in this video we'll discuss sarcomeres now a mirror is like a unit of something and Sarco we mentioned before means flesh or meat basically it means muscle so a sarcomere is a unit of muscle just based on that name sarcomeres must be pretty important and yeah very important so let's look at them revisiting this wild wild diagram of excitation contraction coupling will remind ourselves that when our brain decided we wanted to voluntarily move some skeletal muscle we initiated this series of cascading events that eventually flooded the muscle cell with calcium but why well the secret to understanding that lies in striations but really will see that striations if you look more closely at them are organized into little subunits little sarcomeres so understanding sarcomeres is the key but let's slow down for one slide and look at what are striations well here's a single myofibril and remember each muscle cell is packed with hundreds of thousands of these myofibrils the arrangement of these myofibrils like the stuff that they're made out of is perfectly aligned meaning if here is one muscle cell the striations run the length of the muscle fiber the muscle cell so if we said hundreds of thousands you'd have to draw hundreds or thousands of lines within this muscle cell but everything inside those lines those myofibrils is perfectly aligned meaning that there's some dark stuff in them and some light stuff and dark stuff and light stuff and all the dark stuff perfectly lines up and all the light areas perfectly line up so you get these striations throughout the cell these dark regions within the myofibril are called a vans and let's note that the word dark has an A in it so this is in a band and here's another a band and here is a third event there are also eye bands the lighter colored bands and let's note that the word light as an image so the dark bands are a bands and the light bands are the I bands here's an eye band here's an eye band so what does this have to do with sarcomeres well sarcomeres are these units which just repeat over and over and over down the length of the myofibril so if you find the eye band and you look right in the middle of it you'll see this zigzag structure called the z disk and then if you find another z disk in the next eye band what you've done is you've captured a muscle unit sarcomere this region will repeat both directions again and again and again just copies of this sarcomere thousands or tens of thousands of times down the length of that muscle fiber so if we can understand what happens in a single sarcomere and multiply that thousands and thousands of times that's muscle contraction as a very earliest approach to this concept look at my fingers is there a region that you would call the a band hopefully perhaps here in the middle where my finger tips are found there that is thicker there's more fingers there that is arguably a darker region that would be like an a band and on each side of that would be eye bands and in muscle contraction what will happen is that things will slide over each other remember these sliding filament theory and the sarcomere will shorten I mean on a subcellular scale a very small amount of shortening but you do that eighty thousand times down the length of a muscle fiber and you do that in a bunch of fibers within the cell you can get all kinds of muscle contractions and not just my waving arms but also the expressions on my face all done by sarcomeres contracting so what are in sarcomeres myofilaments which are there are thick ones and thin ones so the thick myofilaments it's kind of a fancy way of saying these muscular filaments that are made of protein or myosin and then the thin myofilaments these thinner muscular filaments that are made of protein are called actin so myosin and actin are found within the sarcomere in a very particular arrangement if we look again at the a band we'll see that there are both red and blue lines on this diagram and the thicker ones that happen to be red our myosin the thinner ones are acting and then if you look in the eye band you'll see only actin we'll look at that again as we start to name some of the other structures on the sarcomere but here's a quick look at myosin and here's actin it's tough to tell here but myosin is a much thicker bulkier protein okay so looking more at this sarcomere it's a slightly different diagram so let's get our bearings sarcomeres always start and end at those z lines or also known as z disks those are in the middle of the I bands here's one sarcomere anywhere you see this thick mile filament in blue in this image that's myosin but I can also see here mixed in with that myosin is actin so the a band has both myosin and very small thin actin filaments mixed together but in some regions there's only myosin so in the a band what do we call that region that has only myosin that's the H band what do we call the very middle where these myosins are all anchored together that's the M line z disks we already discussed a bands and AI bands we've discussed yeah we're looking good so that's just some basic geography of a sarcomere mostly what we're interested in though is the regions of the sarcomere in which the myosin heads see the little bumps on the myosin here come into contact with the acting because that's where the muscle contraction really happens so let's look more closely here's a view of myosin we can see that a single myosin molecule has a long tail a little hinge region and then a globular head and if you can't quite tell there's actually two globular heads on the end of this myosin molecule bunches of those mice and molecules are all wrapped together if you look at an actin molecule maybe these actin subunits it's like two strings of pearls wound around each other that's our actin and the myosin heads really want to grab on to the actin at the active sites and when your muscles are contracting what's basically happening is that myosin head is accessing the active site and it's pulling on it but there are some other proteins accessory proteins called troponin and tropomyosin that are gonna have something to say about whether or not that myosin head can attach to actin so let's look at that on the next slide here we're gonna find out whether or not the myosin heads can pull on the actin and they do that in both directions by the way so those myosin heads they want to attach to the active site on the actin molecule but they can only do it when calcium is present so finally we're understanding why when I think about contracting them voluntary skeletal muscle why does that cause calcium to flood into my muscle cells the answer is because calcium will bind with one of the accessory proteins called troponin and it will cause a structural shift that will then cause a change in the other accessory protein here tropomyosin which will shift aside because right now that tropomyosin this kind of green cable is blocking the active site and you know who's keeping it there troponin troponin is like the glue or the clamp the clamp that's better it's a clamp that's holding tropomyosin in place if you unscrew the clamp with calcium then tropomyosin will shift out of the way and then the myosin head grab the active site and start to contract the muscle and it'll keep doing that until either there's no more calcium or there's no more ATP for energy to cause the myosin head to pull pull pull pull pull you probably won't run out of ATP anytime soon but you might run out of calcium if you're thinking contract contract contract you know I'm flexing and flexing and flexing my brain is firing a signal that's causing the calcium to flood the cell but as soon as I decide no stop contracting my muscles then that whole cascade of events stops the calcium goes back into the sarcoplasmic reticulum and there's none there and then tropomyosin will get clamped back into place no muscle contraction will occur which is basically what's discussed here so that's what happens in just one single sarcomere you calcium you need ATP and then the myosin heads will pull on actin causing contraction in this case flexion of the elbow and if you look closely at a sarcomere you'll see these z-disks will slide closer to each other during that contraction and that explains at an appropriate level of detail muscle contractions work according to the sliding filament theory of muscle contraction we so smart now