Exploring the Skeletal Muscle System

All right, welcome everyone to the skeletal system. So today we're going to start off with skeletal muscle tissue, which is chapter 10. So this is a fairly short unit. We're going to split chapter 11 into two lectures. So what are we going to talk about today? We're going to talk about the properties and functions of muscle tissue. So what's special about muscle? And we're going to focus mostly on skeletal muscle this lecture. We're going to cover smooth muscle and cardiac muscle in those different units. And then we're going to look a little bit about the microscopic anatomy of muscle tissue. So what does it look like under the microscope, which will then lead into how does muscle tissue contract? How do we actually contract our muscles? and what is the innervation that allows or signals our muscles to contract. And then we're going to talk a little bit about the different types of muscle fibers. So maybe some of you have heard of slow twitch, fast twitch, so we're going to talk a little bit more about the actual anatomical terminology for the different muscle fibers. And we'll finish off with disorders and development of muscles. So we said that muscle tissue is actually a composite tissue where we have muscle tissue and we have some areolar connective tissue as well, but it's the primary tissue type in the heart, which is cardiac muscle, and as well as in the walls of hollow organs, such as our digestive tract, and that's going to be our smooth muscle. And then in our muscles, right, the skeletal muscles, so what we actually think of as muscles that are moving our bones. So there's a little bit of terminology we need to understand when we're talking about muscles. So when we say muscle fiber, we actually mean a muscle cell, and this will make a little more sense when we talk about muscle cells or muscle fibers. So We've talked about fibers in terms of connective tissue before, like collagen fiber and elastic fiber. So this is something completely different. So for some reason they term muscle cells muscle fibers, and I sometimes go back and forth between the two, but I try to remind you guys along the way that when we say muscle fiber, we just mean one muscle cell. And we have some prefixes that mean muscle. So myo or mis is going to always refer to muscle, as well as a prefix meaning flesh. So when we see the word sarco, we're referring to flesh, which in this case is muscle. So when we're talking about muscle cells, you know, we didn't really go through the anatomy of cells, but You know, normal cells have a plasma membrane, and the plasma membrane of muscle cells are just called sarcolemmas. Okay, so they have the sarco in there. And then the cytoplasm of muscle cells is called sarcoplasm. So instead of cytoplasm cells, we just give it another name, sarcoplasm. So these are just something to keep in mind as we go along. And then over on the right hand side, we just give a little bit of an idea of what the different muscle tissue looks like. So we're going to focus mostly on skeletal muscle in this lecture and this unit. And again, we'll talk more about cardiac muscle and smooth muscle in those respective units when we talk about the heart and the digestive system. Okay? So what are some special properties of muscle tissue? Why is it special and why can it do what it does? And I want you to know these four properties. So it's excitable. So excitability means that nerve signals can excite the muscle cells. So it actually causes an electrical impulse that travels along that plasma membrane or that sarcolemma and causes a chain reaction. ions to essentially create a contraction within the muscle. Okay, so that's another property is contractility. So the myofilaments that we'll talk about, myosin and actin, are actually responsible for shortening the muscle cells. And if all the muscle cells shorten, then the entire muscle will shorten or contract. And that's how we're able to contract our muscles and move our bones. Now these other two we kind of forget about a little bit and that's extensibility. So extensibility just means the capability of being stretched. So not only can the muscle tissue contract, but it can actually stretch as well. So this will make a little more sense when we talk about muscles in the body, but usually we have a muscle that is doing a prime movement and then we have an opposing muscle that's going to do the opposite movement. Because one muscle can only do one action essentially. So we have to have another muscle that's going to do the equal and opposite reaction or movement. So when one muscle contracts you're going to stretch that opposing muscle and that is that extensibility. So Smooth muscle is also able to stretch. If you think about a big food bolus or even urine going through those hollow tubes, that muscle has to stretch to allow that substance to move through that hollow organ. OK, so a lot of these tissues are able to be extensible. And then last but not least is elasticity. So we've talked a little bit about elasticity when we talked about elastic fibers, but muscle tissue itself has elasticity, which means that it recoils after being stretched. So we said it's able to be stretched, and then it can recoil after being stretched, and that's termed elasticity. So these are the four properties of muscle tissue and what makes muscle kind of cool. So we talked about the properties of muscle tissue, but what are the actual functions of muscle tissue? Well, we know that muscles move our bones, right? So movement is obviously kind of the number one function that we think about muscles doing. And so they're going to move our bones. But also when we think about smooth muscle, it's going to actually squeeze fluids and other substances through those hollow organs. So they produce movement as well. And your heart muscle moves blood, right? Pumps blood. We also open and close body passageways, whether that's going to be voluntary or involuntary. So if you think about voluntary, you can close your eyes and your mouth, right? So these are sphincters that are creating valves, essentially. So they're allowing things to flow. pass through something, right? And this is very important when you're talking about the digestive system as well as the urinary system. So these sphincters are kind of like, you know, passageways or doorways, right, that open and close. Another thing to think about is posture, okay? So it maintains posture either while we're sitting or standing, right? So we already talked about muscles also stabilizing joints, right? So muscle tone helps to stabilize a lot of those synovial joints, especially some of the more unstable ones like our shoulder with that rotator cuff. So we'll learn all about the muscles of the rotator cuff in the next lecture. We also generate heat, right? So these muscles that are contracting are producing energy and so they're going to lose energy as heat, right? So this is really important when we're talking about thermoregulation and maintaining our body temperature. So let's do a little review of the different types of muscle tissue. So we've seen this before and again we'll focus on skeletal muscle today. But it makes up 40 percent of our body weight, so that's almost half, right? So that's a lot. Muscles definitely weigh a lot. They are striated, so the muscle cells themselves are actually striated, meaning they have some lines. And we'll look at that and what causes that striation when we look at the microscopic anatomy of muscle tissue. And these guys are all under the voluntary division of the nervous system, meaning we have control, right? We have conscious control over our skeletal muscles. And that's going to be different when we talk about our cardiac and our smooth muscle. So cardiac only occurs in the heart, right? So we find cardiac muscle only in the heart. Cells are also striated, so they have the same striated appearance as skeletal muscle, but they're branched, so they look a little bit different. They have some other special features, and they're also involuntary, right? So we have no control, either, well, conscious control over our heartbeat, right? So that's under the vol and involuntary division of the nervous system. Something's telling it to beat, right? We'll learn all about that in the heart lecture. Now smooth muscle, okay, is kind of the different one out of all three because it lacks striation, which is why they call it smooth, and this is in the walls of all the hollow organs. So really think digestive system, urinary system, right? And they are also involuntary. So we don't really have control over, you know, movement of the digestive fluid through the digestive system, as well as urine through the urinary tract until it gets to the bladder. And then we obviously control the sphincter in the urethra, right? But there is an involuntary sphincter as well. So we talked a little bit about muscle tissue. Now let's talk about muscles kind of as an organ, right? So obviously they're mostly muscle tissue. And again, we said muscle tissue is a little bit of a composite tissue because we have connective tissue in there as well. But we do have some connective tissue, that dense regular connective tissue, as well as that areolar connective tissue that's kind of in the muscle tissue itself. But we have some other tissue types as well. So we have blood vessels and we have nerves, right? So we're covering a couple different tissue types when we're talking about the muscle as an organ, okay, or muscles as a whole. So first let's talk a little bit about the gross anatomy before we get into the microscopic anatomy of skeletal muscle. So let's talk about how muscles are arranged structurally. So we said that muscle cells are also called muscle fibers. So we group those muscle fibers together and we call them a fascicle. Okay. And then we group those fascicles together to make up one muscle. So if we break it down, if we look over here on our right, we've got one muscle fiber, which is one muscle cell, right? So we group all these muscle cells together to make up one muscle fascicle. And then we group all these fascicles together to make up the entire muscle. Now there are some important connective tissues that are wrapping around each of these layers. So these are going to be sheaths of connective tissue. They bind the muscle fibers together and the fascicles together. And then they all come together and become the tendon. of that muscle. Okay so let's talk about these connective tissues. So we have the epimysium which is just dense regular connective tissue surrounding the entire muscle. Okay so that's the outer layer of connective tissue so that's the epimysium. We also refer to this as fascia. So especially if you're you know in the medical world or the physical therapy world a lot of people talk about fascia or fascial planes and these separate the different muscles okay and that's the epi mesium now the peri mesium surrounds each fascicle okay so around each fascicle we have a peri mesium okay And then remember the fascicles are made up of muscle fibers. So around each individual muscle fiber is a fine sheath of endomysium. Okay. And that's going to be around just the cell, just one single cell is going to be wrapped in that endomysium. Okay. And again, remember, they all come together and create the tendon, which is that how which is how they attach to the bones okay so we'll see that in a little bit So we just looked at all those different connective tissue sheaths, right? So the endomysium around the muscle fiber, the perimysium around the fascicle, and then that epimysium around the muscle itself, all are continuous with the tendon of that muscle, okay? So why is this important? Why are these connective tissues important? So when all those muscle fibers contract together, they're going to exert a force onto that connective tissue and therefore onto that tendon and that tendon is what is attaching to the bone and so it will exert the force onto that bone okay moving it so that's why these connective tissues are important so not only do they make up the tendon but they also provide elasticity So remember we said muscles, muscle tissue is, has elasticity or can be elastic and they bounce back, right? So if it's being stretched, it can return back to its normal shape. And the tendon sheath helps with that, right? So these connective