IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 PDF

Hello and welcome, everyone. This is a year one presentation in the introduction to medical sciences module. It is part of your TBL preparatory materials for the team based learning session entitled Introduction to Neuroscience, Membrane Potential and Muscle contraction. This particular presentati...

Hello and welcome, everyone. This is a year one presentation in the introduction to medical sciences module. It is part of your TBL preparatory materials for the team based learning session entitled Introduction to Neuroscience, Membrane Potential and Muscle contraction. This particular presentation is entitled Sarcomeres and Sliding Filament Theory Part one. My name is Dr Julianna Gal, but please feel free to refer to me as Julianna in any further correspondence that we may have. As I mentioned, this particular presentation is entitled Sarcomeres and Sliding Filament Theory Part one. And it is a portion of a three part presentation series, all relating to sarcomeres and the sliding filament theory. So in part one, we will be considering skeletal muscle organisation and the structure of sarcomeres. Part two will we'll be focusing on skeletal muscle contraction and the sliding filament theory. And then finally, in part three, the focus will be control of skeletal muscle force and the role of calcium. So in respect of this first part, skeletal muscle organisation and structure of sarcomeres, the following overview is appropriate. Firstly, we'll be looking at anatomic organisation of skeletal muscle. Then we will be looking at the more fine ultra structural organisation of skeletal muscle. And then finally we'll be considering the sarcomere and some of the key biomolecules IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 associated with the sarcomere and how they are organised within the sarcomere, particularly with an eye to thinking about how they relate to force production. So in this first slide, what I wanted to get across to you, if you're not already relatively familiar, is the idea that skeletal muscles come in a wide variety of shapes and sizes. And so we have three photographs of dissections of the human body where its skin and subcutaneous fat has been removed. And so revealing the more superficial musculature. On the left panel, we see mostly the upper arm from the shoulder region through to the elbow region, and you can see that the two arrows are pointing to the tendon of origin of the long head of biceps brachii the tendon of origin of the short head of biceps brachii, and you have this distinctly sort of fusiform shaped muscle belly in the middle. You can't quite see it, but this muscle attaches to the radial tuberosity and allows for the flexion of the elbow joint. If we look at this middle photograph, you can see the anterior aspect of the entire thigh region exposed. And I've indicated a number of muscles. So on the lateral aspect, we have the muscle belly here of Tensor fasciae latae which is close to where the hip joint is. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 And so the tendon of origin is relatively short. But in this case, the tendon of an insertion is quite long and fuses with the iliotibial tract, which you can't quite see. But this actually is a structure that is often invisible to the naked eye in real life, in a real human all along the lateral aspect of the thigh. Now, some muscles are very distinctly strap-like such as the sartorius, which is uniform in its thickness along its whole length, which cuts across the diagonal of the anterior thigh. Another muscle I've highlighted here is the iliopsoas, which originates at the Ilium as well as the spine. And this muscle attaches to the femur, so this is very much a hip flexor muscle. A hip adductor muscle is adductor longus. So what you can see here is that you have this shiny sort of white pearlescent sheet of material. This is the connective tissue that has the role of the tendon. It's not shaped like a cord like we saw in biceps brachii. But this sheet like morphology is what we call an aponeurosis. And the fibres attach essentially to the underside of the sheet, inserting on the femur and adducting the thigh. Finally, I've also included vastus medialis, which is part of the quadricep muscle group. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 Here we have you can see the fibres attaching obliquely. So at an angle to this central connective tissue tendinous structure, which ultimately becomes the patellar tendon and then the patellar ligament. And then finally, as a really nice contrast to the muscles that we've looked at so far, we have the right aspect of the dorsal of the human body, the back exposed again, the skin and subcutaneous fat and removed. But what you can see is the site of origin of latissimus dorsi along the spinous processes of the thoracic and lumbar spine and the fibres literally fan from a wide site of origin and converge, as I'm sort of showing at a very concentrated point along the anterior aspect of the humerus, so you can't quite see it in this photograph, but suffice to say that we have a very broad site of origin and a very concentrated site of insertion. So an almost triangular shaped muscle in this respect. So we've seen that skeletal muscles have quite a variety of shape and sizes, but they all share a number of common features in terms of the gross anatomy or the gross organisation of skeletal muscle. They all have a tendon of origin. They all have a muscle belly and they all have a tendon of insertion. So the muscle belly tends to be the reddish brownish tissue that you could see in the previous slide, whereas the tendon material tends to be white pearly in colour. