so now we have looked at various types of muscle let's focus in on skeletal muscle we've seen that we have two types of striated muscle skeletal muscle which is our voluntary muscle that we are able to move on command and it moves the bones of our skeleton like lovers so this is the one as I say that we will be focusing on in this chapter but as a quick review we also have cardiac muscle which is also striated but it is involuntary it is not under our conscious control and then we have smooth muscle which is also involuntary and differs from skeletal and cardiac muscle and then it is not straited and we talked a little bit about why skeletal muscle and cardiac muscle is striated or striped and appearance but we'll see a little bit more detail why that is first let's just have an overview of a skeletal muscle here we have a muscle attached to a bone in this case it's the femur but muscles first of all have to have some kind of attachment if they're going to move bones they have to have an attachment to them and that will be in the form of a tendon typically which is a rope like structure that is made up of dense regular fibrous connective tissue that's basically bundles of collagen now sometimes we will have a muscle that is attached to other several bones or maybe a sheet of connective tissue so sometimes instead of taking the form of a tendon a muscle attachment can take a sheet like appearance and this is called an aponeurosis but either way it is a point of attachment for a muscle either to several bones or to a collection of connective tissues now one of the things we're going to see is that each muscle has what is called an origin and that's the part where it's relatively fixed it doesn't really move and then it has the point of insertion and the point of insertion is going to be the point of attachment that actually moves so that when we looked at our biceps brachii which was this muscle of the arm we had our point of attachment are in our origin up on the shoulder and the point of insertion in our radius so one of the long bones of the forearm so that when we move our biceps brachii we're actually shortening it from the forearm and decreasing that angle between the forearm and the upper arm and so that insertion is the point of attachment that news so we have a tendon is going to be the point of attachment and the point of insertion and sometimes it's an eight-point euro so so as I said both of these are connective tissue ways to link a muscle to above now then we have this mu fashion or fashion that covers the muscle and we consider each muscle it's an organ and so it has a connective tissue sheath this muscular flash o which surrounds it and basically separates it from other tissues within the body now within of the muscle we then have this connective tissue covering called the epimysium and epi as we know means a pun so this is a covering below the fashion that is upon the muscle and it's basically what really kind of holds it together and this epithelium is going to cover the entire outside of this muscle and the muscle itself will be further subdivided into something called fascicles and a fascicle is just a bundle and so if we look here we can see these individual classical's each one of these fascicles will be surrounded by another connective tissue layer called the Paramecium and this is around a fascicle to each one of these and if we extrude out one of these fascicles we can see that the Paramecium is here and it's going to provide a route for blood vessels and lymphatic vessels and nerves here's a nerve here and each one of these fascicles comes further be subdivided into the individual muscle fibers so fascicle means bundle and what it is a buggy off these muscle fibers and each muscle fiber as you recall is an individual muscle cell each muscle fiber is surrounded by something called an endo mesial and that is this very a real er connective tissue layer that basically covers and surrounds an individual muscle fiber it will surround the nuclei and remember we have hundreds of nuclei all along the length of this muscle fiber because these muscle fibers can be several centimeters in length we have blood vessels we have neurons and we have to had a way police use to get to the muscle and so they could travel through this this endomysium or this connective tissue layer that surrounds each muscle fiber now if we look at each muscle fiber we will see that it is further subdivided and something called myofibrils so within an individual muscle cell we have these myofibrils and the myofibrils run the entire length of the muscle cell and the myofibrils are going to be made up of something called sarcomeres you can see one here here is one sarcomere so if we kind of zoom in on that a little bit but you see two sarcomeres here here we have a muscle cell and with extruded out one of our myofibrils and we can see a sarcomere and that's one of these little chambers if you will compared to here and this is the contractile unit of muscle this is the thing that actually contracts and short use the muscle so the functional unit of muscle is the sarcomere and let's have a real quick look at what makes up the sarcomere we have thick filaments made up of myosin and we have thin filaments made up of actin and you'll see that these things do not run the entire length of the sarcomere our myosin filaments are centered along the middle of the sarcomere along what's called the in line which is this protein disk of basically the it's a disk that runs right straight through the sarcomere and it's what allows these myosin filaments to be anchored to it and if you were to look three dimensionally you would see on the side here you would see myosin filaments if you were to look in here you would see that the myosin if you look straight down in here would be surrounded by these actin filaments but only at the edges so you can see that the myosin filaments do not extend