what's up guys dr goodin here and in this video we'll be talking about the structure and function of muscle tissue okay let's roll that intro [Music] before you can really understand training physiology and programming and periodization you have to have at least a basic understanding of muscle physiology a basic understanding of muscle physiology sets you up to better understand some of the principles by which we train some of the neuromuscular mechanisms that we can harness or potentiate during training and it helps us to make logical training decisions based on anatomy and physiology of the athletes that we are training so today we're going to look at a basic overview of the structure and function of muscle tissue okay so let's dive right in okay this comes from chapter one of the essentials of strength training and conditioning textbook put out by the national strength and conditioning association authored by doctors hof and triplet okay so the muscular system we want to talk about both the macrostructure and the microstructure so big picture and small picture of muscle tissue what's happening at a whole muscle level as well as what's happening at the cellular level in muscle we're not going to go too deep i'll save that for exercise physiology but we do need to know the basic overview before we move forward so each skeletal muscle is in itself an organ that contains primarily muscle tissue but also connective tissue nerves and blood vessels right so we need this tissue needs to be innervated it needs to have blood flow for oxygen obviously and then also to remove waste products and it needs connective tissue to kind of hold it all together and give it a resiliency now if we check out this diagram we see three layers of connective tissue the outermost layer is the epimycm and that's going to run along the perimeter of this muscle deep to that we have the perimysium running in between these individual muscle fascicles okay fasciculus is the singular version of that word fascicles and literally the fasciculi that just means a bundle of sticks in i don't know latin or greek or something so it looks like a bundle of sticks right so early anatomists thought oh look at these long muscle fibers it looks like this bundle of sticks let's call it a fasciculus now within this fasciculus we have the endomysium running between individual muscle fibers okay and so those are your three levels of connective tissue now it's important to remember that all of these levels of connective tissue are actually continuous with the muscle tendon so when tension is placed on any individual muscle fiber whether by contraction actively or by passive stretch that tension runs through the length of the muscle and what is what allows the muscle to develop tension from one point of origin to its point of insertion now in this fasciculus we have these individual muscle fibers okay so here's that single muscle fiber coming out of the fasciculus and you can see that it's multi-nucleic right so we have all of these multiple myonuclei or muscle cell nuclei and the reason why it has so many nuclei is because it's such a long cell it has a large domain which each of these nuclei must control and so with a single nucleon nucleus this cell could not manage all of the cellular functions going on and so muscle cells are very unique in that they have multiple nuclei and at some point when we talk about hypertrophy we'll actually talk about how satellite cells donate nuclei to expanding muscle cells in order to increase the number of nuclei and expand the volume of that muscle fiber so as you enlarge your muscles through the process of process of hypertrophy the enlargement of a muscle cell you can actually gain more myonuclei so we see that inside of the single muscle cell we have sarcoplasm which is analogous to cytoplasm of a cell and we have these myofibrils these are the contractile strands within the cell that are themselves composed of myofilaments and the two myofilaments are actin and myosin oops actin and myosin so actin filaments are thin and they're woven together uh with strands of actin and myosin filaments are thick and they have these globular heads that latch on to the actin filaments now i mentioned that we have not only muscle tissue but also nerves and blood vessels and connective tissue so let's talk about the functional unit of the muscle which is actually comprised of both nerve tissue and muscle tissue and it's called the motor unit so the motor unit is the contractile unit of muscle tissue a single motor unit consists of the motor neuron also called the alpha motor neuron and all of the muscle fibers that it innervates okay so here we see a picture of the alpha motor neuron alpha motor neuron and we can see that it is innervating multiple muscle fibers within this muscle cell so each of these little neuromuscular junctions is representing a single fiber that this alpha motor neuron is innervating there are typically several hundred muscle fibers in a single motor unit but that can vary the motor units of your eye for instance contain only a handful of muscle fibers per motor unit while the muscles of your quadriceps those motor units contain hundreds of muscle fibers per motor unit it depends on how finely we need to grade our force output of these muscles now zooming into a muscle cell we see some important distinguishing characteristics that set muscle cells apart from the other cells of your body we can see the mitochondria floating around that's similar to other cells but we have what are called t tubules or transverse tubules and these openings in the cell membrane actually allow an action potential to infiltrate and spread across this long