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 So in terms of their gross organisation, skeletal muscles typically connect one bone to another, crossing at least one joint. And it is the muscle belly that is the site of active force production. By Active, we mean that it's the force that is generated by the active stimulation, by the central nervous system. Now, when stimulated by the nervous system, muscle belly generates a contractile or a pulling type force, sometimes also called a tensile force. The contractile force pulls equally on the tendon of insertion and the tendon of origin, so that pulling effect is uniform along the length of the muscle. And so the pulling effect at either end is the same in terms of the absolute value of force. So what we see is the outcome of a skeletal muscle contraction depends upon the net effect of all the forces at each end of the muscle. What I want to do next is to consider how skeletal muscle actively generates force. So in order to explore that further, we need to look much more carefully at the organisation of skeletal muscle, particularly down right down to the cellular level of skeletal muscle. So here we have a diagram that shows an expanded view of skeletal muscle structures going from the whole muscle right through to the single cell. So here we have a cylindrical sort of portion shown which I'm tracing with the cursor and that is representing the whole of the muscle, IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 a whole muscle belly and the whole muscle belly is surrounded by a connective tissue called the epimysium. Now, the whole of the muscle belly is essentially a group of fascicles. So these circular portions that I'm now pointing to are all a number of muscle fascicles. One of which is kind of expanded to see more clearly. And one fascicle is surrounded by another connective tissue layer now called the perimysium. So each fascicle is surrounded by a fine connective tissue layer called the perimysium. If you expand one of these fascicles, you'll see that essentially they are made up of a cluster or a bundle of muscle fibres. So here's the circular cross-section of the muscle fibres. Three are shown right here, here and here. And at the level of the muscle fibre, which is the same as a muscle cell, in addition to having the muscle cell membrane, just like any cell, will have a plasma or a cell membrane, the muscle cell or muscle fibre also has a very fine connective tissue layer around it. This one is called the endomysium, so the endomysium surrounds each muscle fibre. And if we now expand and magnify a single muscle fibre and look at it in cross-section, we see, for example, represented a nucleus. And we also see that the dominant feature in a muscle cell or muscle fibre is bundles of this linear material shown looking like strings poking out of the cross section. And if we look closely at these myofibrils, which are these strands, IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 we see that they are made up of units that I'm indicating with my arrow and moving along units strung in a line along a chain. And those units are called sarcomeres. So myofibrils. Which form the dominant feature within muscle cells are made up of chains of sarcomeres. And if we actually further increase the magnification to look and see what's inside or what makes up these myofibrils we see very fine strands, very fine linear strands of material. These are called myofilaments, and they are made predominantly of the main contractile and regulatory proteins, actin and myosin. So we have linear bundles of materials right from the cellular level, the muscle fibre or muscle cell right through to the whole of the muscle. And there are connective tissue layers at each of the fundamental layers around the whole muscle is the epimysium. Around each fascicle is the perimysium. And in fact, around the muscle cell or muscle fibre is the endomysium. In this slide, we can see a really nice image that shows the relative scale of the muscle fibre compared to the connective tissue fibres that surround it. So here we have an electron microscope image and this large structure that kind of cuts almost vertically from top to bottom. That is essentially part of a single muscle cell or muscle fibre. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 And the very fine strands that you see cutting across more or less from left to right of variable thicknesses. So some very, very fine, some slightly thicker. Those are the collagen fibres that make up parts of the endomysium layer that surround the muscle fibres. So it's been disturbed a little bit because of the the preparation for the image. But suffice to say that the absolute size of the skeletal muscle fibre is much, much larger than the fine connective tissue layer that surrounds it. But if you have this connective tissue layer at the cellular level, the fascicle level and the whole muscle level. In fact, you have a fair amount of connective tissue in amongst the muscle fibre or muscle cells. So what we want to do in this suite of presentations really is to consider active force production within the muscle. We said it happens within the muscle belly and in particular it happens within the muscle cell or the muscle fibre. So we can consider what type of structures does a muscle cell contain that lends itself to the production of force? So certainly what we saw from the diagram and we'll see further on in subsequent slides that the dominant feature within a skeletal muscle cell is the content of material within the myofibrils. So those myofibrils take up most of the space within a muscle cell. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 The myofibrils contain the structural proteins that are associated with force production, the actin and the myosin. We also saw represented in the diagram that the muscle cell has nucleus. And in fact, muscle cells have nuclei. They are multi nucleate, So there are more than one nucleus per each muscle cell or muscle fibre. The more nuclei that you have, the more DNA that you have, the greater your capacity to synthesise new material that you need for either repair or in the case of muscle cells for adaptation. So we certainly have a sense that muscle cells are highly adaptable to the mechanical environment that they find themselves in. And then finally, one of the other features that is pretty critical to force production and the sustainment of force production is the availability of energy. So we will see that muscle cells certainly contain a fair amount of mitochondria, and the mitochondrion is a structure that is related to energy metabolism. So if a muscle cell has more than one of these organelles, then that is potentially good for energy production within the muscle cell. So we can see that muscle cells more or less have the ingredients present to generate and sustain force. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 So what I want to do next is to consider how actually is the muscle cell organised to produce force. So we know it has the kind of capacity by way of the ingredients. But how is the muscle cell organised to produce force? So here we have a slide where we have two images. The top image is a light micrograph, so an image taken through a light microscope of part of a skeletal muscle fibre. The lower image is an electron micrograph, so image taken through an electron microscope of a similar region across as part of a skeletal muscle fibre. So the first image has a caption with respect to the fact that in the light micrograph, if we look at part of the skeletal muscle fibre, you can see a distinct, albeit relatively faint, but very regular cross banding pattern. So cross striation or cross banding means that if we consider the length in this direction, you can see a faint banding pattern. Going crossways across the muscle fibre, and it is very regular, so the spacing between the light regions and the dark regions is very uniform as you go along the length of the skeletal muscle fibre. Now we have arrows pointing to A and to I which represents the relatively darker region and the relatively lighter region, respectively, which we'll consider in subsequent slides. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 We also see that in the light micrograph, you can see these sort of squished oval structures darkly stained. And these are the nuclei. So the nuclei of skeletal muscle fibre or skeletal muscle cell, are kind of pushed very close to the outer edge of the muscle cell. So the dominant feature in terms of the volume of a skeletal muscle fibre is the contents related to this very fine but very distinct banding pattern. Now, if we take a look at this region that's outlined in rectangle, And expand that in terms of increasing the magnification, we see much more detail with respect to the nature of that banding pattern. So firstly, again, we have identified the nucleus, and in fact, we can also see Mi, mitochondria. They are pushed again very close to the outer edge of the cell. So here we have one skeletal muscle cell and we have another neighbouring one relatively close by that you can just kind of see. But the dominant feature, again, is that with respect to this banding pattern, and now you can see it in much more detail. So you see a very high density, thin dark line, a lighter region, a medium density region, a light region and then not very thin, high density region. And if you look that unit of High density light, medium light, high is repeated again, I can trace with the arrow again, again along the length of the muscle fibre. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 So with respect to the caption, the dark and the light bands are arranged in series, so one after the other along the length of the muscle fibre. But we also see very high alignment of the band pattern if we consider going and cutting across laterally across the muscle fibre. So this comment about...and show lateral registration with adjacent myofibrils means that the regions of the low density material are relatively highly aligned from one myofibril to the next within a skeletal muscle fibre. And the regions of this more broad medium density are lined up and the very fine but very high density regions are lined up as well. So because of this alignment laterally, that is where the cross banding comes from in the light micrograph. So the light micrograph light in dark regions, if you blow them up, you can see the light region, darker region, light region, darker region. Except, of course, in the electron micrograph, we see it in much greater detail. So we call these very high density, very thin lines the Z line, and the Z line is repeated and forms essentially the boundary between each of these repeated units along the length of the myofibril. So this tells us something about the way in which the content of the skeletal muscle fibre is organised relative to neighbouring myofibrils within the cell. So now what we want to do is we want to see if we can make some sense with respect to increasing the detail further. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 So this is 3000 times. Now if we go to 14000 times and will focus on one of these repeated units, this is essentially what we see. So we mentioned very early on in the presentation that the myofibril is made up of sarcomeres that are linked in series and in fact the sarcomere is bounded by these very fine lines of high density material and this is what we call the Z lines or the Z disks. So one sarcomere is from one Z line or Z disk to the next. And, we find that we see a distinct region of relatively low density material, so it appears very light in the electron micrograph. And then as we proceed towards the inside of the middle of the sarcomere, the density increases again to a very high density region in the middle. And then that pattern is reflected, or there's effectively a mirror image about the midline so that the other side of the sarcomere looks the same and has that sort of medium density, higher density light region. And then another z line. So that sarcomere is a very ordered, very repeated sort of structural unit where we can see that material is present. That's relatively low density, so appears light. Then there's regions of relatively higher density, sort of medium density with a high density midline M, and then that pattern is reflected on the other side. So we have like a bilateral symmetry of the sarcomere structure about the centre, IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 and these Z lines or Z disks form the boundary per each sarcomere. The light zones or the light regions are called the I band. The central region, a relatively higher density is called the A band. Within that, we identify a slightly different region called the H Zone. So these bands represent the sort of pictorial representation of what we see when we look at a sarcomere in the microscope. The next thing we want to do is consider what might be the molecular species residing in these areas. And so if we combine what we know about microscopy with the biochemistry, we can start to understand what precisely is going on within each sarcomere in terms of the molecular organisation and why we might get regions of dark versus light in terms of the electron micrographs. So the first thing I want to remind you is that in no way will you ever have to repeat and redraw these diagrams. But I've put them in here because it's fascinating to think that we understand the detail of these molecules in such a way that we can make these molecular models effectively to illustrate precisely what they look like. So in the top right hand side, we see a molecule that's called myosin. So essentially, the myosin has a fairly linear region of helicies wrapped around each other, and this is called the tail region of the heavy chain. