the recall activity here again because I feel like we've had a large volume of new information and I want to make sure we're all up to speed on that content um so I'll just put you into breakout rooms for maybe like five minutes tops on these um so you can answer them together ideally without your notes and then we'll come back as a group the first question is um to list and briefly describe each of the four regulatory proteins on the thin filament so I want to know specifically what are the four different kinds of regulatory proteins found on the thin filament and very briefly what's the function of each of those the second question here is asking about the sliding filament model what is the sliding filament model during the sliding filament model we talk about specific parts of the sarir that change which regions of the sarir change during sub uh during contraction and then alternatively which regions of the sarir stay the same okay so again I'll put you into breakout rooms for about five minutes try and do this without your notes and then we'll come back and we will go over this together let's [Music] see all right all right I think we're all back now so to recap as a group um what are the four regulatory proteins on the thin filament and and let's even take that a step back someone give me an example of one regulatory protein yeah Carter I was just a little bit confused on this because um I think I think it's um it's it looks like there's only two but you know how one of them is split up so yep so so troponin there are actually three different kinds of troponin each of those troponin is its own separate protein yep that's a that's a good question does that make sense so collectively there are four so the three different chonin those are different protein isomers right isomers being different varieties of the same structure good question okay so what's an example of one of those troponin then since I already mentioned that one what's one kind of troponin tropon and I tropon and I very good Haley and which is tropone and I do um attaches to actin very good that attaches that whole protein complex to act in so it's kind of like the anchor on the ship right it's what keeps everything on that thin filament in place very good what's another kind of troponin see which attaches to the tropomyosin very good thank you Caitlyn we also have troponin t which attaches to tropomyosin very good t for tropomyosin um and when it does that it pulls tropomyosin out of the way right and then what's the Third Kind of troponin Carter is your hand up oh I didn't even realize it was it still up but I'll answer it and it's the one that binds to calcium very good there's a third tropon tropon in C C because it binds to calcium very good so when calcium binds troponin C that communicates to troponin T that it's time to attach to tropomyosin tropomyosin of course being the fourth regulatory protein what is tropomyosin so that it doesn't bind to the thi filament very good thank you Emily so it it's all along the actin and it covers the binding site for mein so that the thick and thin filament cannot bind to each other when tropomyosin is in the way very good okay next question what is the sliding filament model what is that Carter um so I think the sliding filament model is um it just explains how um the the the are they myofibrils or just fibers muscle fibers so they're they're inside the myof fibral but we're specifically talking about filaments within that we're talking about the the sarir right those units of sarir and then the thick and thin filaments okay um yeah I just had to there had a little bit of termin te ology um so in the sliding filament model the the filaments don't actually shorten the thick and the thin don't actually shorten themselves but they just um they slide past each other um making those um eye bands and H zones shorter so oh sorry I spoiled no that's good that's good that's very very good so you've answered the next part too right so Carter said a couple of really important key points there number one the sliding filament model states that thick filaments and thin filaments don't actually shorten during contraction instead they slide past one another right they slide past one another when our muscles contract the result is that the H Zone shortens right the H zone is that area around the M line where there is no thin filament it's only thick filament also the eye B band shortens the I band is the region around the zline the anchor of the thin filaments where there's no overlap with thick filament um and then the region of the sarir that stays the same is the a band right the a band is from one tip of the thick filament all the way to the other tip of the the thick filament so collectively the regions that get shorter and the regions that don't change at all that how we know that those filaments slide rather than change their shape right is everyone with me on that okay are there any questions before we move on with new content okay so I'm going to get my laser pointer up right so um kind of what were speaking about was this idea that these um thick and thin filaments slide past one another right um and collectively that causes the whole sarir from Z to Z to shorten but individual elements like the thick filament itself and the thin filament itself those don't change their shape but proportionally different regions change their shape because of that sliding motion