tissue sheaths help with the elasticity. They also carry the blood vessels and the nerves. So we have to bring. blood to the muscle cells and we also innervate the muscle because that's how it tells it to contract right so these blood vessels and these nerves are going to run so if you notice in the picture they're running in between the fascicles right so we have a blood vessel here so you kind of see these blue and red dots in between the fascicles and that's where it's going to run okay same thing With the endomysium, you see the blood vessel little blue dots here, and that's going to be running in between all those muscle fibers. Okay, so speaking of blood supply and nerve supply to muscle tissue, it has a lot of good blood supply, and it does have good innervation as well. So when we're talking about each skeletal muscle, Each muscle is going to be supplied by one, like branches of one nerve and one artery. And you can have multiple veins, right? And that'll make a little more sense when we talk about arteries and veins and nerves and the branches of those arteries and veins. But the cool thing about the arteries, because we said that muscles contract and stretch, and the arteries have to be able to stretch and recoil. coil with that contraction of the muscle. So when you look at this picture it's kind of cool because you can actually see you know the red is the artery coming in and the orange is actually the muscle fiber. So we see a couple muscle fibers here they always run in parallel and there's a few of them here and then you can actually see those branches of the artery in here that are all squiggly right. So that's where they're allowed to be able to stretch and recoil because arteries are fairly elastic themselves. OK, now nerves and vessels, they obviously are going to be branched. Right. So the smallest of the branches are going to serve these individual muscle fibers. So you may have one nerve coming in and innervating that one. muscle, but again, we're going to branch to the individual muscle fibers. Okay, don't worry too much about that. We'll talk more about the innervation and blood supply in other lectures. So now let's talk a little bit about muscle attachments and how they actually attach to the bones and that and therefore how do they move the bones. So most skeletal muscles are going to run from one bone to another and they are usually going to cross one or more joints because the joint is where the movement is actually happening but you have to span that joint to be able to move it. and you're always going to have two points of attachment on the bone. So when we're talking about movement of the bones and what the muscle is actually doing, you're always going to have one bone that will remain fixed, okay? And the other one that's going to actually move. So it's a whole lever system, and we'll talk about levers in a minute. So we have one that's going to have the origin attachment. and this is going to be the less movable attachment or the fixed more fixed attachment okay and then we're going to have a insertion okay so then that's going to be the more movable attachment so there's always going to be two attachments of the muscle so there's always going to be an origin on the less movable bone and then an insertion on the more movable bone okay And muscle tissue itself never actually is the thing that attaches to the bone. It's always going to attach via the connective tissue. Okay, so we already said that all these muscles are surrounded by connective tissue sheaths which are then going to create the tendon. So we actually do have two types of attachments. We do have a tendon which is considered an indirect attachment. because it's usually kind of a longer connective tissue attachment. So they call it indirect, okay, because it looks like the tendon is what's actually attaching to the bone. Whereas we have another type of attachment that we call direct or fleshy attachment, and that's because the connective tissue fibers are really short, okay. So these are usually going to be the origins, okay. So these are usually going to be the origin attachment, whereas those tendons, which have the longer connective tissue fibers or the indirect attachments, are usually going to be the insertions. And where all of these attachments happen on the bone, whether it's an origin or an insertion, that's what's creating those bone markings that we just learned all about. OK, because that's where they're going to be putting the stress on the bone. So the bone is going to lay down some more bone tissue there to reinforce it to counteract the stress, the pulling stress or that tension from the connective tissue by the muscle. Okay so now that we kind of covered the gross anatomy of muscle let's look a little bit at the microscopic anatomy. So we said, and I'm going to keep reminding you, that when we're talking about a muscle fiber, we're talking about a single muscle cell. And these fibers are going to run in very long cylindrical patterns, okay? So these one muscle cell or one muscle fiber can be very, very long. It can be as long as the muscle itself. And how do they do that? Why are they such large cells? Well, each of these muscle cells are actually formed by a fusion of multiple embryonic cells, which is why they're multinucleated. So if you look at one muscle cell or one muscle fiber, you notice that there's multiple nuclei on the periphery. And that's because it actually is made up of multiple cells, multiple embryonic cells coming together. And so again, this is just one. Muscle cell here and it's butted up right against another muscle cell. Okay, so they're stacked on top of each other Okay lined up in parallel, but this is just they continue right so they continue Going left and right. So that's just a fraction of that muscle fiber So let's dive a little deeper into these muscle fibers and where the striations actually come from. And that's really the internal structure of these muscle fibers. So what are muscle fibers actually made up of? Well, the whole muscle fiber actually has a chalk full of these contractile organelles called myofibrils. Okay, so these are specialized organelles. So it's just like any other organelle of a cell, like the nucleus, mitochondria, all those things that make up the insides of cells. Well, muscle cells have these specialized organelles, and they're called myofibrils. So I know this is going to start to get confusing, right, because one muscle cell is called a muscle fiber, and then that muscle fiber is made up of a bunch of myofibrils, okay? So these guys are just these long rods essentially, okay, and they make up about 80% of the cytoplasm. So essentially that's the majority of the muscle cell are these myofibrils. Of course we have those peripheral nuclei, we have some mitochondria thrown in there as well, but this is the majority of the organelles inside the cell. And when we talk about one myofibril, okay, it's made up of this repeating row of sarcomeres. And one sarcomere essentially is the functional unit of muscle tissue. So the sarcomeres are what are actually going to be contracting, which is going to cause the entire muscle cell to contract. Okay, so we'll look at the sarcomeres a little more closely in a minute. But just to kind of break this down for us, we said, you know, so let's go big to small, right? So we said one muscle is made up of many muscle fascicles. One muscle fascicle is made up of many muscle fibers, and those fibers are the cells, right? So then one muscle fiber or one cell is made up of many myofibrils. Then if we actually just look at one myofibril, it's made up of many sarcomeres. So you can kind of go big to small or you can go small to big, but essentially it's kind of a breakdown of the structure of muscle tissue. So again, on the right-hand side, what we've done is we've taken one fiber and we've blown it up and we look inside and we see a... bunch of myofibrils okay and then within that myofibril you see the banding pattern right now the bad the banding pattern are the sarcomeres and we'll look at that in a second okay so now let's break down the sarcomere so i want you guys to know that the sarcomere is the basic unit of contraction for skeletal muscle but let's actually take a look at what the components of the sarcomere are and how it actually contracts. Okay, so what are some special features? So the delineation or the boundaries of a sarcomere are the z-disc to z-disc. Okay, so these are just where the thin filaments come together and we'll talk about the myofilaments in a second. But essentially that is the boundary of the sarcomere. And it looks like a zigzag line. So that's how you can remember a Z disc. Okay. And then the center of the sarcomere is the M line. Okay. So what are these actual myofilaments? Okay. So the myofilaments are what are going to be doing the contracting. So they're the contractile units of that sarcomere. So we have a thin or an actin filament and they start at the Z disc and then extend toward the center of the sarcomere and they are in blue. So here's one actin or thin filament and here's another one. So they start at the Z disc and they go towards the M line. And then the thick or myosin filament starts kind of at the center. the center and it extends outward toward the z-disc on either side. It overlaps with the thin filament so this overlapping between the two myofilaments is what's very important for the shortening. So the myofilaments are actually what are creating the striations in the muscle fibers. So they create a banding pattern. So we call the center section where there is complete crossover between those two myofilaments the A-band. Okay so the A-band is going to be the entire length of the thick filaments with the crossover of the thin filaments. And we term this the dark band because there's more crossover, there's more filament interaction, so it creates a dark band. So if you think A for dark, because there's an A in dark, that's the A band, which is the dark band. Now the I band is the region with only thin filaments. Okay, so here's the I band right here. And the Z disc is the center of the I band. and we also call this the light band because it only has the thin filaments it doesn't have the thick filaments and so it's a lot lighter okay so now we create this light dark light dark banding pattern which creates the striation look okay the vertical striations so because all of these myofilaments are stacked on top of each other the myofibrils are then stacked on top of each other therefore the and then the cells and so on right so if you stack everything on top of each other you're going to get this banding pattern across all of them to create the striations okay so they all line up because we have the myofibrils lined up and it's going to create that banding striation So here's just a blow-up picture of two muscle fibers stacked on top of each other so you can actually get a better sense of the banding pattern and the striations that are being created. So they're vertical striations right up and down and so you can really see that dark A-band next to the light I-band and in the center of the light I-band you can actually see a dark line as well. and your eyes are not playing tricks on you, that is the Z disc. So remember we said that the Z disc is in the center of the light band, and so that is actually the Z disc. And we said that the sarcomere is Z disc to Z disc, so if we look at that, that's actually right here to right here. So it's half of a light band on either side of the dark band, okay? So another structure found in the sarcomere is called titin. And titin, think of like a spring. Essentially, it's going to extend from the z-disc and attach to the thick filament. So it's right here in yellow. And essentially, it resists overstretching of the muscle because we said that muscle can stretch as well, right? It has extensibility. So If it stretches, that means it's stretching and separating these myofilaments. And so we don't want them to stretch beyond reach of each other, right? And so that's what this structure does, this titin, is it's not only going to hold the thick filaments in place, because they're not attached to the Z disc, but when it uncoils, the muscle's being stretched. And then it allows for some elastic recoil when the muscle then comes back to place. Okay, so when it's being