bring that back um and it was pretty thick pretty like a square foot by foot piece of clay or Play-Doh and you took your hand and you made a hole in it every so often you know that clay bends and makes a hole but you still have the surface of the clay surrounding the hole that's very similar to how T- tubules would look um in in a skeletal muscle cell it's not like you have made it a hole in in the cell such that it's an unregulated pore instead you've you've dented in so maybe that helps um T-tubules are just kind of hard to get your head around maybe we should do some more Play-Doh i think I'm going to bring that back just don't eat it right you guys wouldn't eat Play-Doh right maybe some of you would i have some suspicions about some of you i think most of you would not put it in your mouth but anyway um let's talk about what we're going to do in part two part two in part two of this particular set of notes we're going to see how all of this leads to sarcamir contraction or shortening so we're going to see um some detail about measin the measin heads that I've been talking about they bind actin at their specific binding sites interestingly and this is a big area of misconception no ATP is directly required for this binding so I'm going to be I'm going to try to be very careful and very precise about the language we do use ATP during muscle contraction that's not what I'm saying that we don't use it it's just where we use it is a little um unusual or maybe not what you would expect so in other words once this tropomyiain is moved out of the way meosin will bind actin and we don't need ATP for that that is just um a a chemical interaction that is going to happen when our meosin heads do bind actin we're going to get what's called the formation of a a crossbridge and a crossbridge is a good way of thinking about it it's a very temporary bond between these two proteins it can be formed quickly and it can be broken quickly and when we talk about crossbridge cycling that's just the formation of this bond and the dissolution of this bond it has to happen over and over and over again in order to get your sarcimeir to shorten as a result of this crossbridge cycling formation and then dissolution formation and dissolution we will see actin this whole thing is going to be moved towards the center of our sarccomir due to meosin head rotation so meosin it's a very interesting protein there's a lot about it that's just like wow that makes sense but it's not exactly logical so I'm going to take some time to describe the structure of measin it is not just a contractile protein it's an ATPAS which means it has enzymes that hydrayze ATP into its components which would be ADP and PI when we have the meosin heads that have bound to actin and they're going to function as a unit meosin and actin will function as a unit the heads of the meosin will swivel and because actin's attached to it it takes act with it so measin attaches to actin and if meosin moves actin moves this particular part is called the power stroke and as a result of the power stroke the entire actin filament will move towards the mline which is the center of our sarcimeir but everything here is due to the work of act of meosin meosin binds actin and if meosin moves then actin moves it doesn't really get a choice because they are stuck together you don't get a lot of movement per power stroke you get about 10 nanometers of movement that's not much it's not going to do any work it's not going to lift any weights but you have a continuous cycle of measin heads at small different intervals binding and contracting or doing the power stroke releasing getting another grip binding doing a power stroke releasing and so we should as a result get a very graceful shortening of the cell it's it shouldn't be jerky and that's because you have measin heads even in the same sarccomir at different stages of the crossbridge cycling so imagine if you were uh playing tug-of-war you had a giant rope two two teams many people on each side when you play tugof-war it's it's been a while i play with my dog and she wins all the time but imagine teams and you're all gripping the rope at different times and pulling you don't just all grip and release grip and release because you would lose so imagine a team of people playing tug-of-war that's kind of what the masin heads do grip at different times and pull so we get a nice graceful movement towards that mline at the end of that crossbridge cycle when the meosin head has after its bound after it had its power stroke it has a funny habit of not letting go of actin remember measin loves actin has posters of acting on its walls follows it on whatever they do on social media i'm not sure but meosin does this um not like a stalker that would be weird but like you know just really infatuated like yes I wonder what act's doing so meosin has to be convinced to let go of actin so meosin heads will remain attached to actin permanently until something comes along what do you think that is you can't see it yet what are we going to have to tease measin with to get it to let go atp this is where we use ATP we don't need ATP for binding actin we need ATP for letting go and so a lot of this has to do with the chemistry of the meosin heads uh meosin cannot bind actin and ATP at the same time there's a space issue and so that's why we have to basically ask myosin you're going to have to make a decision and luckily it always chooses ATP over actin um so we're going to see how that sort of arrangement works and why it cannot bind actin and ATP at the same time but ATP molecules have to be around it has to be you know plentiful in the cycoplasm and when they are we'll see that they bind measin heads at sort of like a a cleft or a pocket area and that's where the enzyme is that can hydraize it so what happens to muscle if you run out of ATP you're doing it in the midst of a contraction and you run out of ATP what happens it's a cramp pretty bad one you rub it the