this first video on the muscular system will really have two components first there'll be an introduction where we talk about some general concepts of the system and repeat some of the information you've heard previously in the course in the second part when we look at muscle structure we'll just say what happens if you take a muscle and cut it open and look inside what little bits are in there and if you look inside those bits what bits are those bits made of and we'll just keep looking at smaller and smaller bits of muscle until we get down to the cellular level and then the subcellular level and all of this understanding of muscle structure we will build on in the next few videos to understand how muscles contract so let's get started functions of the muscular system there are many but we can start with something basic like movement now the muscular system allows your skeleton your body to be moved around but there are many other types of movement the beating of your heart the movement in the walls of your hollow organs which allow for things like birth ejaculation digestion and so on and we could go on from here like how your hairs are moved to raise up thanks to your erector pili muscle or how your pupils dilate and contract thanks to smooth muscle in your eyes and so on so lots of types of movement the muscular system also helps you maintain your posture and your body position so think about even just subtle shifting in your chair requires quite a lot of muscle contraction and relaxation at the appropriate times to help maintain your position and posture the muscles also stabilize the joint thanks to tendons that span joints and something called muscle tone which means basically that in your muscles some cells are always firing and kind of maintaining tone to your muscles last is this concept of thermogenesis when muscles are contracting they generate heat so you know the three types of muscle tissue from when we study the tissues skeletal cardiac and smooth muscle we're going to talk about what they have in common in some ways that they differ some of this will be repeated from previously but some of this is new so three things that all the things that all three muscle types have in common are these terms here number one excitability so all muscle cells are excitable they will all respond to a stimulus now that can be a nervous system stimulus like when i think about bringing my finger to my mental region and then i do it but it could also be an endocrine message all the muscle types are also extensible they have the characteristic known as extensibility meaning that they can contract over a range of resting lengths even if they've been stretched so meaning you can put many parts of your body in different positions and from all of those positions your muscles are still able to be contracted all muscle types exhibit elasticity they are elastic so they can rebound after contraction or stretching and they exhibit contractility that just means they contract all muscles shorten and exert a pull now think about this do we really need to point out that all muscles contract isn't that the most basic thing that all muscles do and the answer is yes and yes yes that is the most basic thing they do and yes we need to point it out because occasionally the fact that smooth muscle does not have striations will confuse some students into thinking it's somehow different than the other muscle types so all the muscle types contract we will be using skeletal muscle as our template for understanding contraction but cardiac muscle uses a very similar system and smooth muscle also uses a similar system it's just that some things are arranged in a certain way so that the muscle looks smooth it does not show striations but it does contract okay three muscle types may differ in each of these appearance of course the muscles can look different at a macroscopic level like comparing organs in the body and at a microscopic level they also can different how they are controlled so some muscle types are voluntarily controlled some are involuntarily controlled and there are a few interesting areas where muscles like the diaphragm seem to operate on their own but if you want to you can take over conscious control of them the muscle types will differ in their location in the body of course and their function of course so much of this location and function stuff you have already seen in previous videos and so here is a familiar looking table that we did when we talked about muscle tissue back in unit one so you can revisit that unit for that information and much of the information here or all of it is from our tissues lecture as well so please read through these slides if anything here doesn't sound familiar you should go back to that video on muscle tissue from unit one i did want to make sure i point out that we are very used to seeing skeletal muscle with these long parallel fibers kind of like thin threads running parallel to each other and that's true but of course if those fibers are all running together in a pack if i rotate these images or my hand and look at it from a different perspective what i would expect to see are a bunch of cylinders coming out of the computer screen towards me so make sure you realize that skeletal muscle will not always appear as a bunch of parallel striated lines if you're looking at it in a cross-section what you should see are these cylinders often with the peripheral nuclei very clearly pushed out at the edge of these muscle cells here's smooth