stretched, it's uncoiled. When the muscle actually contracts, it coils up. Okay, but the whole job is really to keep from overstretching the muscle. So here's just a little review of our pictures. So we kind of go big to small to see that sarcomere. So the top picture are our two muscle fibers showing the banding pattern with the dark and light A and I bands. And then if we blow up just one of those muscle fibers, then we get the second picture. And so you can see how the muscle fiber is full of those myofibrils. And then if we blow up just one of those myofibrils, we're able to see the sarcomeres in series to each other and how they all line up and we're able to then see the myofilaments and how that sarcomere is put together. Okay, so just a little bit of review. So now let's talk about another specialized structure in the muscle cell and that's the sarcoplasmic reticulum. So it's very similar to the smooth endoplasmic reticulum in a normal cell, but it's specialized in the muscle cell. So essentially there's these interconnected tubules that surround each myofibril. So think of it kind of like a sleeve of channels. okay a sleeve of tubes or a stocking that is wrapped around each of these myofibrils and at the end they kind of come together and this area they come together at is called a terminal cistern and when they come together they butt up against what's called this t-tubule and so i'll explain what a t-tubule is so remember we said that there is a plasma membrane right around each muscle cell. And we just call us, you know, the muscle plasma membrane, the sarcolemma. Okay, so it's just the same thing like a regular plasma membrane. And essentially, it's going to invaginate downward in between some of these micro, these myofibrils. Okay, so it's going to invaginate inward, and then it's going to create this transverse tubule. And wherever we find these T tubules or transverse tubules, either side is going to be those terminal cisterns of the sarcoplasmic reticulum. And this creates what's called a triad. Okay, so a triad essentially is a T tubule with two terminal cisterns on either side. So that's why it's made up of three, right, triad. And so there's a lot that happens at this triad. And this has to do with how the muscle cell actually gets stimulated to contract. Okay, so keep the sarcoplasmic reticulum in mind when you think of contraction. So we said it has something to do with contraction. So what is the actual function of the sarcoplasmic reticulum? Well, essentially think of it like a storage house for calcium ions, okay? And so when the nervous system sends that signal down to contract, the sarcoplasmic reticulum releases all of these calcium ions. And then the calcium ions are going to diffuse all throughout the cytoplasm. And how they're going to do that is through those T-tubules, okay? So this triggers what we call a sliding filament mechanism. So the sliding filament mechanism is the term for contraction. So it's what's actually happening with those myofilaments, the actin and myosin filaments inside the sarcomere. OK, so sarcoplasmic reticulum, just think of it like the storage house of calcium. the calcium is the little signaling ions that's stimulated by the nervous system and essentially those calcium ions diffuse out throughout the cell really quickly and they essentially trigger this whole sliding filament mechanism to contract okay so what is this sliding filament mechanism how does it shorten the muscle fiber So essentially we said that those myofilaments, right, the actin and myosin overlap, okay, they overlap on each other. And so what's going to happen is they're going to slide across one another and that's going to shorten the overall sarcomere and therefore shorten the overall muscle fiber. So what happens is the myosin has these little heads, they look like little knobs. And they are what are going to attach to the actin filament. And what happens is we energize that little myosin head using ATP, and they're going to pull the thin filament towards the center of the sarcomere. So that creates more of a crossover between the two filaments. So therefore, the overall sarcomere shortens. But keep in mind that the thin and thick filaments or the actin and myosin filaments themselves do not shorten. OK, so the filaments do not shorten, but the sarcomere shortens and therefore there's more overlap between the two filaments. OK, but if all the sarcomeres are shortening all at once, right, the whole myofibril is going to shorten and therefore the whole muscle fiber shortens. And if all of the cells, all the muscle fibers in the whole muscle are doing this all at the same time, that whole muscle is going to shorten. And this whole process is initiated by those calcium ions, which are released by the sarcoplasmic reticulum. OK, and again, we need energy for this. Right. So we need that ATP to energize those myosin heads. Muscle cells have a lot of mitochondria to produce that energy because mitochondria are kind of our powerhouses of the cell, so they can produce all of that ATP for this sliding filament mechanism. Let's talk a little bit about how the nervous system signals the sliding filament mechanism. So essentially we have motor neurons, right? So motor neurons are going to come down and innervate the muscle tissue. So this is a voluntary movement, right? So we have control over this. And what they're going to do is create a neuromuscular junction. So that's where the nerve ending is. the muscle fiber itself meet. Okay, so remember we said that those nerves are going to branch and then these branches, the smallest branches, are going to interact with each muscle fiber itself. Okay, and this is going to create this neuromuscular junction. And the place where there's actually the end of the neuron is, or the nerve ending, is called the axon terminal. Okay, so Or you can call it a terminal bouton, but I like axon terminal better. So this is just at the end of the nerve axon that's going to interact with that neuromuscular junction. And the big thing about the axon terminal is that it stores a neurotransmitter. So this is a molecule that's essentially going to be released from the neuron. and it's going to then trigger a cascade of actions. Acetylcholine or ACH is the most common neurotransmitter that there is. There are other ones out there such as epinephrine, norepinephrine, but when we talk about it, usually we're talking about acetylcholine. And so at this neuromuscular junction, we have an axon terminal, which stores that neurotransmitter. And then there's going to be a space in between that axon terminal and the sarcolemma. So we said the sarcolemma is that plasma membrane wrapping around the muscle fiber. And this space is called a synaptic cleft. So it's just some terminology to think about. We'll talk a little bit more when we talk about the nervous system as well. But this is the space that the neurotransmitter has to cross. So let's go through the process a little bit, okay, of what's actually happening. So this picture is from your book, okay, so I've just blown it up. It was the same picture as the last slide, but I've just included the text from the textbook, okay. So if we have the nerve impulse that's coming down the neuron, right, and it's going to reach this neuromuscular junction. So there's three neuromuscular junctions right here because that's where the axon terminal meets the muscle fiber. So what happens is this impulse is going to stimulate the neurotransmitter to be released. So the neurotransmitter is going to be released out of that axon terminal. It's going to have to cross that synaptic cleft. but then what is it going to do it's going to interact with the sarcolemma of the muscle cell okay which that is going to stimulate the release of calcium okay so this is going to travel through those t-tubules so remember we said that those t-tubules are important for triggering this whole mechanism okay so it's carried down those t-tubules so essentially to try to trigger all the myofibrils at the same time, right? So you want everything to contract at the same time, okay? And that's how it does it is through that T-tubule. So it disperses very quickly through the cytoplasm to release those calcium ions which trigger the sliding filament mechanism, okay? So this is just a really cool transmission electron micrograph of the actin and myosin filaments. So you can see the myosin filaments are quite thick and bright white here and then in between them you can see a thin filament right here. And you can actually appreciate the myosin heads here as well. So they're interacting with the the thin filament from the myosin which is kind of cool to actually appreciate that through this very high powered microscope So now that we've triggered that sarcomere contraction from the nervous system, what does that actually look like? And we actually have some pictures here, some microscopic pictures of what's actually happening along with the drawing. Okay, so number one is a fully relaxed sarcomere of a muscle. So this would be a muscle at rest. Okay, and so what happens is is that the titin is slightly stretched out, right? There's some overlap, because we always have overlap between the two myofilaments. And we know that the distance of the sarcomere is from the Z disc to the Z disc, right? So that's fully relaxed, okay? And we can see in the picture, because we just follow it right up, here's our A band, which is dark, right? And then we have half of a light band on either side because that's the Z disc, right? So the Z disc kind of splits that light band in two. So what happens, right? So now we have contraction. So we have shortening of the sarcomere. So what does that look like? And that's number two. So we have much more overlap between the thick and thin filaments or the actin and myosin. Our titin is quite coiled. right because it's not we're not stretching the muscle and that's what its job is to prevent over stretching but look what happens is the z disc comes closer together okay so the sarcomere itself is going to shorten but look what stays the same so the a band is going to stay the exact same width and why is that that's because the a band is the entire distance of the thick filament. So the thick filament is going to stay the same length, right? So is the thin filament, but we're losing some of the I-band because they're overlapping more. So if we look at our picture, our micrograph, and you notice that the A-band becomes a little bit lighter because there's more thin filament in there, which is kind of the lighter filament. right and our eye band shortens significantly on either side right so there's a lot more overlap and the whole sarcomere shortens but again those filaments themselves do not shorten okay so that's what's happening microscopically in the muscle cell during contraction okay So now I've just written everything out for you that we just talked about over the last slide. So how does our striation pattern change during contraction? We said the sarcomeres shorten, but the filaments do not, right? The I-bands shorten, but the A-band does not. The titin is going to coil during contraction, right? Because we're not stretching, we're doing the exact opposite. We're contracting. So I just wrote out everything that's happening during that muscle contraction. So now we can extend muscles. So we said that muscles can stretch, they can extend. We have kind of equal opposite muscles over joints. So if one muscle is doing one motion, the other muscle is going to do the opposite motion. Just think of flexion and extension, right? Think of your elbow. Well, you've got one muscle on the one side of the joint that's going to do flexion, right? Your biceps. And then on the other side of your arm, you have a muscle that's going to do the opposite motion, extension, and that's your triceps, right? So biceps, triceps is kind of a classic opposing muscle movement, okay? So when we talk about the force of contraction, right, all muscles can contract, the greatest force that a muscle can actually produce is when the muscle has been slightly stretched, because you start out with less overlap between the filaments. And that means there's more room for overlap, right? So you can actually overlap more and shorten more. Okay, so that means you have a better force of contraction in that muscle so the best way to do