okay here we're looking at the last piece of muscle contraction this is exactly what happens with the muscle fibers when the muscles actually contract and so you've learned previously about two types of fibers called actin and myosin filaments and the myosin ones have these little heads that kind of look like little ratchets that are literally going to grab onto act in and kind of pull them along like a little arm bending it's pretty nice like that ATP is obviously involved and so we're going to see how this actually works there's another small detail um it's not that small actually but it involves calcium ions and this is why calcium ions are necessary for muscle contraction but we're going to go into that uh right afterwards but first let's summarize in these five steps exactly how muscle contraction actually happens so I've already mentioned that these myosin filaments have little heads like little ratchets that attach to specific binding sites on the actin molecules so starting here with step one the meos and filaments have heads that form cross Bridges when attached to these actin filaments and um this is what happens when it's actually attached and there is no ATP present it actually stays attached when ATP is present as a molecule the ATP which stands for adenine adenosine triphosphate actually binds to the mein head and in the process of the ATP molecule binding to the myosin head it actually causes it to detach from The Binding site on the actin molecule so that's a pretty important detail here it seems kind of counterintuitive but with when ATP is present the ATP will actually be used to help detach this and as it gets detached well then the ATP gets used um hydrolized and gets broken down to adenosine diphosphate and an extra additional in ganic phosphat so what happens is the ATP is broken down and converted to ADP and when that happens it causes the head to actually change angle slightly and if it changes angle then you can see that when it reattaches it's actually going to reattach to a different place and so it's this detaching bending and reattaching at a slightly different place that allows it to actually pull it along and keep on changing so if I'm pulling a rope in a game of tug ofar for example I pull I let go and I hold on to the Rope a little bit further along then I pull I let go with the other hand grab the Rope a little bit further along and I keep doing that process and that actually helps me to pull the Rope towards me and beat my friends and feel very proud of that so you can see that's happening here so when the heads attach to the actin uh filament a little bit further than where they were previously then we're ready to go and then the actual movement the actual Power Stroke as it's called is when that ADP and phosphate is literally released from the head it detaches so these actual molecules detach and that causes the movement of the actual myosin head and that pulls the act the actin sorry the actin filament along so that's called the power stroke and then when you see an image like this you can you can start to understand how we're getting from this relaxed muscle image to this contracted muscle fiber image it's basically all these little heads are attaching and then moving along detaching and then pulling everything back towards the center okay and so that's why the saram Mir actually looks like it's changing in width so the final detail that I wanted to mention here involves calcium ions and uh there are two more proteins that are involved in this so we have actin we have myosin and there's two others called tropinin and tropomyosin and I'm going to use a smiley face and squiggly line and some calcium ions to help illustrate this a little bit um when your brain sends information to your muscle fibers to contract that information is going to be uh delivered through motor neurons and motor neurons are the neurons that are moving that are opposite of sensory neurons and they're the ones that are actually causing your muscle to do things so there's sending signals to eector organs things that secrete hormones or things that will muscles that will actually move and so at the very end of that muscle there will be something that at the end of the signal path there will be something called the sarcoplasmic reticulum which is going to be a little bit difficult to illustrate in this entire thing here but the sarcoplasmic reticulum will release calcium ions so calcium ions are very important for muscle contraction and what's what happens is these calcium ions are going to actually help move something out of the way in order to allow this attaching to actually happen so wrapped around these actin filaments is a protein called tropo myosin okay we're going to see this here all right tropomyosin is like a long fibrous protein that wraps around the actin films we already have some filaments here but there's more stuff that's wrapped around and literally this tropomyosin may actually be blocking The Binding SES so if you if you see these two dark darker colored circles here and that's the binding site for the myosin head uh it may actually be blocked by this particular uh protein called tropomyosin which is wrapping it up so you can think of it as this is these are myosin heads so I would think of this as tropomyosin it's trying to trip up trip up myosin trip up myosin myosin can't bind because it's being tripped up by tripo trip trip up meos or tropo myosin okay so we got to get the tropomyosin out of the way and that's actually the role of these calcium ions because they come down and they bind to these smiley faces another protein these smiley faces are representing tropinin and then when the calcium actually binds to tropinin so let's pretend like calcium is binding to troponin check out what happens The Binding of the calcium to the troponin actually causes the tropomyosin a slightly shift revealing these binding sites for myosin to actually join in okay so the calcium ions come in they bind to a protein called tropinin The Binding to tropinin causes tropomyosin to undergo some kind of confirmational change and then it actually ends up revealing these active sites so no calcium no muscle contraction that's why you got to have calcium besides for healthy bones and teeth as we learned in public service announcements growing up as kids so uh that summarizes all of that together so please take a look and one more question to leave you with uh please pause the video if you're if you want to think about this I'm just going to cut the video short the correct answer for this is actually C for cowboy all right have a nice day