in this video we're going to focus on the microscopic anatomy of the muscle fiber and get into greater detail of what's actually happening in the myofibril so when we look at the typical muscle fiber or muscle cell it has it's a cell that is very large so it can be a very long cell so if you look at your bicep for example the length of the bicep is a single muscle cell now there can be lots of muscle cells within the bicep muscle but lengthwise it goes all the way from insertion to origin the diameter however the muscle fiber can vary anywhere from 10 to 100 micrometers and this has to do with how many sarcomeres are added to it to increase the load capacity of the muscle so there are some very unique adaptations to the muscle fiber cell that allow it to coordinate contraction so that the entire muscle cell contracts sort of as a unit instead of just having bits and pieces of the muscle contract so if i look at the plasma membrane which by the way there are a lot of muscle specific terms so in your reading the plasma membrane or ordinarily it's called the plasmalemma the plasma membrane in muscle cells you'll oftentimes see it's called the sarcolemma so anytime you see s-a-r-c-o sarco that's going to indicate muscle so when we look at the plasma membrane separating the interstitial fluid from the cytosol you have these invaginations of the plasma membrane and we're we call these invaginations transverse tubules or t tubules for short and these are going to be very important to allow electrical signals to go deep into the muscle cell so action potentials will go deep into a muscle cell to help coordinate muscle contraction muscle fibers also have a very intricate and heavily modified smooth endoplasmic reticulum now of course we don't call it the smooth endoplasmic reticulum we're going to use the muscle specific name the sarcoplasmic reticulum or sr for short and it is heavily heavily branched and the ends of the sarcoplasmic reticulum are very coincident with the t tubules so its modified function is to harbor a lot of calcium ions so this is where a lot of calcium ion storage is happening so in your textbook the smooth endoplasmic reticulum or parts of it these are given specific names now i just want you to be aware of these terms but i'm not going to be testing you the very ends that are next to the t tubules these are known as the terminal cisterna so you have a terminal cisterna on either side of the t tubule and then you'll come across the term triad which is basically the two terminal cisterna along with the t tubule now i'm happy if you just remember the sarcoplasmic reticulum this is where you store the calcium now there are some other modifications so the proteins there are special proteins in many muscle fibers one specifically myoglobin this is a cousin of hemoglobin and this allows for temporary oxygen storage by the muscle because there's a lot of aerobic respiration that's going to occur in the muscle sometimes the cardiovascular system can't keep up so you have this little extra insurance policy now for muscles that are darker in color you've probably looked at a chicken dark mate versus light meat the darker meats are usually associated with muscles that are very active you're going to have a lot more myoglobin there hence the coloration of that muscle you also have special special vesicular storage structures for glycogen so remember this is the polysaccharide that you can break down into glucose so that you have a ready supply of glucose for metabolism so there are other crucial structures and the most crucial structures are the sarcomere so i'm just going to draw a very small section of the sarcomere it's a much larger structure that i'm than what i'm drawing here but the sarcomere is going to consist of two main components the thin filaments and the thick filaments and each of these filaments is dominated by one specific protein the thin filaments you have a protein called actin and in fact it's sort of a globular version of itself and the way actin looks like because it's globular just imagine it's a spherical protein and based on its three-dimensional structure it has this binding site so this is a binding site for a motor protein called myosin now remember actin was a very important protein in the cytoskeleton it formed the basis of the microfilaments here it's co-opted to generate this contracting unit the main component of the thick filaments is a motor protein called myosin and so myosin consists of four polypeptides you have this long tail and this bulbous head and on this bulbous head you have a binding site and this binding site is specific to actin there is a second binding site this is the part of the motor protein that is atpase so this is a region that will hydrolyze atp and use that energy to move right remember mechanical work and in that mechanical work the tail doesn't move but this head region can go backwards and forward sort of in a ratcheting mechanism okay so when we look at the sarcomere you actually have a bunch of myosins where their tails are overlapping and the heads form these little sort of they almost look like little oars and then on the other side the same thing so the myosins multiple myosins together form the thick filaments now in between you have a very large protein that kind of coils at the end so it acts