area that has the cramp what are you actually doing that works what are you actually increasing in that area blood flow blood flow which brings oxygen which allows the cell to make ATP so it's not going to be instant but that's what that's why if you get a cramp it's sort of a natural propensity to rub that area and it actually does work if you've ever seen um animals on the side of the road that have been hit and their legs are straight out that's rigamortis and the reason that happens is because the animal is out of ATP because it's no longer alive and rigam mortise sets in pretty quickly without ATP around and it can break down over a series of hours if not days depending on environmental temperature do I have any deer hunters in this room deer hunters yeah so when you get a deer you hang it from a rail and that's to break the bonds here because you don't want to eat meat that's in rigor mortise you couldn't cut into it that's the same reason why when you uh I don't know if any of you have ever toured a slaughter house but they hang cows on a rail as they extinguinate for the same reason you're wanting gravity to break these bonds um so there's a lot of interesting things that you have benefited from that had to do with trying to break these bonds after an animal has died so you can't eat meat that is has had rigor mortise it's inedible hard to cut can't eat it tastes terrible anyway let's talk about how we're going to get this ATP in the meosin heads and how we're going to let it um work its magic so it lets go of actin so we have some convincing to do I guess you should say so we're going to take a look at the structure of measin we're going to be looking at a large protein it's got um several components to it some of these are structural some of these swivel or move and some of them bind to actin other parts bind ATP so it's a very complex molecule different forms of meosin exist um what actually makes a muscle cell slow twitch or fast twitch actually comes down if you boil it down and go okay but why okay but why um it comes down to how fast these meosin heads can hydrarolyze ATP at the end of the day from a biochemical standpoint that's the difference between slow twitch and fast twitch slow twitch muscle cells do not have the ability to hydrayze ATP very quickly they are that's why they're called slow that's why they are less fatiguable they just can't burn even if they have tons of ATP their little heads can't hydrayze it that fast on the other hand fast twitch muscle cells they're fast twitch very powerful because their measin heads are biochemically a little bit different they still participate in contraction of course they're still part of a sarcimeir but they have the ability to hydrayze ATP extremely quickly and so that provides a lot of force but it also depletes your ATP pool much more quickly so that's really the difference between slow twitch and fast twitch other cell differences exist like you know cell diameter access to blood that also differs but still if you drill down and you're still curious I know I don't know if you're like me but you're like I have to know the reason why at the most atomistic level why is it like that that is that's why so when you think about you know you watch athletes on TV professional athletes and you see them just absolutely masters at their particular event it comes down to a lot of its genetics did they get were Were they born with meosin heads that could hydrayze ATP quickly or were they born with a lot of more measin heads that were very efficient sort of the slow burn more of the marathon type muscle yeah when you say head what are you Oh Cindy that's a great question i feel like this was a leadin that I asked you to ask me before class so it's such a perfectly timed question i'm so glad you asked we're gonna talk about that right here perfect on time so let's talk about what is a measin head we're gonna see on this drawing that each measin molecule has and again I'm speaking very generally i'm not talking about slow twitch or fast twitch meosin at this point just what is meosin like what is this thing that does all this work each meosin molecule have a long tail and that long tail has to reach all the way towards the center of the sarcimeir when we braid a lot of them together it causes measin to look thick we're going to see that it has um basically what we're going to call a neck these are more biochemically known as heavy chains and light chains i'm just going to call it a neck um and then we're going to see that it also has two heads that swivel so they can act together they're going to close on something and then they're going to swivel so we have some moving parts I guess you could say so let's take a look at what what this thing looks like this is a single um turn this around kind of distracting um this is a single what we're going to call molecule of of myasin and it has some interesting features so as you're drawing this um make sure you appropriately place your ATP binding site in the middle and as you're drawing this realize that these heads sort of um fold up around actin or up around ATP so the ATP would fit in this pocket you're just sort of seeing the halves um so these work together in tandem so this tail is pretty thick pretty long and it reaches all the way towards the M line and so the center of a sarccomir would be that way and your zline the end of the sarcimeir would be over here these are pointing anyway so you'd have many many of these in a sequence making up a measin filament so that's our tail we've got what we're going to call a neck and then this neck has some hinges and those hinges allow the meosin heads to once they have clamped onto something specifically actin and they have an energy source that has already been hydrayed it allows these heads to swivel and that's what moves actin so again imagine this as a clamp we're just looking at the inside so this would be a mirror image these two heads so we're gonna see that we