it is slightly extend the muscle before contracting to get the most force okay and of course when we talk about muscle extension you're always thinking about the titan which is going to keep it from over stretching okay Now when we talk about contraction, there's really two main types of contractions. And one you think of every day and the other one you may not realize is actually muscle contraction. Okay, so concentric contraction is the classic muscle shortening. So the muscle is actually going to shorten while it's doing its work, right? That's our classic biceps curl. right? The biceps is shortening, you're doing work, right? But we have another one called eccentric contraction. And this is actually a muscle generating force as it lengthens. So think of a muscle as putting on the brakes in eccentric contraction. So it's going to resist gravity or a weight. It's kind of the down portion of a push-up. Okay, you're resisting that Gravity, you're resisting falling to the ground, but your muscle is actually lengthening. So I know that one's a little confusing, and concentric contraction is more the classic thing of what we're talking about, of what we think of as contraction. So concentric and eccentric contraction. So two types of mechanisms for contraction. So now we're going to finish up the topic kind of with the muscle fibers. Okay, so we have muscle fibers in every muscle has all of these types of fibers. Okay, so there's three main types of skeletal muscle fibers and it just depends on the muscle with how many of each type it has. Okay, because what is the job of that muscle and we'll talk about that in a minute. So we have two questions we have to ask ourselves to decide what type of muscle fiber it is. One, how do they manufacture energy? How does that muscle fiber get energy? Do they use oxygen? So do they undergo cellular respiration or not? Or anaerobic respiration? So that's number one to ask, right? So the difference is, yes, we use oxygen and that's oxidative fibers. Okay, so they're going to produce their energy using oxygen or called aerobically. If we don't use oxygen, it's called glycolytic fibers. So they do produce energy, but they don't use oxygen. They use sugar, which is also called glycogen, right? So the unit of sugar is glycogen. The monosaccharide. And so that's termed anaerobically, without oxygen. So they're either going to be oxidative or glycolytic. Now, the second question we have to ask ourselves is how quickly do they contract? Do they contract fast or slow? And that'll give us our three different categories. And again, just to reiterate. All skeletal muscles have all three types of fibers. They just may have more of one kind than another, just depending on their job. So here are our three classes of muscle fibers. So again, we have our slow oxidative, right? So we use oxygen, but we're slow. Fast glycolytic, we use sugar instead of oxygen, and we're fast. and then our last one is fast oxidative so we use oxygen and we're fast okay and I'll talk a little bit more about the red and white or the colors of the fibers in a second okay so let's look at them individually So our first one is slow oxidative. So these guys are more red because they have a lot of myoglobin. So it's very similar to hemoglobin in the blood and hemoglobin is red in color and so is myoglobin. And that's what binds the oxygen. Okay so we have lots of myoglobin because we're using oxygen to create our energy. So lots of mitochondria, lots of capillaries in there to supply the oxygen. Our contraction is slow, right? So they contract slowly, but something that's really good about contracting slowly is you're very resistant to fatigue. So these muscles don't tire out very fast, okay? So they kind of contract continuously for long periods of time. Now you don't get a lot of power from them, right? So it's kind of like the turtle. right the slow turtle it can kind of go for a long period of time but it may not be very powerful or very fast right and the fibers tend to be slow in diameter okay so what type of muscles do you think maybe would have more slow oxidative fibers so kind of think about it for a second That's right, so postural muscles. So think muscles that are going to have to contract for longer periods of time. So think of your trunk, right? Your posture muscles. They're going to have to contract for a long period of time to keep your posture, right? So now let's look at the second kind, which is fast glycolytic. So they're white because it doesn't have much myoglobin. right if we're not using oxygen or using sugar we don't really need that much myoglobin and we don't need that much mitochondria either because we're getting our energy from anaerobic pathways because we use glycogen or sugar now these guys contract quite fast so very rapidly but the problem with doing that is you tire quickly it's like the sprinter right so they can't go very far but they can go really fast okay because the fibers are quite large so they're about twice the diameter of a slow oxidative fiber and they contain more myofilaments right so if we have more myofilaments that means more sarcomeres that means more power okay so what muscles might have more fast glycolytic fibers That's right, our upper limb muscles. Okay, so these guys are quite powerful, right? So they're powerful but they tire very quickly. They're prone to fatigue. Okay, now last but not least is our fast oxidative fiber. So they are kind of intermediate in their color. They do have quite a bit of myoglobin because they use oxygen, but they're quite larger. They're larger fibers. So they do have a good capillary supply as well. So these guys use oxygen to make their energy. Okay. Now they're different from our slow oxidative because they are fast. So whereas slow oxidative contracts slowly, these guys contract quickly. But they are less oxidative. resistant to fatigue than are slow oxidative. So they're fast like our glycolytic, but they are a little more resistant to fatigue than the fast glycolytic because they're using oxygen, right? So there's a good supply of oxygen, whereas if you just use up the sugar right away, that's all you got, right? So they're a little, they're not as big as the fast glycolytic, so they're pretty powerful but not quite as powerful as the fast glycolytic. So just think of everything kind of in between the other two, right? So they're kind of a good mix of the two. They're a little faster, they're a little more resistant to fatigue, but they're not the two extremes. So what type of muscles do you think would have more fast oxidative fibers? well our last ones right so our lower limb muscles so these guys have to do locomotion because they have to be a little more powerful to propel our body weight forward but they don't have to be they have to be more resistant to fatigue as well so they're quite powerful but maybe not as powerful as our upper arm muscles but they are more resistant to fatigue so they can last longer right because locomotion can require a longer distance or performance okay so they're kind of in the in-betweeners the intermediate So now we're going to finish up with some disorders and development. So what are some disorders of muscle tissue? Well muscular dystrophy is kind of a group of diseases and they're all inherited so they're genetic and they essentially destroy muscle tissue. So the muscles degenerate and what happens is the muscle tissue is replaced with fat and connective tissue. So if you replace muscle tissue with fat and connective tissue it no longer functions, right? So muscular dystrophy is quite a degenerating disease and this is the picture on the right. If you look and you have see these normal muscle cells and then on the right hand side you see kind of few muscle cells with a lot of connective tissue kind of in between and replacing a lot of the normal muscle cells. So there's a couple different types of muscular dystrophy, but we won't go into the different types. Now myofascial pain syndrome is essentially pain caused by tight bands of muscle fibers. So essentially a lot of the fascia. Okay, so this is can be caused by problems with the connective tissue with the muscles. Okay, so myofascial muscle and fascia. So essentially the connective tissue can be too tight and cause some pain. Now fibromyalgia is kind of a mysterious disease or disorder causing chronic pain. So what happens is the brain processes things as painful. It affects mostly women, but a A common symptom is widespread musculoskeletal pain. So is it really a problem with the musculoskeletal system? Probably not. It's probably having to do more with the neuromuscular system. But it's kind of a disease of ruling out all the other problems. So if you've ruled everything else out, You could call it fibromyalgia if you have these symptoms. So again, there's not a ton known about it, but it is quite a crippling disorder in terms of pain. So how do muscles develop? And this is kind of cool. This is how we get those multinucleated muscle cells. So they develop from myoblasts. Okay, so myoblasts are kind of the stem cells of muscle tissue. Okay, so we said we have those germ lines, right? If we go back to our tissue lecture, we have our ectoderm, mesoderm, and endoderm. And the mesoderm is going to develop into these myoblasts. And the myoblasts are what are fusing together to form one skeletal muscle fiber. Okay, so that's how we get these multinucleated cells. Now what's cool is that satellite cells are actually sitting around each muscle fiber. So a thing is that muscle cells cannot divide. Okay, so they can't divide. They're kind of like neurons, okay, like the nervous system cells. So they can't divide. So they're actually really kind of not very good at healing because they can't divide. But what they do have are these satellite cells. And they're very similar to myoblasts. They're kind of embryonic or stem cell-like cells. And what happens is if a muscle cell gets damaged, then it's going to fuse that satellite cell to the brain. And that's what to the muscle cell itself to help them grow and repair. So this is kind of the whole idea behind weight training and muscle development is you're actually making micro damages in the muscle cells so that these satellite cells start adding to your muscle cells. And so then they can grow and get bigger, okay? So kind of an interesting idea, but in terms of tissues that heal really well, muscle is not actually one. It's kind of in the middle because they can't divide. So they're definitely not the best at healing, which is something that we, it's kind of a misnomer. So muscles actually start contracting even by week seven of development. So that's pretty early on, right? And the cardiac muscle is even earlier than that. So the cardiac muscle is actually pumping blood three weeks after fertilization. So that's super early, right? So the muscle system is very early in its development. We have to get things moving around. Now, there is a difference kind of when we talk about the percent of body mass, because we said that, you know, muscle makes up about 40% of our body weight. And that's a little bit different in males versus females. It's a little bit more in males and a little less in females. That's why we kind of average it out to be 40%. And the difference is due to androgens, right? It's due to the male hormone. Now muscles age, right? So we tend to increase the amount of connective tissue in muscles and kind of decrease the muscle tissue itself. So the number of muscle fibers actually decrease. But that's a good thing about exercise as we age to try to stimulate that. those satellite cells to continue to kind of lay down cells and keep healing and growing those muscle fibers. Okay, and we do get a loss of muscle mass with aging, and that's, you know, estimated to be about 50% by the age of 80. So we really decrease that muscular strength as we age. But again, you can kind of combat that with good exercise. we age. So here are our learning objectives for the day. And next we're going to be talking about the muscles of the body. So we're really going to be splitting that up into two lectures. So our first one we're going to be talking more about the head, neck, and torso or the trunk. And then the next lecture will focus on the limbs right kind of shoulder and pelvis and the upper and lower limbs okay see you guys next time