like a little shock absorber and this large protein is called titan and it's going to anchor the thick filaments to a group of proteins which we're not going to talk about except give their name called the z disk and this central area where i drew the little dot dot dots there's also proteins there um specifically myomecin but that's not a name you'll need to know uh the name i want you to remember is this middle point is called the m lime okay so these are the two main proteins in the thick filaments the actin on the other hand sort of anchor to the z disc the actins will sort of link together like so and notice how not every single myosin overlays with the actin okay there is sort of an optimal resting length if you will for the sarcomere now there's a protein that is blocking the actin and the myosin from interacting the name of this protein is tropo myosin so it blocks actin and myosin interaction the last protein we need to be aware of is a protein that is associated with that tropomyosin and what's really significant about this protein well it's significant in general is that this is a calcium binding protein so the name of this protein is troponin and it is a calcium binding protein so if calcium is in the cytosol it's going to interact with this protein and cause the activity of this protein to change what happens in muscle physiology is when the calcium binds and let's actually draw a little a mini version of this right so you have basically the actin and you have the myosin head but you can't interact because tropomyosin is blocking that interaction so let's say this is at rest okay when you have activity and you have calcium in the cytosol the interaction is actually going to change the tropomyosin is shifted away so it's no longer blocking and that's because the calcium bound to the troponin and the troponin joint the tropomyosin away as a consequence the myosin can now very strongly interact so here we have a very weak interaction here we have a strong interaction that we see during contraction so there are many other proteins some of which i actually have listed here nebulon myomecin dystrophin these are mentioned in your textbook but these five proteins myosin titan actin tropomyosin and troponin this makes up the basics of the sarcomere now of course you have many of these units here that will cumulatively form a sarcomere but you can stack sarcomeres end to end right especially if you're dealing with a muscle that's very long and you could also stack sarcomeres sort of top to bottom if you want a muscle that's thicker that has a greater capacity to generate tension so the last topic within this video that i want to discuss discuss is the sliding filament model and this model gained a lot of acceptance when we look at the histology differences between a muscle that is at rest versus a muscle that is contracting but we've arrested it in that contracted state now earlier when we talked about histology we discussed how there was this periodicity of light dark light dark you actually saw striations now your textbook goes on to name specifically these zones but these are zones that we're not going to be too worried about at least for our exam when you have overlaps so just imagine i'm looking down at this muscle fiber when you have an overlap of both the thin and the thick filaments you're viewing through more stuff more proteins so these regions will appear darker because they're sort of screening out a lot of the light however when you're looking through and let's say you just have thin filaments or you just see thick filaments well there's less stuff to look through these will appear light so the light dark light dark light dark periodicity that you see in muscle striations has to do with the organization of the proteins of the thin and the thick filaments so when i look at again the names of the the bands are sort of irrelevant right the age zone here is light the eye band here is light the overlap between actin and myosin it's not the a band but the overlap here would be dark so when you arrest a muscle cell you actually have a thicker dark region so why do you have a thicker dark region well it has to do with the actin and myosin interaction the myosin the only part of myosin that moves is the head it goes backwards and it goes forwards that's the only part of the thick filament that moves the thick filament remember because of titan was anchored to the z disc what's going on with the actin let's say this actin is interacting with this myosin the myosin will interact with this actin and as it shifts backwards and forwards it's going to cause this actin to shuffle that way and when it shuffles that way it pulls the thin filament closer to the m line so the heads are moving for the myosin but in essence it's really the thin filaments that move across the rotating heads of myosin from the thick filaments so as a consequence the thin filaments are shunted closer to the m line that's why you get this overlap so in the next video we're going to focus on the overall physiology but remember the overlap that we see here that thick band the more overlap the more interaction we have between actin and myosin and we're going to call these interactions [Music] cross bridges so the more cross bridges you have the more tension that muscle is going to be capable of