the entire length of the sarcomere nor did the ex actin filaments the actin filaments are anchored at either end of the sarcomere at this area here called the Z line also called the Z disk and remember this is a three-dimensional structure so while we're looking at the end of this thing here the sarcomere sort of along the z line we would see that we have this kind of arrangement of actin filaments or thin filaments arranged in this kind of pattern in here and if it were to look straight down in there like this were tube and we're looking straight down into it we would see eventually these thick filaments and the thick filaments are anchored at the em line and you can think of em for middle and you can think of Z as like the end of something and what we'll see as what causes the muscle contraction will be the interaction between these thick filaments and these thin filaments and we will see that the thick filaments anchored at the em line pretty much stay in place but they have these heads on them they are made of a protein called myosin and the myosin have these heads that kind of wrench it in and out and pivot and these heads are going to be able to grab onto the actin filaments and then pull them towards the in line and in doing so we see that the actin filaments are connected to the Z lines that's going to bring the Z line closer to the m1 so basically this whole sarcomere will shrink it will pull together and these z lines will come closer to in line and that's going to shorten this whole thing and you can imagine that of all the circum users in this entire myofibril contracted at once the myofibril would do that and when you consider that the muscle cell is made up of myofibrils if all the myofibrils are doing that it wants the muscle shorten all the muscle cells that are being stimulated will shorten at once and so this interaction the these thick and thin filaments is what causes the muscle contraction and because we have the thick filament anchored at the m-line the only part of it that moves really our heads the little things that pivot that grab onto the actin and pivot towards the in line and pull the act in towards the in line so the actin then will slide over the myosin and what we call the sliding filament theory of muscle contraction so basically what's going to happen is the actin will be pulled by the myosin closer to the M line and that's going to bring the Z lines closer together now when you look at one of these things through a microscope as we've done we see that we have the thick filaments and the thin filaments that create these bands these striations and if we look closer still we would see that we have an a band which comes from a word and isotropic which is a microscopic Ramon microscopy drum we're not gonna get too worried about what it means but think of a isn't dark because the thick filaments are thick so not as much light passes through them and so they're going to appear darker so the a band is the dark band and then where we have our actin from here to here or there's no overlap we have what's called the eye band which literally means isotropic but in our case we can think of I for light where I like I in the middle of light and these are comprised mostly of thin filaments so we don't have as much blockage of light coming up through the slides so they appear lighter in appearance so basically our eye band corresponds to where we have actin only and our dark band our a band is going to correspond to where we have myosin now at some point we're going to have a zone of overlap and we'll see that we have the zone of overlap right here and this is where we have on either end of the myosin filaments we have a little bit of overlap with the actin filaments because these myosin filaments going to have to grab onto these actin filaments and pull them there's going to have to be some zone of overlap where they can actually reach these actin filaments and reach up and grab them if there were no zone of overlap there could be no interaction now we'll see that we have in the middle here we have the H band and that's basically where there's no overlap so that is the part of the dark band where there's no overlap and I'm not going to get too terribly picky about that but suffice it to say that we do have an area and a resting muscle where there is no no overlap there's just myosin and so this would be our H band so now let's look at what this thing looks like under a microscope so here we have a true micrograph and this is a little bit differently stained from the one that we saw in our muscle slide but we can see that we've got a different stain as well as we're more closely zoomed in so here we can see the defining edges of our sarcomere that is our Z lines so our z lines are here in black our light band or I band extends on either side of it because this is where we had actin filaments only or thin filaments only then we have our dark band centered around the middle of the sarcomere on the in line and the dark band is this purple band that is myosin and where we have the lighter purple it is myosin only and where we have sort of it gets darker still that's where we have the overlap of myosin and actin or the overlap of the thick and thin filaments so if we're doing quick cartoon rendering of this thing so that we can talk about the function a little bit I see the M line which is our anchor point for our thick filaments and we see our Z lines which are anchor points for our thin filaments and as the thick filaments interact with the thin filaments they're going to pull those actin closer to the M line which is going to have the effect of pulling the Z line with it because our actin filaments are anchored to that z line we also have this a wavy protein right here and this for the core of the thick filament and this is called Titan G itin and this is an elastic type of protein that has two major functions one of which is to keep the