muscle which often we see as these swimming minnow appearances these cells all running together seeming to cross the screen in front of us but if you could rotate those swimming minnows and look at them in cross section you would expect to find something more like what's shown here it's basically like if a smooth muscle cell is a swimming minnow and if you could take a minnow and cut it in half with scissors and then look at what was left you would have more or less like a cheerio or a donut shape the body of the minnow on the outside all the organs and guts in the middle so smooth muscle keep an eye out for this longitudinal appearance or the cross-section appearance the good news is smooth muscle is almost always in two layers so you'll see both if you can recognize one of them look for the other and that will confirm you've found some layers of smooth muscle and cardiac muscle we don't generally look at it from the different perspective although you can typically you'll be looking at the normal branching pattern of cardiac muscle okay so let's take a muscle and cut into it and you've done this perhaps at your home or place of work uh you can take a chicken breast which is just a pectoralis major muscle and tear into it and you'll see all these string like strands so what are those are they different from the ones you'd also see in meat i don't know what this weird looking steak is but you can tell here also some things are compartmentalized here so to better understand your culinary experiences let's just start with a diagram of a basic muscle so here's a muscle maybe it's a biceps brachii you know and what we've done is just taken a pair of scissors and cut right through it if you do that you look closely at this muscle what you'll notice is that there is an outer layer on the muscle called the epimysium anytime you see mys or mus you should be thinking muscle so epimysium is the muscular layer on top of the muscle this connective tissue but when you cut into the muscle you can also see that inside them the muscle is made of a bunch of subunits so what are these subunits called well if we look at one up close that is a bicycle these are the little strands or threads that you see when you rip open a chicken breast they're fascicles each fascicle also has a connective tissue layer in this case called the perimysium around it every one of these fascicles is separated by connective tissue here's the thing if you cut into this fascicle what you'll see are a bunch of little subunits so what are these subunits called well we'll pull one out and hopefully it already looks familiar i can see some striations i can see some peripheral nuclei this indeed is a muscle fiber meaning a muscle cell each muscle cell is also wrapped in a connective tissue layer called the endomysium so the topmost connective tissue layer the middle and the innermost connective tissue layers all have almost the same name epimysium perimysium and endomysium so we have started to look deep inside a muscle and in fact we're already down to a cellular level muscles are made of fascicles which are made of fibers but we're not done there by a long shot we're just getting going we need to look inside one of these muscle fibers to really understand how muscles work so here's a good looking muscle fiber i can see the striations the peripheral nuclei and so on let's look a little more closely it turns out that these muscle cells are full of little cylindrical subunits as well these are called myofibrils and if we look more closely at a myofibril or in this case half a dozen or so myofibrils we can see that these are these uh repeating units stuffed into the muscle fibers and each one of them does not have a connective tissue sheath around it because that wouldn't really make sense right we're inside a muscle cell now so there's there can't be connective tissue inside a cell because connective tissue is made of cells and fibers and ground substance so it's not connective tissue but there is a special structure called the sarcoplasmic reticulum that wraps around each myofibril and we're going to get into those in another video when you get into the details of muscle contraction but it is nice to realize that all of these units starting with muscle and all the way now down to myofibrils they each have something wrapped around them that should learn about now in these myofibrils if we look very closely you can see these little dots inside because our final step in this process is to realize that myofibrils are packed with also thin little strands called myofilaments more specifically these are protein strands called actin and myosin and if you can see in the image that there are some red lines in one area and blue in the other so those are the two types of myofilaments so now we've completed our list we know that mussels are made of fascicles which are bundles of fibers and fibers are packed with myofibrils and myofibrils are packed with myofilaments these tiny protein strands called actin and myosin and to give you a little preview myosin is just this protein that's a long twisted strand with two heads on the end and actin is this kind of thin basic protein strand and what will happen during muscle contractions is that myosin will tug on the actin not unlike the way that tug of war works with your hands pulling on the rope myosin will pull on actin and that's the basics on how muscle contraction works to understand the details of how muscle contraction works we're going to need at least two more videos