right um and then I went on to show you some really cool scanning em images of skeletal muscle when it's relaxed versus when it's contracted so you can see these differences right so here's a sarir up at the top that's relaxed so here is one zline and here's the other Z line um and if we compare that to when it's relaxed we see a couple of really important things number one we see that those Z lines are closer to each other so collectively the entire sarir has shortened in addition to that we see these I bands which are the region on either side of those Z lines the eye bands have shortened right we see the H Zone in the middle has shortened that one's a little bit harder to see right but it has shortened and then we see this a band the end of one thick filament all the way to the end of the other um or to the other side of that thick filament that has not changed right so again this is what's demonstrating to us that those filaments slide past one another they don't actually shorten and change their length individually okay so now we're going to talk about the crossbridge cycle and how the muscle generates force and I have a picture on the slide of my family and I in a canoe a couple of years ago um which very much felt like a sinking ship at the time um so your book makes a refence reference to the crossbridge cycle and it's kind of like an ore paddling a boat um but I feel like this is Northern Minnesota so we should use the reference of a canoe and a canoe paddle right um and this is a really chaotic scene there's me and there's my three children and my husband and the dogs in the front and I look very concerned and worried that we're going to tip and everyone else is having a good time um this is kind of like what's going on in the skeletal muscle every time it contracts right we've got this mechanism that's kind of akin to a new paddle moving along in the water um and it seems chaotic there's so much going on but there's actually some synchronicity to the Rhythm right there's actually some sort of synchronicity going on so let's talk about what's going on here it's not actually halfhazard and chaotic like this scene here so this crossbridge cycle is referring to it's a cycle right it's a referring to a cyclical formation that links between the actin and the my right the thin filament and the thick filament resulting in that sliding of thin filaments toward the mline of the sarir okay so that's kind of a mouthful there's a lot being stated there but this is all stuff that you've already learned right the actin and the mein connect right that mein head it's got that binding sight on it and as soon as tropomyosin moves out of the way on the actin that actin head or that mein head excuse me attaches to the the actin on the thin filament and the result of that is that the thin filaments get pulled toward the center of the sarir which is that mline remember the midline and I said that this is analogous to rowing a boat through a water or through water or I think more appropriately analogous to paddling a canoe right and so to that end um we can think of the crossbridge itself is kind of like the or paddle or the new paddle every time we link together that thick filament with the thin filament so every time the mein head binds to the mein binding site on actin that's kind of like when the Cano paddle makes that initial contact with water right then we have something called the power stroke during the power stroke the masin head actually moves moves so it changes its position right and I'll show you on the next slide so you have a visual of this but the the mein head actually its head back and then it moves forward so it moves its head its position in space and that's what propels that thin filament toward the center of the muscle toward the mline or the midline right this is analogous to the ore or the canoe paddle propelling the boat forward in the water right so you've got your canoe paddle you stick it in the the water and then you pull it back right that's what's analogous to this power stroke then the thick and the th the thin filaments detach from each other they separate kind of like when the canoe paddle breaks contact with water and then the mein head returns to its initial position and the entire cycle starts over again okay so this entire process only happens when calcium is present right so this is very much dependent on calcium calcium is kind of like the All-Star player in the skeletal muscle and in muscle in general right because when calcium binds troponin c troponin t moves tropomyosin out of the way and that allows the mein head to contact the actin right so this only happens in the presence of calcium so let's take a look at a visual of what's actually going on during this crossbridge cycle and did everyone get a chance to write this stuff down do I need to pause for another second okay okay so here we've got the collective crossbridge cycle um as it happens in the skeletal muscle and I'm going to go through this figure step by step to help make some sense of it Oh I thought I had this animated in okay I guess I don't that's okay though um so we'll just start here at number one right so this first step is The Binding of mein to actin so let's look at the figure that goes along with it so here's our thick filament remember the thick filament has all of these mein heads attached to it and the mein head is connecting with that little black dot right