could clamp on something and that would either hydrayze ATP or if we don't have ATP we could clamp onto actin but because of where the ATP binds and the actin that's why you can't bind ATP and act at the same time it just does not work it's a space issue it doesn't fit it doesn't clamp so meosin cannot absolutely just cannot do it bind ATP and actin at the same time remember we don't need ATP for myosin and actin interaction and here's where people are like well how does it get its energy then if they can't bind at the same time how does it get the energy to swivel and the answer to that is meosin because it is an ATP ace which means it's an enzyme that can hydrayze ATP so meosin heads can hydrayze ATP to its components which would be of course ADP plus PI you can hold on to those for later use and it will gets a lot of people on this exam adp and PI structurally are not the same thing as ATP so that's why you you can hold on to your energy source but not have your space issue i think it's um important to just kind of be hyper clear about this because otherwise it it sounds like I'm talking out of both sides of my mouth i'm like it can't bind ATP and act at the same time but then I'm going to come over here and talk about remember that energy we got from ATP well look it's still here and now it's bound to act and so I think that could be really confusing if we don't remind ourselves that once we hydrayze ATP it structurally is different than what we broke it down to so we can hold on to these components no trouble those components do not interfere with actin's ability to bind or meosin's ability to bind act questions on this so far so good so now we're going to go through crossbridge cycling steps we're going to see we're going to put all the pieces together we're going to see what it does to the circum talk about a little bit of force generation any questions that you'd like clarification on all right well seeing none I'm gonna sort of reset this go to the right side and again we're going to start putting everything together taking all the steps and components that we've been working on and see how they actually do something or make it work question of question and you may have answered this while I was writing but once it hydraizes ATP does it just rebind to actin or does it let go until it's Oh that's a good question so the question is once it hydrayes ATP does it bind to actin immediately is that what you asking does it just rebind right back since it likes yeah it's going to be close there's a little bit of time in between um where we're going to see the meosin heads when they hydrayze ATP they actually have a confirmational shift that happens before they combine actton and that's because we have a lot of moving pieces in play here but it looks really like it it's going to look it happens so fast you don't you you couldn't see it but there is a small bit of time where meosin is going to have ADP plus PI in its cleft the pocket there and it has not yet bound to act but that what happens after that is so fast it it almost looks simultaneously so that's a good question other question okay give me just a minute to get reset What are we doing oh okay all right soon as I can get this focus we get on with it here we go all right so we're going to take a look at what happens during crossbridge cycling or as a result of crossbridge cycling so again this is a temporary we can think of it as a temporary bond that exists between actin and meosin we need to form it we need it to be structurally sound or efficacious while we have swiveling and then we need it to dissolve so we can do it all over again um so the first thing I want to do before we go into the steps is just take a little bit of time to remind you about the details of a sarcimeir um I covered this in pretty good detail in human body one so if this looks familiar that's great it's meant to uh if it's been a while that's fine and I also think it's kind of good to present it again because the meaning of the structure is likely changed because we've just talked about all these different proteins and sometimes it's helpful to see the big picture like remind yourself why are we talking so much about actin and measin so I'm going to talk about briefly the details of a single sarccomir we're going to see how it can shorten and that is the sort of the heart of the sliding filament theory that's been in existence for quite some time i think this really came around in the 1950s if you can imagine what a discovery that would have been at the time because they didn't really have the best technology i mean they they thought they did right because they didn't know but looking back it's like it's crazy what they could figure out with these pretty rudimentary um microscopes and assays so we're going to start by looking at a single sarccomir and a single sarcimeir is bordered or has its boundaries as z lines so that's what these are we've got actin and we've got myasin i'll give you a moment to draw this as you draw I think it's good to extend your actin into the next sarcimeir you kind of have you know components of two additional sarcimeirs on either side this is going to be the focus though so take some time to draw that as you draw this circum is um going to be fairly relaxed and we're going to see that the meosin heads are ready to go with actin as soon as they can so just take some time to draw this and then we'll be out of it it's okay okay that's All right if you have your drawing you feel pretty good about it see what you can recall from human body one if that's not much it's okay it's not like most people wake up in the morning and refresh their memory on you know sarcimeir structure so just think about what have we talked about in the past what have you seen in the past can you recall some of that what are the myofilaments what is their name actin and meosin so if you're like what that's actin and meosin so I'm going to do this first uh in this circum very um small