spacing between the thick and thin thin filaments so they don't bump into one another and do this kind of thing but where they keep their spacing to where they can have their optimal interaction but also to bring over stretching muscle because we want to make sure that we have some area of overlap between dickenson filaments so that the myosin heads can actually grab on to the actin and start interacting with them so this Titan as you see it looks kind of wavy and elastic is there to prevent so that if the muscle gets over stretched and the sarcomeres get pulled out so long that the actin and myosin could no longer interact but give it a little bit of a recoil and that's going to pull the sarcomeres back to the right length where we can get this zone of overlap so that the actin and myosin filaments can interact with one another now there are some terminologies that we need to know when we talk about muscle so when we talk about a muscle fiber remember that is a muscle cell and cells as we know have a cell membrane well in the case of muscle we have a very special name for the cell membrane and the cell membrane is called the sarcolemma and anytime we see the word Sarco it has to do with muscle likewise any time we see my o as in myofilaments over here our myofibrils this has to do with muscle as well so typically we will see the term Sarco and Myo associated with muscle so our sarcolemma is our cell membrane of muscle and lemma means sheath comes from the Greek word sheet so it really means this outer covering that is the sheath of the muscle fiber we have the cytoplasm inside the muscle cell is called the sarcoplasm and then we have the endoplasmic reticulum inside the muscle cell is called the Sarco plasmic reticulum so these are just special terms for cellular components as they pertain to muscle cells now we can also look at the arrangement of our sarcoplasmic reticulum the sarcoplasmic reticulum is arranged in a network around myofibrils and we will learn that the sarcoplasmic reticulum has an abundance of calcium ions inside and this will become very important and we will see that around the zones of overlap we have these bulges and these bulges are called cisternae and some texts will call them terminal cisternae we will also notice another feature we will notice these things right here these are called transverse two booties and if you look out here they have their openings out on the sarcolemma and what they are is they are an extension of the sarcolemma deep into the muscle so and the reason that we will need to extend these the sarcolemma via these t2 Bewley's deep inside the muscle cell is that we will learn that the sarcolemma is an excitable membrane what this means is it can conduct an electrical signal and it can propagate an electrical signal along its length and so if we look at muscle cells we will find that they have a voltage associated with them that is they already have electrical charge and what we call a potential and a potential is a separation in charge and so we will find that we have positive ions and negative ions both inside and outside cells in general muscle cells like almost every other cell in the body have much more sodium outside the cell so you my owns much more potassium ions inside the cell both of those are positively charged ions however we have other ions and more specifically we have negatively charged proteins inside the muscle cell didn't make it more negative on the inside so that means we have a separation of charge across this membrane across the sarcolemma and that separation of charge is called a potential and its measured in millivolts and on a muscle cell it is typically negative 90 millivolts so in a resting muscle cell that's not doing anything we have a charge that could be we stuck a little electrode in there we would measure about 90 millivolts now the other thing that's interesting as I said is that this is an excitable membrane so it can conduct an electrical signal and that electrical signal is what we call an action potential and the action potential will be a reversal a very brief reversal of this electrical charge so it will become momentarily positive now this is what is going to stimulate the muscle cell we'll see how the muscle cell is stimulated we'll see how it is stimulated a little bit later on by what we call a motor neuron and we will see how an action potential is propagated through the muscle cell well in order to get inside the muscle cell to trigger the contraction we need a way to bring the action potential inside the cell and that is the job of the transverse stimulus and we will see the transverse duties are arranged in these what we call triads and that is where we have a transverse stimulate being surrounded by one of these terminal cisternae on either side of the sarcoplasmic reticulum and we will see that it's very important that we have this arrangement because where we have our terminal cisternae is where we have an abundance of calcium ions the electrical signal that is the action potential that is propagated across the cell membrane the sarcolemma would be brought into the cell where it will cause the release of calcium from the sarcoplasmic reticulum and this will be key as we see later on when we study how the muscle contract if we look at another part of the muscle cell we have a very specialized part and that is where we're going to get our signal from so obviously the muscle has to generate an action potential but where does it get the signal to do this well in order to do that we have what is called a motor neuron and the motor neuron is literally a neuron that comes out of your spinal cord comes to your muscle and it receives a signal from neurons in the brain and these neurons in the you're going to send this signal to a motor neuron that will then leave the spinal cord and contact almost a cell and it's going to stimulate that muscle cell so when you think