that's the mein binding site on the thin filament you should notice about the thin filament a couple of things number one there's this little green circle attached to the troponin complex that little green circle is calcium so calcium in this figure calcium is bound to troponin C and as a result troponin T has pulled tropomyosin off of that mein binding site so you'll notice all of these little black dots all along the length of the thin filament those meas and binding sites they're all exposed that's really important okay and that is only like that because calcium is present so just to orient you to what's going on in the thin filament first okay so again we've got mein the mein head specifically is bound to the mein binding site on the thick filament right that's that first step that's kind of like when the canoe paddle initially makes contact with the water okay you'll notice that the other functional site of the mein head that's that atpa remember we said that's the site where there's enzyme activity to hydroly or break down ATP which releases energy so you'll notice that in this first step in this first um portion of the cycle ATP is not on the mein head here instead it's ADP and an inorganic phosphate so that tells you that that reaction has already occurred right ATP has already been hydrolized okay so moving from the first step to the Second Step that inorganic phosphate is released from the masin head and now the mein head goes from being cocked back like this to moving forward okay does everyone see that in this figure here in Step One the mein head is cocked back and here in step two it's gone from this position to this position okay that's really important that is called the power stroke so that's like when the canoe paddle is in the water and we paddle ourselves forward right at this stage at this stage masin the head is considered to be in its low energy form meaning this is its preferred State masin prefers to be cocked forward like this rather than cocked back like this okay so it's low energy State cocked forward that's its preferred State because it doesn't require energy to maintain that state it's kind of like the default in this state where it's like this this is said to be rigger has anyone heard of this word before has anyone heard of the term rigor mortise so rigor mortise is this idea that um after following death and this is true in all different animals following death the body becomes very stiff the reason for that is because in order for muscles to relax it actually requires energy so when the body and stop synthesizing new ATP it's unable to relax the muscle because ATP is what is required in order for muscle to relax so as myosin is in this preferred state where its head is kind of cocked forward like that that is called rigger and so the ADP has been released and it's just stuck to the act in thin filament contracted and pushed forward so then right so we've kind of come in into the midline here right the the sarir is contracted at this point so then a fresh molecule of ATP comes in and binds to that binding site on the mein head right that atpa site when that happens mein releases from actin right it comes off of the thin filament so those two filaments become separated this is like when the canoe paddle when we're back here here when it comes out of the water and we're about to start the we're starting to um do the cycle over again right so we've come off of the thin filament and then this ATP because it's bound to an atpa this ATP becomes hydrolized meaning it gets broken down to ADP and an inorganic phosphate and in doing so you should notice a distinct differ difference in the mein head here in stage four as compared to here in stage five what do we notice that's different over here other than ATP has been hydrolized do you notice anything different about the orientation of that Myas yeah Carter um so yeah it's it's like it's not I don't know how to explain it's um it looks like it's relaxed but it's not it's just it's brought closer to the um the the other filaments under it um and sure and right so it's not extending out toward the thin filament but what about the orientation of the heads here those masin heads do you notice anything different about those I'm just wondering if there's any time where does it lay flat at any time or so the M heads are always going to stick out they'll always stick out yeah so look at the angle of the head here versus the angle of the head here what I want you to notice is that in stage four when ATP comes in and binds to it the head is like this right it's in that low energy preferred State and then in stage five after ATP has been hydrolized and energy has been released the head is now cocked back like this right it's like we've loaded a spring it's gone from here in its low energy State now it's harvested that energy from ATP hydrolysis and boom it's cocked back into this high energy state does everyone see that look at the angle of this head here so here it's pointing backwards versus here it's pointing forward does everyone see that difference so that's really important to notice and again this meas and head being cocked back that's said to be a high energy State meaning that protein does not prefer to be in that position it prefers to be like this right but it's cocked back right now in that high energy State because it's harnessed that energy from ATP so now that it's in this state it