sarcimeir most would have far more units than what we see here but we'd have act up here it's going to be in blue notice actin spans a zline i'll come back and talk about the complexities that that can cause and obviously we have um myasin trying to label it where it's going to be out of the way measin here looks thick because we have all these heads protruding and all their tails coales in the center break together makes a pretty thick filament holding the sarcimeir together are a handful of what are called structural proteins titan and nebulin are big ones there's a third one I'm going to add called actinin talk about titan first titan gets its name because it's a very big protein but Titan attaches to the ends of measin and it um tethers tethers this measin to the Zline and the point of this or the significance of Titan other than it has a really cool name so of course we're going to talk about it but um it prevents myin from wobbling during a contraction everything here moves the sarcimeir shortens the meosin heads are rotating that's a lot of movements and that doesn't work too well if structurally things could sort of bounce or wiggle and so tighten tighten a very large protein helps stabilize measin on the ends uh there will be another stabilization protein that I'll talk about in a minute when we get down to this let's talk about um nebulin nebulin is associated with actin and nebulin has um some strange roles it's not completely fully elucidated what nebulin does i'm going to draw so nebulin usually starts at the Zline and it winds around it can be confused for tropomyiain and drawings don't let that be you but this is nebul nebulin helps stabilize actin during contraction nebulin they think also helps guide if you wanted to add another gactin monomer protein to actin they think nebulin helps that new monomer know where to go and so actin can grow and it can shrink um especially in other cells that are not so much concerned with sarcimeirs but nebulan's not just a stabilization protein it's also sort of a a guiding um force that lets the new molecules know where they go so that's nebul and like I said there's another um protein that I want to mention it seems to be popping up on the MCAT so I thought well we should talk about it and that's actin so nebulan kind of ends at the Zline an actin or actcttenin depending on how you want to pronounce it is going to take that nebula and sort of intertwine with it and it winds up and sort of down our Zline so this was not on my labeling list but actin so these proteins like actin and nebulan actually help form part of the Zline and the Zline of course is the boundary of border with sarcimeir but it's also a conglomerate of different proteins so actin helps hold actin to the Zline speaking of Zline I think that's next up to label it's um pretty straightforward gets its name because it looks wavy not much more to say about that but the Mline talk about the Mline this gets its name m stands for myomisin which is another structural protein it has um kind of a nice sort of serves as a reference point so going down the center of a sarccomir that's the M line and a lot of times you'll just see this as just like a dotted line just like I drew and they don't really go into like what is this like is it just a reference point like a spot on a map what is this but this is actually in addition to these other structural proteins like Titan nebulan actin this M line here actually is represented by a protein called myioin myin it too is a structural protein and it helps stabilize measin in the middle so sort of to recap meosin needs help being stable because it's got to move it's got to swivel it has to work and so it needs to be tethered on its ends and it needs an anchoring point in the middle so that it does not move or wobble if it did you're going to waste energy um having ATP be hydrarolyed but you don't get nice efficient compact contraction of your sarccomir so that's going to be useful so that's just some details about a sarccomir see if you can remember the zones of a sarccomir and then we'll talk about why do we need to be concerned about zones or bands usually we talk about I band and a band h band is sometimes called Hzone so if you see it called Hzone on an exam it's not a trick it just has several different names i usually use Hzone because that's how I learned it so I'm going to be using I band and A band and probably Hz zone but H band is also fine i think sometimes people think it's a trick and it's not it's just different names so let's talk about where we would put these bands or zones try to find some different colors here that'll show up not sure what's low on the color options here so first let's talk about eye band this is the band runs up and down and it's actin only actin only so I'll probably come over here and label it because I don't have so much stuff we're not talking or worried about structural proteins so I guess technically it could include Titan but we're just talking about bands are just specific to measin and actin so this area here and it would go into the next circum as well this is the eye band it is an area that is actin only again we don't worry about what structural proteins may be around but this is our actin only area the A band is actin and meosin so we'll see some overlap and there's two different ways you could draw this um the A band truly is represented by the length of myin but it also includes areas where actin could overlap so this from this tip of a meosin all the way to the tip of that same molecule but the other end that's the A band the A band is actin and meosin actin and meosin H band or H zone that is the meosin only zone so if we look up and down the sarcamir again moving vertically and not worrying about other structural proteins um in the middle here this would be our H zone or H band and it is meosin only question to think about why do we spend time talking about bands and zones it's because it helps us know what