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 And then there are two branches coming off of the tail region with globular portions at each. So the myosin tends to be called the thick filament, and this thick filament has a heavy tail region and then a hinge region and then myosin heads at the ends of these hinged regions. When we take a look at the bottom left-hand diagram, we see that there are three main proteins that are named. Admittedly, it looks pretty complex, but on the face of it, what you see is a very linear sort of piece of material going essentially from left to right. So actin, troponin and tropomyosin are collectively important in regulating force production and allowing force production. So the actin is shown as a gentle, linear helix that spans, sort of, left to right between these dashed lines shown in this sort of yellowy and this tropomyosin is similarly a gentle helix wrapping around in close proximity to the actin. And then finally, troponin complex is again a gentle helix that wraps around in close proximity to the actin. But has this slightly complex region. Don't worry about the letters for now, suffice to say that that's the troponin portion that's important in the regulatory process, which we will look at in subsequent slides in the rest of this total presentation. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 So we have very distinct molecules involved. Now, this diagram is going to introduce two more important structural proteins. So in terms of force production and regulation of force production, actin and myosin are key. Troponin and tropomyosin are important in the precise regulation. But the act force production really can't take place without the actin and the myosin. So in this diagram, we have the bounds of an entire sarcomere, so we have the Z disc at each end and I'm tracing around the whole sarcomere. So what we have is the actin. Remember, it was a very sort of gentle, linear molecule that had a gentle helix that wound around for the length. So the actin is anchored at the Z disc and protrudes in towards the M line, the midline of the sarcomere. So we have a number of them shown here. Kind of in this yellow colour. What we also see is this thick filament myosin. Spanning across the midline and poking out towards the Z disc at either end, and we had a linear portion to each of these myosins with a bulbous sort of hinged portion sticking out. And in this case, the head of these hinged portions poke out towards the actin. So that's on the right hand side, we just have the actin and the myosin shown, and the whole idea is that they're held in very close registry relative to one another. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 So that potentially. They can make contact with one another. Now, if we look at the left hand side of this diagram, we have two extra molecules shown. So the first of these is called nebulin, and nebulin is a structural protein that is anchored at the Z disk, and it runs along the length of the actin in very close association with the actin. And in fact, it is thought to form a stabiliser for the actin, keeping the actin in alignment relative to the myosin. So this nebulin molecule kind of helps to make sure that the actin is held in the right position relative to the myosin. The other important molecule we see is one called titin, and this is shown in this sort of wavy line running right through from the Z disc right through to the midline. And in fact, we'd have the same thing happening on the right hand side of this diagram. And so essentially, we have a case of this titin molecule, which is a large structural protein running the entire length of the sarcomere, In close association with the myosin. And in fact, this molecule is thought to play a role again in the support this time of myosin, particularly in returning the myosin back to its original position relative to the actin, following any sort of distortion or shortening of a sarcomere. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 So we have two big molecules, nebulin and titin, that form a behind the scenes supportive role for the actin in the myosin. So they are like almost like scaffolding proteins that hold the organisation of the sarcomere in register. And so this partly explains why we have a very regular pattern of sarcomeres when we look through the electron microscope. It's partly all held together, not in a rigid way, but in a sort of a loose association to make sure that the actin and the myosin are in the correct position for force production. So finally, what I'd like to just take a look at is if we really crank up the magnification now to two hundred and seventy seven thousand times, we can literally see these molecules so we can see the thick filament myosin, we can see the thin filament actin, and we can literally see the hinge region where the bulbous head of the myosin pokes out from the linear portion. And this bulbous head of the myosin is what forms an attachment with the actin what we call a cross bridge. So here we have the thick filaments, for example, one thick filament, here's another thick filament that I'm following the thin filament you can see in between. And importantly, you can see the heads of the myosin sticking up from the thick filament and making attachments to the thin filament. So it's just fascinating, really, that you can actually see these IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1 molecular species with such detail, you can literally see the cross ridges, and as we'll see in subsequent presentations, the cross bridges are the critical aspect of the generation of force in in skeletal muscle. So by way of summary, we can consider the following. Skeletal muscle contains a significant amount of connective tissue, so we have the endomysium surrounding the muscle cell, the perimysium surrounding the muscle fascicle, and the epimysium surrounding the whole of the muscle. A skeletal muscle fibre is, in fact, a skeletal muscle cell, so these are amongst the largest cells in the body. Sarcomere is the site of force production, so having these units, sarcomeres, along the myofibres will say something about how force is produced and how it's transmitted along the muscle fibre. But the fundamental unit of force production is within the sarcomere, and that the sarcomeres contain important molecules, including myosin and actin. troponin, tropomyosin, nebulin and titin. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 1