can pull thin filaments from over here toward the midline okay so we can imagine that over here is the zline and over here is the midline right so if I take a thin filament that's closer to the zline and I Snap It Forward closer to the midline that shortens the entire length of the sarir Carter did you have another question oh you're muted does so does it want because it looks like it wants to be perpen like the uh the filament itself and then the the mein head it looks does it want to be per like perpendicular so yeah so the the head okay so the filaments themselves are parallel to one another but the head is in these different states of being sort of perpendicular right it's it's not perfectly perpendicular in either of these because that would just be pointing straight up okay the high energy phase is pointing backward toward the zline the low energy phase is pointing forward Ward toward the midline toward the center of the sarir neither is perfectly perpendicular theoretically it's some point during the power stroke it will be perpendicular for a very brief moment as it transitions from here to here does that make sense yeah I think I'm just having a hard time looking at this diagram okay so what other questions do we have who else is having a hard time with this figure or what what questions do we have about what's going on here so after it un like attaches does it just go back to the resting state to act in yeah so after it unattached so it unattached before it goes back into the resting state the the low energy preferred State sorry let me rephrase that because I said that backwards but before it unattach it um it goes back into this high energy state where the head is cocked back after the filaments detach so it's in this low energy State during rigger ATP comes in and that pulls the mein head off of the thin filament but it stays in that low energy State while it's being pulled off and then once it's pulled off the atpa hydrolyzes ATP releases energy and then that the head back into its high energy State I don't know if that answered your question especially because I misspoke at the beginning does that make sense a little bit but what point does like the actin does the actin move or is it just the the filment yeah good question right so the actin moves very good so the only thing on the thick filament that's moving is that mein head it goes from here to here here here to here but the thick filament itself doesn't move at all the head on the mein it grabs onto the thin filament and as it does this power stroke as it does that it pulls the thin filament with it right so it goes from here to here and that takes both ends so it happens in opposite directions on either side of the midline so that both ends of the Z lines come closer to the m line Carter did you have a question a followup uh no I just had my hand up on accident okay Haley what question did you have yeah so um when it says like the cocking of the masas head when it's like leaned back and it's in high so is every time it's leaned back it's high energy or is that not right yes that is absolutely right every time it's cocked back like that so it's in this position it's pointing toward the zline pointing toward the end of the sarir and every time it's like that that's its high energy form and we know it's high energy because it's taken on the energy from this ATP right that ATP has been broken down and seven kilal of energy have been released and that energy has been used to to point that mein head back to to get the spring load ready to go does that make sense um does it does high energy form mean it has high energy or needs it means good question it means it has high energy right it's taking on energy to put itself into that position so that is a high energy state which is important because in order for it to move forward to do the power stroke that's work right that is the work that's being done every time your skeletal muscle contracts does that answer your question yeah okay Carter what question did you have okay so I think I got a better it looks like Yeah because it look so it looks like the mein head when it's relaxed is you can see that from the filament that it's attached to that and it's relaxed form it's it looks like it's you know makes a perpendicular 90° angle and then when it gets cocked back it becomes it doesn't come completely parallel to the actual filament but it like you know it that's how it stores its powers that it back and then it's almost parallel but then it you know as it attaches to the ACT and it gets um then yeah when it attached to them the Acton and then the energy is released then it goes back to that perpendicular state of sorts and then right acting along that's what I very good very good yes so we can good so um I really want everyone to spend some time with this figure over the next 24 hours um and make sure you understand these different components here right ATP is really important because energy from ATP is what pushes that mein head back right it's like spring loading a gun or something I don't really know mechanics of a gun but um I I guess it's like it's like a springload I do know that I do know that um so pulling it back that's like a springload here right it's it's got all of this stored potential energy that's the high energy form and then once it grabs on to the thin filament that's when it engages in the power stroke and boom it pulls the thin filaments