muscle cell very very rapidly so that you can have a more synchronous contraction of all of the contractile elements within the muscle cell we also have what's called the sarcoplasmic reticulum so in the sarcoplasmic reticulum is where the muscle cell stores and releases calcium calcium is super important because it's a second messenger the release of which is signaled during or after an action potential in order to travel into the contractile elements and allow actin and myosin to bind together as we'll see in a moment the membrane of a muscle cell is called a sarcolemma instead of a plasmalemma and then of course we see the myofibrils that we talked about previously so the main function of muscle tissue is to contract and to generate tension between two different points on the skeletal system to create movement and motion the way that muscle tissue accomplishes this is through what we call the sliding filament theory this slide here shows the multiple levels of detail that through which we can observe the sliding filament theory okay so let's zoom further into this image and go through step by step so first we see the whole muscle here and if we zoom in on the fasciculus here and then even further into an individual muscle fiber there and finally to the level of the myofibril then we see within these myofibrils what are called sarcomeres and a sarcomere is the contractile unit of the muscle cell and they run from z line to z line and in between those z lines is the boundary of the sarcomere so we see here the z lines and so what we're looking at is actually two sarcomeres in this image and we notice that sarcomeres have this striped or striated appearance and that is caused by the overlapping regions of the different contractile elements actin and myosin so remember myosin are the thick filaments and they're shown down here the pink ones and the actin are those lighter colored filaments so going back to our pictures of the sarcomere then we have the m line which is the anchor point for the myosin easy to remember m for m line and for myosin and we also have what's called the h zone and the h zone is where you only see myosin okay so it's going to be pinker then the a band is the region within which that those myosin filaments extend from end to end the a band is where you have myosin filaments the i band is where you have only actin filaments but no overlap with the myosin filaments so the i band is lighter in color and it's only the actin filament filaments and in between there you have your z lines now when we look at the myosin filaments close up we see that they have these globular heads and it is these heads that complete what's known as the power stroke right the latching and then power stroking and then breaking and then re-latching on the actin filaments to actually complete the contraction and the shortening of a sarcomere and in between those we have the braided actin filaments and they're actually covered with this troponin tropomyosin complex that you see here which blocks the binding sites for the myosin heads until calcium enters the cell so remember we talked about how the sarcoplasmic reticulum stores calcium and then releases calcium when an action potential infiltrates the t tubules on the muscle cell and allows that action potential to then allow this release of calcium into the myofibrils which then causes the troponin tropomyosin complex to move and allow the myosin heads to bind on the actin and then in the presence of atp which is the body's energy currency then we have the power stroke and the re-latching occurring okay so fairly complex however if we look at it from this uh big picture view zooming in from the whole muscle down into the individual sarcomere i think it gives us a better understanding of what's actually happening during a muscular contraction as you develop tension in the muscle down to the sarcomere vantage point and this has implications into how we understand things like the stretch shortening cycle or different types of muscle contraction eccentric concentric and isometric or into how we program rest periods based on which energy system that we're using so very important to understand this fundamental underlying basis of muscle physiology so the key point here is that it's the discharge of an action potential from a motor nerve that signals the release of calcium from the sarcoplasmic reticulum into the myofibril causing tension to develop so that was a very succinct way of explaining in one sentence everything that we've just talked about from the alpha motor neuron down to the individual contraction of a sarcomere that's the path that it takes now to summarize the sliding filament theory of muscular contraction we would say that actin filaments at each end of the sarcomere anchored to those z lines slide inward on the myosin filaments because of the globular myosin heads performing the power stroke and that pulls the z lines toward the center of the sarcomere and thus shortening the muscle fiber so when we have all of these different sarcomeres and they don't always shorten all at once within a muscle but you have various sarcomeres shortening and pulling and remember they're bound by three different layers of muscle fascia so as these sarcomeres shorten that fascia is generating tension and the tension because it's continuous with the muscle tendons anchored to your skeletal system that generates tension and force on your bones and that force creates leverage on the skeletal system and allows you to move your body and external objects okay so we have the big picture view of muscles moving on bones and the very microscopic view of what's happening at the sarcomere level now