hey everybody Professor Bob long here doing your anatomy and physiology lectures if you see me wearing the same clothes I'm doing multiple videos in one day a short window to try to get some of this stuff done and I'm not Britney Spears or Cher I don't have multiple wardrobe changes so nonetheless I'm gonna be doing another muscle lecture this is muscle lecture I believe it's the most lecture 5 or 6 anyway this is the member will be on the video on my YouTube page and on canvas nonetheless this muscle lecture is going to be over what we call myosin cycling or the cross pitch crossbridge cycling of the myosin molecule during muscle contraction it's going to be rather detailed lecture so I'm going to set us up where we left off last time essentially what we're going to see here is we're going to see a piece of the sarcoplasmic reticulum I'm just going to label it s R for soccer plasmic reticulum which we know the sarcoplasmic reticulum is loaded with calcium ions it is a calcium reservoir and stores tons of calcium ions now there are calcium ion channels here that are waiting to release the calcium and there's calcium pumps that are going to pump the calcium back in that is sitting over our actin molecule now I'm not going to draw this out every single time because we're going to draw a series of steps across the board here but essentially what we're going to see is when the muscle is at rest meaning we're not contracted we are relaxed I'm going to have my little actin filaments or my G actin molecules with their subunits there and their active sites sitting here like this I think I drew one too many but who cares now we have them all in the protein tropomyosin sitting over the active site we have troponin sitting here waiting for calcium to bind and we have our myosin heads I'm only going to draw one myosin molecule but I'll have my myosin head sitting here like this these are the myosin cross bridges before I get too far there's a little bit of setup the myosin molecule is actually an enzyme that can hydrolyze or break down ATP molecules so sometimes we call that an ATPase an enzyme that can break down ATP as you guys know I'm going to draw a quick review here of ATP but ATP is essentially a double-ring molecule of adenosine that's going to have three little phosphate groups attached to it is in these chemical bonds that we store energy as I covered earlier in the semester when we form a bond it takes energy to make a bond when we break the bond it can release that energy some bonds release more energy than they make then it takes to make them and so on and so forth as you guys are aware if I if I break this last phosphate molecule off when I digest it again this is ATP adenosine triphosphate with three phosphates linearly here I can break that down into a molecule of adenosine and the adenosine would only have two phosphates and it would release an inorganic phosphate the one that's not attached organic molecule when I break down the ATP into ADP plus the phosphate some of the energy in that bond is released into the atmosphere or into the universe the surrounding area in some form so breaking down ATP into ADP releases energy anytime I see ATP bound to something I know the energy went somewhere knowing that as a quick review when I look at my molecule of myosin the myosin heads they have ADP plus phosphate bound to them and the energy that went into that that was in that molecule is actually transferred to the hinge I always think of the hinge like the string on a bow and arrow actually I like the bow on a bow and arrow or like the hammer on a pistol if I use the energy in my muscles to pull on a bowstring I'm loading that energy into the boat it's called potential energy it has the potential to do work if I release the string the energy and the bow is released and shoots the arrow or the same thing when you when you pull the hammer back on a pistol you've taken some energy and put it into that little spring and it's sitting there waiting to fire you just have to release it by pressing the trigger so now the myosin head has a high affinity for ADP it has a higher affinity for the active site on actin given the opportunity it would release the ADP in mind actin actin is not available it just holds onto the ATP it has an even higher affinity for the molecule ATP so if I give it three choices ATP actin the active site or ADP it will choose ATP first then the active site on actin and ATP last with nothing else available this is what it buys now as you guys know from the previous video all I need is an action potential which I always abbreviate as ap to come down and shock these calcium channels into opening when calcium channels open the calcium ions would leak out bind to troponin and cause it to change shape prior to that happening we have this set up the myosin head is sitting here holding on the ATP troponin and tropomyosin recovering the active site on actin so the first step and all this is going to be called active site exposure and this is outlined in the book and in my notes say an active site exposure in order to expose the active site all I eat is the calcium to bind a troponin troponin will change shape and pull tropomyosin off the active site I'm not going to draw both sides of this molecule in the entire setup every time so that it's a little easier for you to follow if you're drawing this so here's one row of my actin subunits but if you notice now that calcium is bound the tropomyosin has been pulled off to the side because troponin has calcium bound to it now that the active site is exposed by myosin head which would be sitting here on the cross the bridges binding to ADP and phosphate will now release some of the energy here it caused the hinge to reach up and attach to the active site on actin which is the next step in the process is called cross bridge attachment and all that's going to happen in the situation I'm not even going to draw the troponin to the tropomyosin I'm just going to draw my little actin subunits here with their active sites exposed essentially what's going to happen is the hinge is going to allow the myosin heads to to reach up and bind and when they do they release the ATP plus the phosphate and we can put a little