So what are we going to talk about today? We're going to talk about the properties and functions of muscle tissue. So what's special about muscle?

And we're going to focus mostly on skeletal muscle this lecture. We're going to cover smooth muscle and cardiac muscle in those different units. And then we're going to look a little bit about the microscopic anatomy of muscle tissue.

So what does it look like under the microscope, which will then lead into how does muscle tissue contract? How do we actually contract our muscles? and what is the innervation that allows or signals our muscles to contract.

And then we're going to talk a little bit about the different types of muscle fibers. So maybe some of you have heard of slow twitch, fast twitch, so we're going to talk a little bit more about the actual anatomical terminology for the different muscle fibers. And we'll finish off with disorders and development of muscles.

So we said that muscle tissue is actually a composite tissue where we have muscle tissue and we have some areolar connective tissue as well, but it's the primary tissue type in the heart, which is cardiac muscle, and as well as in the walls of hollow organs, such as our digestive tract, and that's going to be our smooth muscle. And then in our muscles, right, the skeletal muscles, so what we actually think of as muscles that are moving our bones. So there's a little bit of terminology we need to understand when we're talking about muscles.

So when we say muscle fiber, we actually mean a muscle cell, and this will make a little more sense when we talk about muscle cells or muscle fibers. So We've talked about fibers in terms of connective tissue before, like collagen fiber and elastic fiber. So this is something completely different.

So for some reason they term muscle cells muscle fibers, and I sometimes go back and forth between the two, but I try to remind you guys along the way that when we say muscle fiber, we just mean one muscle cell. And we have some prefixes that mean muscle. So myo or mis is going to always refer to muscle, as well as a prefix meaning flesh.

So when we see the word sarco, we're referring to flesh, which in this case is muscle. So when we're talking about muscle cells, you know, we didn't really go through the anatomy of cells, but You know, normal cells have a plasma membrane, and the plasma membrane of muscle cells are just called sarcolemmas. Okay, so they have the sarco in there.

And then the cytoplasm of muscle cells is called sarcoplasm. So instead of cytoplasm cells, we just give it another name, sarcoplasm. So these are just something to keep in mind as we go along. And then over on the right hand side, we just give a little bit of an idea of what the different muscle tissue looks like.

So we're going to focus mostly on skeletal muscle in this lecture and this unit. And again, we'll talk more about cardiac muscle and smooth muscle in those respective units when we talk about the heart and the digestive system. Okay?

So what are some special properties of muscle tissue? Why is it special and why can it do what it does? And I want you to know these four properties. So it's excitable. So excitability means that nerve signals can excite the muscle cells.

So it actually causes an electrical impulse that travels along that plasma membrane or that sarcolemma and causes a chain reaction. ions to essentially create a contraction within the muscle. Okay, so that's another property is contractility.

So the myofilaments that we'll talk about, myosin and actin, are actually responsible for shortening the muscle cells. And if all the muscle cells shorten, then the entire muscle will shorten or contract. And that's how we're able to contract our muscles and move our bones.

Now these other two we kind of forget about a little bit and that's extensibility. So extensibility just means the capability of being stretched. So not only can the muscle tissue contract, but it can actually stretch as well.

So this will make a little more sense when we talk about muscles in the body, but usually we have a muscle that is doing a prime movement and then we have an opposing muscle that's going to do the opposite movement. Because one muscle can only do one action essentially. So we have to have another muscle that's going to do the equal and opposite reaction or movement.

So when one muscle contracts you're going to stretch that opposing muscle and that is that extensibility. So Smooth muscle is also able to stretch. If you think about a big food bolus or even urine going through those hollow tubes, that muscle has to stretch to allow that substance to move through that hollow organ. OK, so a lot of these tissues are able to be extensible.

And then last but not least is elasticity. So we've talked a little bit about elasticity when we talked about elastic fibers, but muscle tissue itself has elasticity, which means that it recoils after being stretched. So we said it's able to be stretched, and then it can recoil after being stretched, and that's termed elasticity. So these are the four properties of muscle tissue and what makes muscle kind of cool.

So we talked about the properties of muscle tissue, but what are the actual functions of muscle tissue? Well, we know that muscles move our bones, right? So movement is obviously kind of the number one function that we think about muscles doing. And so they're going to move our bones.

But also when we think about smooth muscle, it's going to actually squeeze fluids and other substances through those hollow organs. So they produce movement as well. And your heart muscle moves blood, right?

Pumps blood. We also open and close body passageways, whether that's going to be voluntary or involuntary. So if you think about voluntary, you can close your eyes and your mouth, right? So these are sphincters that are creating valves, essentially.

So they're allowing things to flow. pass through something, right? And this is very important when you're talking about the digestive system as well as the urinary system.

So these sphincters are kind of like, you know, passageways or doorways, right, that open and close. Another thing to think about is posture, okay? So it maintains posture either while we're sitting or standing, right? So we already talked about muscles also stabilizing joints, right?

So muscle tone helps to stabilize a lot of those synovial joints, especially some of the more unstable ones like our shoulder with that rotator cuff. So we'll learn all about the muscles of the rotator cuff in the next lecture. We also generate heat, right? So these muscles that are contracting are producing energy and so they're going to lose energy as heat, right? So this is really important when we're talking about thermoregulation and maintaining our body temperature.

So let's do a little review of the different types of muscle tissue. So we've seen this before and again we'll focus on skeletal muscle today. But it makes up 40 percent of our body weight, so that's almost half, right?

So that's a lot. Muscles definitely weigh a lot. They are striated, so the muscle cells themselves are actually striated, meaning they have some lines. And we'll look at that and what causes that striation when we look at the microscopic anatomy of muscle tissue. And these guys are all under the voluntary division of the nervous system, meaning we have control, right?

We have conscious control over our skeletal muscles. And that's going to be different when we talk about our cardiac and our smooth muscle. So cardiac only occurs in the heart, right?

So we find cardiac muscle only in the heart. Cells are also striated, so they have the same striated appearance as skeletal muscle, but they're branched, so they look a little bit different. They have some other special features, and they're also involuntary, right? So we have no control, either, well, conscious control over our heartbeat, right?

So that's under the vol and involuntary division of the nervous system. Something's telling it to beat, right? We'll learn all about that in the heart lecture.

Now smooth muscle, okay, is kind of the different one out of all three because it lacks striation, which is why they call it smooth, and this is in the walls of all the hollow organs. So really think digestive system, urinary system, right? And they are also involuntary.

So we don't really have control over, you know, movement of the digestive fluid through the digestive system, as well as urine through the urinary tract until it gets to the bladder. And then we obviously control the sphincter in the urethra, right? But there is an involuntary sphincter as well.

So we talked a little bit about muscle tissue. Now let's talk about muscles kind of as an organ, right? So obviously they're mostly muscle tissue.

And again, we said muscle tissue is a little bit of a composite tissue because we have connective tissue in there as well. But we do have some connective tissue, that dense regular connective tissue, as well as that areolar connective tissue that's kind of in the muscle tissue itself. But we have some other tissue types as well. So we have blood vessels and we have nerves, right? So we're covering a couple different tissue types when we're talking about the muscle as an organ, okay, or muscles as a whole.

So first let's talk a little bit about the gross anatomy before we get into the microscopic anatomy of skeletal muscle. So let's talk about how muscles are arranged structurally. So we said that muscle cells are also called muscle fibers.

So we group those muscle fibers together and we call them a fascicle. Okay. And then we group those fascicles together to make up one muscle. So if we break it down, if we look over here on our right, we've got one muscle fiber, which is one muscle cell, right? So we group all these muscle cells together to make up one muscle fascicle.

And then we group all these fascicles together to make up the entire muscle. Now there are some important connective tissues that are wrapping around each of these layers. So these are going to be sheaths of connective tissue.

They bind the muscle fibers together and the fascicles together. And then they all come together and become the tendon. of that muscle.

Okay so let's talk about these connective tissues. So we have the epimysium which is just dense regular connective tissue surrounding the entire muscle. Okay so that's the outer layer of connective tissue so that's the epimysium. We also refer to this as fascia.

So especially if you're you know in the medical world or the physical therapy world a lot of people talk about fascia or fascial planes and these separate the different muscles okay and that's the epi mesium now the peri mesium surrounds each fascicle okay so around each fascicle we have a peri mesium okay And then remember the fascicles are made up of muscle fibers. So around each individual muscle fiber is a fine sheath of endomysium. Okay. And that's going to be around just the cell, just one single cell is going to be wrapped in that endomysium.

Okay. And again, remember, they all come together and create the tendon, which is that how which is how they attach to the bones okay so we'll see that in a little bit So we just looked at all those different connective tissue sheaths, right? So the endomysium around the muscle fiber, the perimysium around the fascicle, and then that epimysium around the muscle itself, all are continuous with the tendon of that muscle, okay?

So why is this important? Why are these connective tissues important? So when all those muscle fibers contract together, they're going to exert a force onto that connective tissue and therefore onto that tendon and that tendon is what is attaching to the bone and so it will exert the force onto that bone okay moving it so that's why these connective tissues are important so not only do they make up the tendon but they also provide elasticity So remember we said muscles, muscle tissue is, has elasticity or can be elastic and they bounce back, right?

So if it's being stretched, it can return back to its normal shape. And the tendon sheath helps with that, right? So these connective tissue sheaths help with the elasticity.