let's take another look at it going step by step through the contraction of a myofibril so in this first phase we'll just call this a this is a stretched muscle and the i bands and h zones are elongated because there's not very much overlap between the myosin and actin filaments okay that's the overlap so the i band which is only actin filaments and the a band which is the or sorry in the h zone which is only myosin filaments those are stretched out there's also a low force potential why do you think there would be a low force potential well it's because it's the globular heads of these myosin filaments acting on the actin filaments that produces force within a muscle the contractile force of a muscle can be really reduced down to the number of myosin and actin binding sites at any given moment and so if there's very low potential for binding between the myosin and actin filaments then there's low potential for muscle force now in the second phase of contraction this uh this is when muscle is contracting right partially it's not fully contracted to its shortest length but maybe maybe it's at its most optical optimal length for force production and that's because again we have all of these cross bridge potentials now there's a much greater potential for cross bridges to happen because all of these myosin globular heads can theoretically be bound to the actin in performing a power stroke okay so this is where we get into optimal length tension relationships you'll notice that muscle performs best or is strongest at certain lengths and it's different for each muscle depending on the orientation of its fibers and the fiber typing etc and then also depending on its origin and insertion and what type of lever it's creating but for muscle tissue itself just for the force of a single muscle fiber it is there is some sort of happy medium where there is the maximum number of myosin and actin cross bridges that can form and then finally once muscle is completely shortened um there is a v there's again a low force potential due to the reduced cross bridge uh potential and we see here that there's and we see here that there's actually an overlapping of the actin which is and the and the myosin is actually running into the z lines over here and so for instance if you've ever tried to do a bicep curl and you hold it at the top in that maximally shortened state you don't have there's not a lot of force you can produce there because your muscles already shortened or if you have ever done any type of like hamstring curl or a standing uh you know or a standing knee bend where you flex your knee and bring your heel up to your butt and you try to hold that for as long as you can those muscles are maximally shortened and they'll begin to cramp because they're actually running into each other the sarcomeres are are physically out of room they can't go any further and therefore the muscle is maximally shortened okay so let's recap really quick the phases of the sliding filament theory of muscular contraction first we have the resting phase right this is when there's no action potential no calcium has been released from the sr and then we have the excitation contraction coupling phase this is when an action potential has traveled down the alpha motor neuron it has infiltrated the t tubules released calcium from the sarcoplasmic reticulum and the calcium is now going into the myofibrils we have the contraction phase and this is when the coupling of the actin and myosin is happening cross bridges are forming atp is present in allowing the cycling of the myosin head and calcium's in there as well removing the troponin tropomyosin complex from the binding sites to allow this to happen okay and fourth we have the recharge phase and this is when we require that atp to release the myosin head so that it can engage again and perform another power stroke and finally the relaxation phase the signal has stopped coming down the axon of the alpha motor neuron and so now calcium is being taken back up into the sarcoplasmic reticulum and that covers the binding sites and so the myosin heads are no longer able to bind to the actin and therefore tension is reduced force is reduced within the muscle and it relaxes okay so key take-home point here that i've already said but i'll reiterate the number of cross-bridges that are formed between actin and myosin at any instant dictates the force production of a muscle so actin and myosin cross bridges dictates force production the more potential you have for actin and myosin uh to bind the more force so that means a larger muscle which has more contractile elements shoved into each of those muscle fibers will be a stronger muscle okay all other things considered now yes there are things like rate coding and neuromuscular factors that we're going to talk about in the next lecture but all other things considered the larger the muscle the more opportunities for actin and myosin to bind and therefore the stronger that muscle is the more force it can produce another key point calcium and atp are absolutely necessary for cross-bridge cycling with actin and myosin filaments so you need calcium in order to physically allow the contraction to happen and you need atp to allow those globular heads to detach so that they can complete another cycle okay and that was a basic overview of the sliding filament theory of muscle contraction i hope that was helpful to you in the next video we'll talk about the neuromuscular system factors related to recruitment and to motor units uh so i'll catch you guys on the next video but until then move well live well keep teaching other people to do the same this is dr goodin thanks for watching [Music] you