closer toward the center of the sarir which shortens the entire length of the sarir so again spend some time with this figure over the next 24 make sure you understand each of these different steps make sure you understand the different energy states of that mein head the low energy State versus the high energy State and how the the the main difference is just the position of that head is it forward or is it cocked back and also make sure you understand that none of this happens if calcium isn't around calcium must be present otherwise mein can't grab onto the thin filament okay all right any lingering questions here before we move forward okay and these have all been really good questions by the way so keep asking them as you have them so the next concept that we're going to speak about is connecting this contractile activity in the muscle crossbridge cycling and then the sliding filament that happens with it we're going to talk about how that's all connected to activity in the motor neuron so remember the motor neuron it's this peripheral neuron that's a part of the sematic nervous system the voluntary branch of the nervous system that is all very important for determining when muscle contracts versus when it relaxes we're going to talk about a concept called excitation contraction coupling which just means how excitation in the motor neuron is coupled or connected to contraction in the muscle right how the neuron is going to determine whether or not muscle contracts um so again this is a sequence of events whereby an action potential in the motor neuron is going to trigger an action potential in the sarcolemma right the membrane of the skeletal muscle to cause contraction so this is all dependent on neural input from that motor neuron and it requires calcium release from the sarcoplasmic reticulum so remember the sarcoplasmic reticulum that comes off of the lateral Sachs right that are connected to that t tubal system and there's a lot of stores of calcium in there and remember we need calcium because it binds to troponin C which moves troponin T and tropomyosin out of the way okay so these are the steps of excitation contraction coupling again how muscle contractions are turned on and how they are turned off how that's all determined by that motor neuron so the first thing that happens um actually the first thing that happens that's not on here is there's an action potential in the motor neuron so you could even write that in your notes that that's really like the the pre-step um the prequel to the series of activities here there's got to be an action potential in the motor neuron because of that action potential in the motor neuron that's going to release acetyl choline in that neuromuscular Junction and that's going to stimulate an action potential in the sarcolemma and again that sarcolemma that's just referring to the plasma membrane in the skeletal muscle in addition to that action potential in the sarcolemma that's going to carry that action potential will continue and it will move down into the T tubules so remember the T tubules they go into the belly of the muscle right so it's kind of spreading that electrical change that electrical signal into the belly of the muscle dhp receptors in the T tubules this is just a kind of receptor it's dihydroxy oh I forget what the P stands for off the top of my head you don't need to know that just know it's a dhp receptor that's going to um it's going to communicate that Action Potential from the T tubule to open up calcium channels in the sarcoplasmic reticulum those calcium channels in the sarcoplasmic reticulum are called ryanodine receptors so this is something called calcium induced calcium release right calcium from these dhp receptors um a membrane potential change from those dhp receptors is going to cause ryanodine receptors to release calcium collectively all of this action potential activity is going to cause a surge of calcium into the cytool calcium binds to troponin specifically it binds to troponin C and then troponin T will shift tropomyosin out of the way and then crossbridge cycling occurs okay so that myosin head will back it will move the thin filament right all of this happens because of an action potential that starts in the motor neuron and that carries a change in membrane potential down into the belly of the skeletal muscle and calcium gets released from the sarcoplasmic reticulum which opens up that binding site for mein to connect to actin all right so I've got a visual of this here on the next screen and this one I do believe oh shoot I thought that was animated I must have taken all these animations out um again that's okay we can just go through it together here um so we've got our motor neuron right that's going to communicate with the skeletal muscle at the neuromuscular Junction remember the neuromuscular Junction it happens along a region of the skeletal muscle called the motor end plate remember the skeletal muscle those fibers they're really long cells the entire cell does not communicate with the motor neuron it's just one particular region and that region is called the motor end plate and that's where we have all of these different receptors that bind acetyl choline remember those are nicotinic col energic receptors so we've got an axon or an action potential excuse me that comes screaming down the axon of the motor neuron right that causes acetylcholine to be released into