happens during a contraction so here's your question to think about before we go any further what bands or zones shorten during a contraction think about that for a minute amen justice All right let's just go through these um Again this question helps make I think this information more useful and not just like another detail that might just seemingly seem like it's for nothing but um let's take a look at the eye band act only do you think it shortens during contraction as Z lines move closer to each other does the eye band shorten act only yes it does the I band shortens so what bands shorten one of them is the eye bands that's because as these Z lines move closer together actin is pulled towards the center due to the work of the measin heads and that shrinks the the actin only zone in fact in a really forceful contraction the actin only area or eye band could disappear completely as it's brought in there is one other zone or band that shrinks during contraction what is that h zone good the Hzone or band also shrinks during contraction and that's because actin is being brought towards the center shouldn't go past we shouldn't see act overlap nebulin should stop that it prevents this from overstretching but we could see it go right like toeto toe with the other actin so the ho would shrink the A band does not change um length area of contraction because the A band is represented by how long is the measin filament remember during sliding filament theory or according to that theory actin slides past measin but it's not as though meosin crumples up meosin stays the same length all that happens is its head swivel they just swivel but it stays the same length they did originally think that measin and actin somehow crumpled up and then somehow expanded and then they're like "Wait that doesn't make any sense Bob." So we're going to change that so it's the bands that shorten or shrink now we're ready to talk about how does this happen and talk about steps of crossbridge cycling we're going to go into the relationship between measin and act i'm going to be giving you the sequence of events on half of this column and then kind of a a visual representation on the other half i've isolated this down to just the interaction between one meosin and one actin filaments because we don't need to draw this like over and over uh it's not it's not very efficient use of time now when you're studying if you find that to be more helpful go for it but we'll just be looking at a very zeroed in area understanding that we're going to have this reaction happen over and over and over again so step one we are starting at the end of the last cycle for reasons that I will explain to you when we're done with the sequence this thing repeats and so we're going to start by looking at how we're going to get myin and actin to dissociate after the last cycle so we're going to start with myin heads being firmly bound to act and we're just going to chart this one meosin molecule as it moves along its little meosin binding site i just put the little bit of a Zline here for reference i think it kind of helps you know how this is moving and where it's going so this measin heads clamp down it honestly um clamps onto the actin binding site as though it was like a a hand coming up and clasping uh actin and you can think of it another way is actin moves through the center or is residing in the center between these two heads so it's a pretty firm bond and the only way we're going to get this to let go is to tease it off with some ATP if we don't have ATP then we cannot get this bond to dissolve it would stay and that could lead to rigor so before we see the next step ask yourself where would ATP fit in this drawing where is our target is it on actin is it on tropponin is it in the masin head cleft that's probably a good option mosin head cleft so let's take a look at what happens we're going to add some ATP to this cleft area right here and remember meosin can't bind ATP and actin at the same time so the ATP once it's in the cleft so it's going to bind to the meosin heads in that cliff that we looked at over here and it's going to cause measin to basically let go of actin so right now we don't have myofilament interaction in this particular this very small instance there is a little bit of space here there would be ATP in the pocket and you can probably see where this thing is about to be aiming for it's let go of its current binding site and it needs to get a grip or a bite on the next binding site so that it can move our actin a little bit more so it sort of when it lets go it kind of already sets itself up for success but it doesn't have enough ump or energy to quite get there so we're going to take a look at what happens as a result of this ATP binding so ATP once it binds to this cleft here does not last very long it is immediately hydraized or broken apart but and this is very strange about measin it hydrarolyes that ATP or breaks it apart but it holds onto its components usually when you talk about you know ATP being hydrayed you see images of the ADP and PI kind of exploding out of the site where it's hydrayed this is not like that um so ATP is going to be hydraized or broken apart by our meosin heads but they hold on to the components it's the hydraysis just the breaking apart again we hold on to the components but the breaking apart um the heads towards the Zline so remember these heads swivel they have a a hinge or a swivel as part of their neck region and it's going to swivel cause those heads the hydraulysis sort of blows the heads back a little bit a swivel and you can see how it's about to it it's really close to its next target which is the next binding site so if we could you know take a really close look at what's going on in this masin head we would still see in this pocket or cleft right here we would still have ADP plus PI and that's different than ATP and because it's different we don't have the space issue remember ATP is different than ADP