They also carry the blood vessels and the nerves. So we have to bring. blood to the muscle cells and we also innervate the muscle because that's how it tells it to contract right so these blood vessels and these nerves are going to run so if you notice in the picture they're running in between the fascicles right so we have a blood vessel here so you kind of see these blue and red dots in between the fascicles and that's where it's going to run okay same thing With the endomysium, you see the blood vessel little blue dots here, and that's going to be running in between all those muscle fibers. Okay, so speaking of blood supply and nerve supply to muscle tissue, it has a lot of good blood supply, and it does have good innervation as well. So when we're talking about each skeletal muscle, Each muscle is going to be supplied by one, like branches of one nerve and one artery.

And you can have multiple veins, right? And that'll make a little more sense when we talk about arteries and veins and nerves and the branches of those arteries and veins. But the cool thing about the arteries, because we said that muscles contract and stretch, and the arteries have to be able to stretch and recoil. coil with that contraction of the muscle. So when you look at this picture it's kind of cool because you can actually see you know the red is the artery coming in and the orange is actually the muscle fiber.

So we see a couple muscle fibers here they always run in parallel and there's a few of them here and then you can actually see those branches of the artery in here that are all squiggly right. So that's where they're allowed to be able to stretch and recoil because arteries are fairly elastic themselves. OK, now nerves and vessels, they obviously are going to be branched. Right. So the smallest of the branches are going to serve these individual muscle fibers.

So you may have one nerve coming in and innervating that one. muscle, but again, we're going to branch to the individual muscle fibers. Okay, don't worry too much about that. We'll talk more about the innervation and blood supply in other lectures. So now let's talk a little bit about muscle attachments and how they actually attach to the bones and that and therefore how do they move the bones.

So most skeletal muscles are going to run from one bone to another and they are usually going to cross one or more joints because the joint is where the movement is actually happening but you have to span that joint to be able to move it. and you're always going to have two points of attachment on the bone. So when we're talking about movement of the bones and what the muscle is actually doing, you're always going to have one bone that will remain fixed, okay?

And the other one that's going to actually move. So it's a whole lever system, and we'll talk about levers in a minute. So we have one that's going to have the origin attachment. and this is going to be the less movable attachment or the fixed more fixed attachment okay and then we're going to have a insertion okay so then that's going to be the more movable attachment so there's always going to be two attachments of the muscle so there's always going to be an origin on the less movable bone and then an insertion on the more movable bone okay And muscle tissue itself never actually is the thing that attaches to the bone.

It's always going to attach via the connective tissue. Okay, so we already said that all these muscles are surrounded by connective tissue sheaths which are then going to create the tendon. So we actually do have two types of attachments.

We do have a tendon which is considered an indirect attachment. because it's usually kind of a longer connective tissue attachment. So they call it indirect, okay, because it looks like the tendon is what's actually attaching to the bone. Whereas we have another type of attachment that we call direct or fleshy attachment, and that's because the connective tissue fibers are really short, okay. So these are usually going to be the origins, okay.

So these are usually going to be the origin attachment, whereas those tendons, which have the longer connective tissue fibers or the indirect attachments, are usually going to be the insertions. And where all of these attachments happen on the bone, whether it's an origin or an insertion, that's what's creating those bone markings that we just learned all about. OK, because that's where they're going to be putting the stress on the bone.

So the bone is going to lay down some more bone tissue there to reinforce it to counteract the stress, the pulling stress or that tension from the connective tissue by the muscle. Okay so now that we kind of covered the gross anatomy of muscle let's look a little bit at the microscopic anatomy. So we said, and I'm going to keep reminding you, that when we're talking about a muscle fiber, we're talking about a single muscle cell.

And these fibers are going to run in very long cylindrical patterns, okay? So these one muscle cell or one muscle fiber can be very, very long. It can be as long as the muscle itself. And how do they do that?

Why are they such large cells? Well, each of these muscle cells are actually formed by a fusion of multiple embryonic cells, which is why they're multinucleated. So if you look at one muscle cell or one muscle fiber, you notice that there's multiple nuclei on the periphery. And that's because it actually is made up of multiple cells, multiple embryonic cells coming together. And so again, this is just one.

Muscle cell here and it's butted up right against another muscle cell. Okay, so they're stacked on top of each other Okay lined up in parallel, but this is just they continue right so they continue Going left and right. So that's just a fraction of that muscle fiber So let's dive a little deeper into these muscle fibers and where the striations actually come from. And that's really the internal structure of these muscle fibers. So what are muscle fibers actually made up of?

Well, the whole muscle fiber actually has a chalk full of these contractile organelles called myofibrils. Okay, so these are specialized organelles. So it's just like any other organelle of a cell, like the nucleus, mitochondria, all those things that make up the insides of cells.

Well, muscle cells have these specialized organelles, and they're called myofibrils. So I know this is going to start to get confusing, right, because one muscle cell is called a muscle fiber, and then that muscle fiber is made up of a bunch of myofibrils, okay? So these guys are just these long rods essentially, okay, and they make up about 80% of the cytoplasm.

So essentially that's the majority of the muscle cell are these myofibrils. Of course we have those peripheral nuclei, we have some mitochondria thrown in there as well, but this is the majority of the organelles inside the cell. And when we talk about one myofibril, okay, it's made up of this repeating row of sarcomeres.

And one sarcomere essentially is the functional unit of muscle tissue. So the sarcomeres are what are actually going to be contracting, which is going to cause the entire muscle cell to contract. Okay, so we'll look at the sarcomeres a little more closely in a minute.

But just to kind of break this down for us, we said, you know, so let's go big to small, right? So we said one muscle is made up of many muscle fascicles. One muscle fascicle is made up of many muscle fibers, and those fibers are the cells, right?

So then one muscle fiber or one cell is made up of many myofibrils. Then if we actually just look at one myofibril, it's made up of many sarcomeres. So you can kind of go big to small or you can go small to big, but essentially it's kind of a breakdown of the structure of muscle tissue. So again, on the right-hand side, what we've done is we've taken one fiber and we've blown it up and we look inside and we see a... bunch of myofibrils okay and then within that myofibril you see the banding pattern right now the bad the banding pattern are the sarcomeres and we'll look at that in a second okay so now let's break down the sarcomere so i want you guys to know that the sarcomere is the basic unit of contraction for skeletal muscle but let's actually take a look at what the components of the sarcomere are and how it actually contracts.

Okay, so what are some special features? So the delineation or the boundaries of a sarcomere are the z-disc to z-disc. Okay, so these are just where the thin filaments come together and we'll talk about the myofilaments in a second. But essentially that is the boundary of the sarcomere. And it looks like a zigzag line.

So that's how you can remember a Z disc. Okay. And then the center of the sarcomere is the M line.

Okay. So what are these actual myofilaments? Okay.

So the myofilaments are what are going to be doing the contracting. So they're the contractile units of that sarcomere. So we have a thin or an actin filament and they start at the Z disc and then extend toward the center of the sarcomere and they are in blue.

So here's one actin or thin filament and here's another one. So they start at the Z disc and they go towards the M line. And then the thick or myosin filament starts kind of at the center. the center and it extends outward toward the z-disc on either side. It overlaps with the thin filament so this overlapping between the two myofilaments is what's very important for the shortening.

So the myofilaments are actually what are creating the striations in the muscle fibers. So they create a banding pattern. So we call the center section where there is complete crossover between those two myofilaments the A-band. Okay so the A-band is going to be the entire length of the thick filaments with the crossover of the thin filaments.

And we term this the dark band because there's more crossover, there's more filament interaction, so it creates a dark band. So if you think A for dark, because there's an A in dark, that's the A band, which is the dark band. Now the I band is the region with only thin filaments.

Okay, so here's the I band right here. And the Z disc is the center of the I band. and we also call this the light band because it only has the thin filaments it doesn't have the thick filaments and so it's a lot lighter okay so now we create this light dark light dark banding pattern which creates the striation look okay the vertical striations so because all of these myofilaments are stacked on top of each other the myofibrils are then stacked on top of each other therefore the and then the cells and so on right so if you stack everything on top of each other you're going to get this banding pattern across all of them to create the striations okay so they all line up because we have the myofibrils lined up and it's going to create that banding striation So here's just a blow-up picture of two muscle fibers stacked on top of each other so you can actually get a better sense of the banding pattern and the striations that are being created. So they're vertical striations right up and down and so you can really see that dark A-band next to the light I-band and in the center of the light I-band you can actually see a dark line as well. and your eyes are not playing tricks on you, that is the Z disc.

So remember we said that the Z disc is in the center of the light band, and so that is actually the Z disc. And we said that the sarcomere is Z disc to Z disc, so if we look at that, that's actually right here to right here. So it's half of a light band on either side of the dark band, okay?

So another structure found in the sarcomere is called titin. And titin, think of like a spring. Essentially, it's going to extend from the z-disc and attach to the thick filament.

So it's right here in yellow. And essentially, it resists overstretching of the muscle because we said that muscle can stretch as well, right? It has extensibility. So If it stretches, that means it's stretching and separating these myofilaments.

And so we don't want them to stretch beyond reach of each other, right? And so that's what this structure does, this titin, is it's not only going to hold the thick filaments in place, because they're not attached to the Z disc, but when it uncoils, the muscle's being stretched. And then it allows for some elastic recoil when the muscle then comes back to place.

Okay, so when it's being stretched, it's uncoiled. When the muscle actually contracts, it coils up. Okay, but the whole job is really to keep from overstretching the muscle. So here's just a little review of our pictures. So we kind of go big to small to see that sarcomere.

So the top picture are our two muscle fibers showing the banding pattern with the dark and light A and I bands. And then if we blow up just one of those muscle fibers, then we get the second picture. And so you can see how the muscle fiber is full of those myofibrils. And then if we blow up just one of those myofibrils, we're able to see the sarcomeres in series to each other and how they all line up and we're able to then see the myofilaments and how that sarcomere is put together.