the synaptic CFT here at the neuromuscular Junction acetyl choline binds to these nicotinic colonic receptors and that causes an action potential in the skeletal muscle that action potential is going to propagate along the sarcolemma so all along the outside of that muscle fiber membrane and then it's going to carry down through this T tubal system so remember the calma kind of wraps the outside of the cell the outer membrane the T tubule is like a tunnel system that goes into the belly of the muscle right it's said to invaginate into that belly of the muscle so that action potential gets carried down through the T tubal system and then that triggers calcium release from the sarcoplasmic reticulum so we've got these lateral sacks here that extend out into the sarcoplasmic reticulum right as the action potential comes down through the T tubal system calcium is released into the cytool calcium binds to troponin C and that causes tropon and T to grab tropomyosin pull it off of the mein binding site and then crossbridge cycling will begin so as long as calcium is present in the cytool here theoretically this will continue ATP also has to be be available when um when the skeletal muscle relaxes calcium gets pumped back into the sarcoplasmic reticulum and that causes the muscle to relax is everyone with me so far what causes it to relax again calcium gets um pumped back into the sarcoplasmic reticulum that's done by a protein called sarcoplasmic reticulum actually sarcoendoplasmic reticulum atps that's a mouthful um it's abbreviated Circa and I think I have it listed on the next slide or the the slide after that so we we'll talk about in a second the Protein that's involved in pumping that calcium back okay so again just to remind you what's going on with the regulatory proteins on the thin filament calcium during contractile activity calcium is released from that sarcoplasmic reticulum and it binds to tropon and C tropon and C having calcium bound to it signals to tropon and T hey you better grab that tropomyosin and move it out of the way so that the mein head can bind to the thin filament okay so that's how the muscle gets turned on right it's all about calcium being released into the cytool and that's connected to an action potential in the monotor neuron that spreads to the skeletal muscle that spreads down the t- tual system so the other piece of this is relaxation right how do we stop Contracting and as I mentioned previously that happens by getting calcium out of the cytool and back into the sarcoplasmic reticulum so calcium must leave troponin when calcium is pulled off of troponin tropomyosin will move back to cover that binding site for myosin on the thin filament and then to get calcium out of the cytool we have a calcium atps in the sarcoplasmic reticulum that is that Circa receptor that I was telling you about and I'll actually type it on the screen here so that everyone's Ultra clear on this Okay so we've got a Ciro receptor it's an atpa so that tells you that ATP is required to pump calcium into the sarcoplasmic reticulum in other words it takes energy for muscle to relax okay so I've got a video on the next screen here that um kind of nicely wraps all this up and puts it together and I'm going to just turn my video off so I don't get kicked out here typically a single motor neuron arising in the brain or spinal cord conducts Action potentials that travel to hundreds of skeletal muscle fibers within a muscle the sequence of events that converts Action potentials in a muscle fiber to a contraction is known as excitation contraction coupling if we look at a single muscle fiber we see that an action potential travels across the entire sarcolemma and is rapidly conducted into the interior of the muscle fiber by structures called transverse tubules transverse or t tubules are regularly spaced in foldings of the sarcolemma that Branch extensively throughout the muscle fiber at numerous Junctions the t- tubules make contact with the calcium storing membranous Network known as a sarcoplasmic reticulum or Sr where it abuts the T tubule the SR forms sacklike bulges called terminal syy one portion of a t tubule plus two adjacent terminal cyy is known as a Triad the membranes of the T tube and terminal syy are linked by a series of proteins that control calcium release as an action potential travels down the T tubule it causes a voltage sensitive protein to change shape this shape change opens a calcium release channel in the SR allowing calcium ions to flood the cytoplasm this rapid influx of calcium triggers a contraction of the skeletal muscle fiber thus calcium ions are responsible for the coupling of exitation to the contraction of skeletal muscle fibers okay so it is 8:50 we're g to stop here we'll wrap up our conversation on this packet of skeletal muscle on Wednesday we probably won't get through the entire packet so don't worry about that um so we'll spend the first half of class reviewing we'll probably get through the twitch um an isometric and isotonic contraction and then we'll spend the second half of class reviewing for the exam if you have questions that you um want to ask or if you want to review any topics with me before Wednesday please come to office hours um you can also reach out via email and set up an appointment or just stop by my office um good luck studying uh take care of yourselves and I will see you all on Wednesday thank you thank you