eye on there for inorganic phosphate but now just like I talked about at the last video if I asked you to hold a couple of footballs under your arms and I threw you a live baby since you have a higher affinity to save a person you would release the footballs and grab the baby and then you would change your confirmation well as soon as the myosin head attaches to the active site on actin it releases the ADP and it was it's gonna do what's called a pivot which is going to be the third step in this process which is going to be called pivoting or myosin pivoting and all that you're going to see is this exact same picture that I already have here's the same seven the actin subunits I think I drew eight one two three four five six not like that so but if you notice the myosin hinge has allowed to pivot or pulled this way and since the Z line would be at the end of this actin filament it's going to pull the Z line towards the end line which would be over here essentially we're looking at this happening and we're seeing myosin heads attach while others are reaching out and as soon as they release their ATP they're going to pull the Z lines towards the M line and they would do the same thing on the opposite side here pulling the actin filaments pulling the Z lines towards the M line contracting the sarcomere now because every circle mater on the myofibril will have calcium available to it due to the structure of the muscle then every sarcomere will contract throughout the entire length of the myofibril the myofibril contracts the muscle cell contracts and so on and so forth now once we tipping we have a step that's going to be cross bridge detachment in order for the cross bridge to detach because the myosin heads our cross bridges have a much higher affinity for ATP than they do the actin if a molecule of ATP floats by they'll let go of the actin and grab the ATP and detach and that's exactly what you're going to see in this step is that they will actually bind a molecule of ATP and release from the actin subunits and then as they break down the ATP they're going to reactivate and essentially what we're going to see in the next step and the last step is going to be reactivation of the myosin molecule we're going to see it go back to the shape where it has ADP plus the phosphate mound and here's my actin subunits now if the action potential is still causing the release of calcium when I reactivate if I'm here if the active sites are exposed it will cycle again the myosin heads will reach up and attach then the molecule will release the ADP in pivot it will bind ATP and detach it'll break down the ATP and reactivate and it'll continue cycling like this one myosin head cycling over and over and over as long as the active sites are exposed and I can have several myosin heads cycling this way burning tons of molecules of ATP if you look at the circle of mirrors on the myofibril models in lab and you try to count the number of sarcomeres on one myofibril plus the number of myofibrils in the muscle cell think about the length of the size of one of those muscle cells how many millions of molecules of ATP were burning a refraction of a second you can understand why we get tired when we have lots of muscle contractions we call this the myosin cycling or the crossbridge cycling for a muscle contraction and it explains where all the energy goes again if we're sitting here at rest the myosin heads holding ATP the active sites are recovered as soon as an action potential hits the muscle cell travels down the circle level down the T tubules and shocks the sarcoplasmic reticulum into releasing calcium calcium binds to troponin troponin changes shape and pulls tropomyosin off the active site once the active site is exposed the cross bridge will attach release a BP and pivot bind ATP and detach breakdown ADP ATP and the ATP in and reset it'll go back to either step 1 if the active sites are still exposed or if we've ended the release of calcium it will just sit here waiting for the next opportunity for you to contract a muscle that's how we burn energy and muscle contraction that also needs us somewhat to understand the process of what's called rigor mortis rigor means difficult or tough or stiff and mortis comes from the word left they're more thing which means dead so it means dead stiff whenever you see an animal that has stiffened up and it's dead and the legs are sticking in the air or a human being is all stiffened when someone first dies as our cells start to use up all the ATP or their lack of oxygen and carbon lack of oxygen and glucose stops the mitochondria and muscle cells from producing ATP then the availability of ATP goes down with no ATP available I can't have the pumps functioning so some of the calcium could slowly leak out expose the active sites the myosin heads would bind release the ADP and pivot but they cannot detach so the muscles don't curl up and you don't cramp up they just tighten in whatever position you're in and we cannot release that takes a period of time for the onset of rigor mortis over time enzymes from vesicles inside the cell lysozymes will break down all the proteins and if I chopped up all the proteins eventually the person would go limp again or the animal so that explains what's going on with rigor mortis which is some further support for the concept or the theory behind this cross bridge cycle anyway as usual I hope you learned something I hope you had as much fun as I did for those of you that are in my class this is in the note set this is following along on page Believix page 56 page 55 was the last lecture where we went through the muscle contraction it's all spelled out for you please look up the worksheets that will be provided if they're not up yet be patient I'm working on all this fill in the worksheet understand the concept and there'll be some quizzes coming up online in the near future again this is all due to this coronavirus outbreak and we're having to do everything online so I'm doing a series of videos this way I hope this is helpful those of you that are not in my class maybe you learned something maybe is a little easier to understand or maybe my lectures are terrible I don't know whatever you think alright see ya