and PI so in the next step our components even though they're broken apart they're still present within this cleft region and at this point because we don't have ATP we don't have a space issue our measin heads can there's a couple different ways that's you know this is called in the literature but we can think about it as the measin heads lightly bind actin on their next binding site this is not a firm grip yet they're getting close it's not there yet but we're getting really close to having a very strong temporary interaction known as a crossbridge that's going to form between these measin heads and act let's bring in some details that we talked about in the past um remember calcium is still sort of an omnipresent requirement here if we don't have calcium then then none of this really would happen but I just like to remind people assuming we have you know enough ATP for meosin to let go of actin and and hydrayze get another bite and assuming we still have calcium present such that tropomyosin is still rotated off of this thing so two big ifs um but assuming all that is in place then our meosin heads will tightly bind actin and that tight binding is the crossbridge the bond that I talked about this is pretty strong even though it's temporary it's pretty strong and at this point um some simultaneous things happen i'm going to try to break it down so you get an idea of what those are but in reality all this happens so quickly it's hard to to you know like you're not going to be able to observe this um easily even with the best equipment so all of our pieces are in place all we have to do is let me do its thing it's going to have a power stroke and measin heads naturally like to uh flop towards the Mline that's their natural confirmation so because they don't have energy keeping them sort of cocked towards the Zline they're going to take sort of their natural inclination and swivel towards the Mline and since they're attached to actin act goes with them they don't have actin doesn't have a choice so what we see as a result of everything being in place you know we had plenty of ATP plenty of calcium um the meosin heads really they're just buried into this actin at this point it's pretty forceful bond um but they're going to rotate towards the Mline which would be over here opposite of the Zline during this rotation during this power stroke um ADP and PI described as it falls out of the cleft it's not useful anymore um so we would at this point have our power stroke and all this work everything we just worked on here we moved the Zline uh 10 nanometers and that's you know kind of like a let down because that was a lot of work for 10 nanometers but you'd have millions of measin heads doing this as a cycle again not all at the same place in the cycle because that would be very jerky rigid movement but like a nice sort of you know very fluid cyclical um chronology to these measin heads and so what we've just followed is one so that's called a power stroke and then we're kind of back to the beginning but to get this to continue what is required for measin to let go of actin what are we going to have to add atp and once we do that once we add ATP we're back to the beginning of this drawing we're back to the beginning of the cycle back to where we started follow-up question how do we stop all this we spend a lot of time talking about how do we get it to happen how does it happen and we don't get to spend as much time usually just based on time constraints of any course what do we need to do to stop this because at some point we got to stop that contraction what's the what what do we need to do what is required for actin and meosin to interact in the first place calcium so the first thing we have to do to stop muscle contraction as in myofilament interaction we have to remove calcium from the cycloplasm how do we do that what organel is going to resequester calcium cycloplasm reticulum ask yourself where did it come from well it came to the cycoplasm reticulum we got to put it back so we're going to remove calcium from the cycloplasm so the tropparonin complex that would have been on your act here that's full of calcium we're going to have to put it back where we find it so we remove the calcium from the cycloplasm so we're going to return it that's an E return the calcium to the SR that requires energy because if you'll remember when we talked about the SR it always contains more calcium than it lets out so we have a concentration gradient problem so to return the calcium to the SR we have to use pumps these are called circa pumps they use ATP circa stands for sarco endopplasmic reticulum or SR calcium ATPAS is literally a calcium pump stuck in the SR membrane so that's good when we remove calcium from the cycoplasm what rotates and covers the measin binding sites on actin tropomyosin so without calcium we don't have an active tropponin complex tropomyiain is gonna flop back down and block it so we got to get rid of that calcium it's an expensive thing because we're using pumps for that we also at the same time so um at the same time simultaneously the sarmma needs to be repolarizing so we're going to have to remove the sodium reestablish our ion gradient very similar uh distribution of ions that we saw in neurons so stopping muscle contraction in many ways just backtrack just follow yourself backwards well what caused myofilament interaction calcium what's the first thing we need to do to stop myofilament interaction get rid of calcium where does it go back to where we found it so we're just putting things back the way they were so we're ready for the next muscle contraction okay okay at this point um I'm going to end skeletal muscle system we had one more page of notes to get to i rarely get to it so not a big deal there on Tuesday I will be starting the cardiovascular system but you have quite a bit of background from lab please do your uh prelab reading and activity for tomorrow if you have any questions let me know