Okay, so just a little bit of review. So now let's talk about another specialized structure in the muscle cell and that's the sarcoplasmic reticulum. So it's very similar to the smooth endoplasmic reticulum in a normal cell, but it's specialized in the muscle cell. So essentially there's these interconnected tubules that surround each myofibril. So think of it kind of like a sleeve of channels.

okay a sleeve of tubes or a stocking that is wrapped around each of these myofibrils and at the end they kind of come together and this area they come together at is called a terminal cistern and when they come together they butt up against what's called this t-tubule and so i'll explain what a t-tubule is so remember we said that there is a plasma membrane right around each muscle cell. And we just call us, you know, the muscle plasma membrane, the sarcolemma. Okay, so it's just the same thing like a regular plasma membrane.

And essentially, it's going to invaginate downward in between some of these micro, these myofibrils. Okay, so it's going to invaginate inward, and then it's going to create this transverse tubule. And wherever we find these T tubules or transverse tubules, either side is going to be those terminal cisterns of the sarcoplasmic reticulum.

And this creates what's called a triad. Okay, so a triad essentially is a T tubule with two terminal cisterns on either side. So that's why it's made up of three, right, triad. And so there's a lot that happens at this triad. And this has to do with how the muscle cell actually gets stimulated to contract.

Okay, so keep the sarcoplasmic reticulum in mind when you think of contraction. So we said it has something to do with contraction. So what is the actual function of the sarcoplasmic reticulum? Well, essentially think of it like a storage house for calcium ions, okay?

And so when the nervous system sends that signal down to contract, the sarcoplasmic reticulum releases all of these calcium ions. And then the calcium ions are going to diffuse all throughout the cytoplasm. And how they're going to do that is through those T-tubules, okay?

So this triggers what we call a sliding filament mechanism. So the sliding filament mechanism is the term for contraction. So it's what's actually happening with those myofilaments, the actin and myosin filaments inside the sarcomere. OK, so sarcoplasmic reticulum, just think of it like the storage house of calcium. the calcium is the little signaling ions that's stimulated by the nervous system and essentially those calcium ions diffuse out throughout the cell really quickly and they essentially trigger this whole sliding filament mechanism to contract okay so what is this sliding filament mechanism how does it shorten the muscle fiber So essentially we said that those myofilaments, right, the actin and myosin overlap, okay, they overlap on each other.

And so what's going to happen is they're going to slide across one another and that's going to shorten the overall sarcomere and therefore shorten the overall muscle fiber. So what happens is the myosin has these little heads, they look like little knobs. And they are what are going to attach to the actin filament. And what happens is we energize that little myosin head using ATP, and they're going to pull the thin filament towards the center of the sarcomere. So that creates more of a crossover between the two filaments.

So therefore, the overall sarcomere shortens. But keep in mind that the thin and thick filaments or the actin and myosin filaments themselves do not shorten. OK, so the filaments do not shorten, but the sarcomere shortens and therefore there's more overlap between the two filaments. OK, but if all the sarcomeres are shortening all at once, right, the whole myofibril is going to shorten and therefore the whole muscle fiber shortens.

And if all of the cells, all the muscle fibers in the whole muscle are doing this all at the same time, that whole muscle is going to shorten. And this whole process is initiated by those calcium ions, which are released by the sarcoplasmic reticulum. OK, and again, we need energy for this. Right. So we need that ATP to energize those myosin heads.

Muscle cells have a lot of mitochondria to produce that energy because mitochondria are kind of our powerhouses of the cell, so they can produce all of that ATP for this sliding filament mechanism. Let's talk a little bit about how the nervous system signals the sliding filament mechanism. So essentially we have motor neurons, right? So motor neurons are going to come down and innervate the muscle tissue. So this is a voluntary movement, right?

So we have control over this. And what they're going to do is create a neuromuscular junction. So that's where the nerve ending is. the muscle fiber itself meet. Okay, so remember we said that those nerves are going to branch and then these branches, the smallest branches, are going to interact with each muscle fiber itself.

Okay, and this is going to create this neuromuscular junction. And the place where there's actually the end of the neuron is, or the nerve ending, is called the axon terminal. Okay, so Or you can call it a terminal bouton, but I like axon terminal better. So this is just at the end of the nerve axon that's going to interact with that neuromuscular junction.

And the big thing about the axon terminal is that it stores a neurotransmitter. So this is a molecule that's essentially going to be released from the neuron. and it's going to then trigger a cascade of actions. Acetylcholine or ACH is the most common neurotransmitter that there is.

There are other ones out there such as epinephrine, norepinephrine, but when we talk about it, usually we're talking about acetylcholine. And so at this neuromuscular junction, we have an axon terminal, which stores that neurotransmitter. And then there's going to be a space in between that axon terminal and the sarcolemma.

So we said the sarcolemma is that plasma membrane wrapping around the muscle fiber. And this space is called a synaptic cleft. So it's just some terminology to think about. We'll talk a little bit more when we talk about the nervous system as well.

But this is the space that the neurotransmitter has to cross. So let's go through the process a little bit, okay, of what's actually happening. So this picture is from your book, okay, so I've just blown it up.

It was the same picture as the last slide, but I've just included the text from the textbook, okay. So if we have the nerve impulse that's coming down the neuron, right, and it's going to reach this neuromuscular junction. So there's three neuromuscular junctions right here because that's where the axon terminal meets the muscle fiber.

So what happens is this impulse is going to stimulate the neurotransmitter to be released. So the neurotransmitter is going to be released out of that axon terminal. It's going to have to cross that synaptic cleft. but then what is it going to do it's going to interact with the sarcolemma of the muscle cell okay which that is going to stimulate the release of calcium okay so this is going to travel through those t-tubules so remember we said that those t-tubules are important for triggering this whole mechanism okay so it's carried down those t-tubules so essentially to try to trigger all the myofibrils at the same time, right? So you want everything to contract at the same time, okay?

And that's how it does it is through that T-tubule. So it disperses very quickly through the cytoplasm to release those calcium ions which trigger the sliding filament mechanism, okay? So this is just a really cool transmission electron micrograph of the actin and myosin filaments. So you can see the myosin filaments are quite thick and bright white here and then in between them you can see a thin filament right here.

And you can actually appreciate the myosin heads here as well. So they're interacting with the the thin filament from the myosin which is kind of cool to actually appreciate that through this very high powered microscope So now that we've triggered that sarcomere contraction from the nervous system, what does that actually look like? And we actually have some pictures here, some microscopic pictures of what's actually happening along with the drawing.

Okay, so number one is a fully relaxed sarcomere of a muscle. So this would be a muscle at rest. Okay, and so what happens is is that the titin is slightly stretched out, right?

There's some overlap, because we always have overlap between the two myofilaments. And we know that the distance of the sarcomere is from the Z disc to the Z disc, right? So that's fully relaxed, okay? And we can see in the picture, because we just follow it right up, here's our A band, which is dark, right?

And then we have half of a light band on either side because that's the Z disc, right? So the Z disc kind of splits that light band in two. So what happens, right?

So now we have contraction. So we have shortening of the sarcomere. So what does that look like? And that's number two.

So we have much more overlap between the thick and thin filaments or the actin and myosin. Our titin is quite coiled. right because it's not we're not stretching the muscle and that's what its job is to prevent over stretching but look what happens is the z disc comes closer together okay so the sarcomere itself is going to shorten but look what stays the same so the a band is going to stay the exact same width and why is that that's because the a band is the entire distance of the thick filament.

So the thick filament is going to stay the same length, right? So is the thin filament, but we're losing some of the I-band because they're overlapping more. So if we look at our picture, our micrograph, and you notice that the A-band becomes a little bit lighter because there's more thin filament in there, which is kind of the lighter filament.

right and our eye band shortens significantly on either side right so there's a lot more overlap and the whole sarcomere shortens but again those filaments themselves do not shorten okay so that's what's happening microscopically in the muscle cell during contraction okay So now I've just written everything out for you that we just talked about over the last slide. So how does our striation pattern change during contraction? We said the sarcomeres shorten, but the filaments do not, right? The I-bands shorten, but the A-band does not.

The titin is going to coil during contraction, right? Because we're not stretching, we're doing the exact opposite. We're contracting. So I just wrote out everything that's happening during that muscle contraction.

So now we can extend muscles. So we said that muscles can stretch, they can extend. We have kind of equal opposite muscles over joints. So if one muscle is doing one motion, the other muscle is going to do the opposite motion.

Just think of flexion and extension, right? Think of your elbow. Well, you've got one muscle on the one side of the joint that's going to do flexion, right? Your biceps. And then on the other side of your arm, you have a muscle that's going to do the opposite motion, extension, and that's your triceps, right?

So biceps, triceps is kind of a classic opposing muscle movement, okay? So when we talk about the force of contraction, right, all muscles can contract, the greatest force that a muscle can actually produce is when the muscle has been slightly stretched, because you start out with less overlap between the filaments. And that means there's more room for overlap, right? So you can actually overlap more and shorten more. Okay, so that means you have a better force of contraction in that muscle so the best way to do it is slightly extend the muscle before contracting to get the most force okay and of course when we talk about muscle extension you're always thinking about the titan which is going to keep it from over stretching okay Now when we talk about contraction, there's really two main types of contractions.

And one you think of every day and the other one you may not realize is actually muscle contraction. Okay, so concentric contraction is the classic muscle shortening. So the muscle is actually going to shorten while it's doing its work, right?