about making an action you pick up something and you make that movement then you're sending a signal that originates from your brain and eventually it's going to reach a muscle via one of these motor neurons and again the motor neuron also has an excitable membrane and we'll talk about it in a lot more detail when we study the chapter on the nervous system but suffice it to say the motor neuron has an excitable membrane that can conduct an action potential and that is this electrical signal and once that action potential reaches the end of the motor neuron is going to cause a series of events that will cause the muscle cell to generate an action potential which is a brief electrical signal that will be propagated all over the muscle the muscle fiber via its sarcolemma or the muscle cell membrane which is able to conduct this action potential now we will see that this part of the sarcolemma here where the motor neuron makes contact has a very special name we call this the motor endplate the motor inflate is a specialized part of the sarcolemma which is going to be signaled by the motor neuron and the motor neuron where it makes this junction with the motor endplate we call this entire thing than our own muscular Junction and so when we study the function of muscle we'll see that the muscle will be signaled by a motor neuron to contract and it will do this via an action potential and it'll act like a relay will have an action potential from the motor neuron and then we'll have a series of events at the neuromuscular Junction and that will trigger an action potential within the muscle cell and that will lead to the contraction of the muscle before we get into that let's look a little bit more closely at the anatomy of the sarcomere because as we know the sarcomere is the contractile unit or the functional unit of the muscle so we already know that we have thick filaments that comprise that are comprised of myosin and ten filaments that are comprised of actin and we know that the myosin will pull on the actin and shorten the sarcomere now let's have a closer look still here we have a thick filament these individual things here are myosin molecules and myosin molecules have these heads and the heads would interact with the actin and typically when we have a resting muscle they're actually prevented from interacting with the actin but let's look very closely we have several hundred of these myosin subunits or myosin proteins that are bound together to form a single thick filament and we'll see that we have these heads that sort of jut out from this thick filament and they go all the way around it and remember that the actin filaments are surrounded or I'm sorry the myosin filaments are surrounded by the actin filaments so the thick filaments are surrounded via the thin filaments which are made primarily of actin we look and we see that the actin has this interesting shape here and actin has several components to it and the act in this play case are these sort of white blobs here and these these sort of silver white blobs in this picture there are two components of actin reeling we have something called filamentous actin and we have two fibers of filamentous actin that are wound around an inner core called nebula I don't think your book shows it it's not that important but you will notice that the filamentous actin sometimes abbreviated f-actin you've got two strands of it wound around each other and they are made up of these little pearl like structures these little blobs these are globular proteins that make up the filamentous actin and they're called G actin for globular actin and each one of them if you see right here as this little slight on it called the active site and it's sort of purple here but as you can see it's covered up by this molecule right here so the actin is this white part then we have something called tropomyosin and troponin to turn trope means to turn so this tropomyosin is sitting on top of the actin and it's covering up these active sites and what these active sites are our area is that these myosin heads would grab onto if they could but right now they're covered up so these two things cannot interact and we'll see a little bit later that we will have a shifting of position or a rotating as the term Tropo myosin truck means turn where these Tropo myosin molecules will rotate away from these active sites on the actin molecules and that will allow for the interaction of the myosin hands with the actin molecules well they can't do it while the tropomyosin is sitting on top of them in fact the tropomyosin is kind of like a gate and right now on the gate is blocking the interaction between myosin and actin and it's doing so because it's sitting on top of the active sites on actin that these myosin heads wouldn't want to grab onto if they weren't blocked so we consider tropomyosin much like a gate the way this blue molecule here this is called troponin now if tropomyosin is a gate that is closed right now over the actin filaments then the troponin is the lock that holds the gate into place so we can think of tropomyosin like a gate right now it's covering or blocking the active sites on actin and so we could say the gate is closed and it's being held closed or held in position by this lock which is troponin and tropomyosin molecule right here and we will see that it is going to interact with calcium so calcium will be the key we know calcium is that ion the stored in the sarcoplasmic reticulum that is released upon stimulation by an action potential so you can see how if we have calcium being the key binds with troponin which is going to change the confirmation or the shape of that molecule much like unlocking a lock then the tropomyosin gate can now swing open and that's going to reveal the active sites on actin and that's going to allow the interaction of the myosin heads with the actin filaments and we'll learn about that in the next section