That's our classic biceps curl. right? The biceps is shortening, you're doing work, right? But we have another one called eccentric contraction.

And this is actually a muscle generating force as it lengthens. So think of a muscle as putting on the brakes in eccentric contraction. So it's going to resist gravity or a weight. It's kind of the down portion of a push-up. Okay, you're resisting that Gravity, you're resisting falling to the ground, but your muscle is actually lengthening.

So I know that one's a little confusing, and concentric contraction is more the classic thing of what we're talking about, of what we think of as contraction. So concentric and eccentric contraction. So two types of mechanisms for contraction.

So now we're going to finish up the topic kind of with the muscle fibers. Okay, so we have muscle fibers in every muscle has all of these types of fibers. Okay, so there's three main types of skeletal muscle fibers and it just depends on the muscle with how many of each type it has.

Okay, because what is the job of that muscle and we'll talk about that in a minute. So we have two questions we have to ask ourselves to decide what type of muscle fiber it is. One, how do they manufacture energy? How does that muscle fiber get energy? Do they use oxygen?

So do they undergo cellular respiration or not? Or anaerobic respiration? So that's number one to ask, right?

So the difference is, yes, we use oxygen and that's oxidative fibers. Okay, so they're going to produce their energy using oxygen or called aerobically. If we don't use oxygen, it's called glycolytic fibers.

So they do produce energy, but they don't use oxygen. They use sugar, which is also called glycogen, right? So the unit of sugar is glycogen. The monosaccharide.

And so that's termed anaerobically, without oxygen. So they're either going to be oxidative or glycolytic. Now, the second question we have to ask ourselves is how quickly do they contract? Do they contract fast or slow? And that'll give us our three different categories.

And again, just to reiterate. All skeletal muscles have all three types of fibers. They just may have more of one kind than another, just depending on their job.

So here are our three classes of muscle fibers. So again, we have our slow oxidative, right? So we use oxygen, but we're slow.

Fast glycolytic, we use sugar instead of oxygen, and we're fast. and then our last one is fast oxidative so we use oxygen and we're fast okay and I'll talk a little bit more about the red and white or the colors of the fibers in a second okay so let's look at them individually So our first one is slow oxidative. So these guys are more red because they have a lot of myoglobin. So it's very similar to hemoglobin in the blood and hemoglobin is red in color and so is myoglobin. And that's what binds the oxygen.

Okay so we have lots of myoglobin because we're using oxygen to create our energy. So lots of mitochondria, lots of capillaries in there to supply the oxygen. Our contraction is slow, right?

So they contract slowly, but something that's really good about contracting slowly is you're very resistant to fatigue. So these muscles don't tire out very fast, okay? So they kind of contract continuously for long periods of time. Now you don't get a lot of power from them, right?

So it's kind of like the turtle. right the slow turtle it can kind of go for a long period of time but it may not be very powerful or very fast right and the fibers tend to be slow in diameter okay so what type of muscles do you think maybe would have more slow oxidative fibers so kind of think about it for a second That's right, so postural muscles. So think muscles that are going to have to contract for longer periods of time. So think of your trunk, right?

Your posture muscles. They're going to have to contract for a long period of time to keep your posture, right? So now let's look at the second kind, which is fast glycolytic.

So they're white because it doesn't have much myoglobin. right if we're not using oxygen or using sugar we don't really need that much myoglobin and we don't need that much mitochondria either because we're getting our energy from anaerobic pathways because we use glycogen or sugar now these guys contract quite fast so very rapidly but the problem with doing that is you tire quickly it's like the sprinter right so they can't go very far but they can go really fast okay because the fibers are quite large so they're about twice the diameter of a slow oxidative fiber and they contain more myofilaments right so if we have more myofilaments that means more sarcomeres that means more power okay so what muscles might have more fast glycolytic fibers That's right, our upper limb muscles. Okay, so these guys are quite powerful, right?

So they're powerful but they tire very quickly. They're prone to fatigue. Okay, now last but not least is our fast oxidative fiber.

So they are kind of intermediate in their color. They do have quite a bit of myoglobin because they use oxygen, but they're quite larger. They're larger fibers.

So they do have a good capillary supply as well. So these guys use oxygen to make their energy. Okay.

Now they're different from our slow oxidative because they are fast. So whereas slow oxidative contracts slowly, these guys contract quickly. But they are less oxidative. resistant to fatigue than are slow oxidative. So they're fast like our glycolytic, but they are a little more resistant to fatigue than the fast glycolytic because they're using oxygen, right?

So there's a good supply of oxygen, whereas if you just use up the sugar right away, that's all you got, right? So they're a little, they're not as big as the fast glycolytic, so they're pretty powerful but not quite as powerful as the fast glycolytic. So just think of everything kind of in between the other two, right?

So they're kind of a good mix of the two. They're a little faster, they're a little more resistant to fatigue, but they're not the two extremes. So what type of muscles do you think would have more fast oxidative fibers? well our last ones right so our lower limb muscles so these guys have to do locomotion because they have to be a little more powerful to propel our body weight forward but they don't have to be they have to be more resistant to fatigue as well so they're quite powerful but maybe not as powerful as our upper arm muscles but they are more resistant to fatigue so they can last longer right because locomotion can require a longer distance or performance okay so they're kind of in the in-betweeners the intermediate So now we're going to finish up with some disorders and development.

So what are some disorders of muscle tissue? Well muscular dystrophy is kind of a group of diseases and they're all inherited so they're genetic and they essentially destroy muscle tissue. So the muscles degenerate and what happens is the muscle tissue is replaced with fat and connective tissue.

So if you replace muscle tissue with fat and connective tissue it no longer functions, right? So muscular dystrophy is quite a degenerating disease and this is the picture on the right. If you look and you have see these normal muscle cells and then on the right hand side you see kind of few muscle cells with a lot of connective tissue kind of in between and replacing a lot of the normal muscle cells. So there's a couple different types of muscular dystrophy, but we won't go into the different types. Now myofascial pain syndrome is essentially pain caused by tight bands of muscle fibers.

So essentially a lot of the fascia. Okay, so this is can be caused by problems with the connective tissue with the muscles. Okay, so myofascial muscle and fascia.

So essentially the connective tissue can be too tight and cause some pain. Now fibromyalgia is kind of a mysterious disease or disorder causing chronic pain. So what happens is the brain processes things as painful.

It affects mostly women, but a A common symptom is widespread musculoskeletal pain. So is it really a problem with the musculoskeletal system? Probably not. It's probably having to do more with the neuromuscular system.

But it's kind of a disease of ruling out all the other problems. So if you've ruled everything else out, You could call it fibromyalgia if you have these symptoms. So again, there's not a ton known about it, but it is quite a crippling disorder in terms of pain. So how do muscles develop?

And this is kind of cool. This is how we get those multinucleated muscle cells. So they develop from myoblasts. Okay, so myoblasts are kind of the stem cells of muscle tissue.

Okay, so we said we have those germ lines, right? If we go back to our tissue lecture, we have our ectoderm, mesoderm, and endoderm. And the mesoderm is going to develop into these myoblasts.

And the myoblasts are what are fusing together to form one skeletal muscle fiber. Okay, so that's how we get these multinucleated cells. Now what's cool is that satellite cells are actually sitting around each muscle fiber.

So a thing is that muscle cells cannot divide. Okay, so they can't divide. They're kind of like neurons, okay, like the nervous system cells.

So they can't divide. So they're actually really kind of not very good at healing because they can't divide. But what they do have are these satellite cells. And they're very similar to myoblasts.

They're kind of embryonic or stem cell-like cells. And what happens is if a muscle cell gets damaged, then it's going to fuse that satellite cell to the brain. And that's what to the muscle cell itself to help them grow and repair.

So this is kind of the whole idea behind weight training and muscle development is you're actually making micro damages in the muscle cells so that these satellite cells start adding to your muscle cells. And so then they can grow and get bigger, okay? So kind of an interesting idea, but in terms of tissues that heal really well, muscle is not actually one. It's kind of in the middle because they can't divide. So they're definitely not the best at healing, which is something that we, it's kind of a misnomer.

So muscles actually start contracting even by week seven of development. So that's pretty early on, right? And the cardiac muscle is even earlier than that. So the cardiac muscle is actually pumping blood three weeks after fertilization. So that's super early, right?

So the muscle system is very early in its development. We have to get things moving around. Now, there is a difference kind of when we talk about the percent of body mass, because we said that, you know, muscle makes up about 40% of our body weight.

And that's a little bit different in males versus females. It's a little bit more in males and a little less in females. That's why we kind of average it out to be 40%. And the difference is due to androgens, right? It's due to the male hormone.

Now muscles age, right? So we tend to increase the amount of connective tissue in muscles and kind of decrease the muscle tissue itself. So the number of muscle fibers actually decrease.

But that's a good thing about exercise as we age to try to stimulate that. those satellite cells to continue to kind of lay down cells and keep healing and growing those muscle fibers. Okay, and we do get a loss of muscle mass with aging, and that's, you know, estimated to be about 50% by the age of 80. So we really decrease that muscular strength as we age. But again, you can kind of combat that with good exercise.

we age. So here are our learning objectives for the day. And next we're going to be talking about the muscles of the body.

So we're really going to be splitting that up into two lectures. So our first one we're going to be talking more about the head, neck, and torso or the trunk. And then the next lecture will focus on the limbs right kind of shoulder and pelvis and the upper and lower limbs okay see you guys next time

Transcript for:Exploring the Skeletal Muscle System

Transcript for:
Exploring the Skeletal Muscle System