IMS 1 TBL 4 Sarcomeres and Sliding Filament Theory Part 3 PDF

Hello and welcome. This is a year one presentation in the introduction to Medical Sciences module. This is part of your TBL preparatory materials for the team based learning session entitled Introduction to Neuroscience, Membrane Potential and Muscle Contraction. This presentation is entitled Sarc...

Hello and welcome. This is a year one presentation in the introduction to Medical Sciences module. This is part of your TBL preparatory materials for the team based learning session entitled Introduction to Neuroscience, Membrane Potential and Muscle Contraction. This presentation is entitled Sarcomeres and Sliding Filament Theory Part three. My name is Dr Julianna Gal, but please feel free to refer to me as Julianna in any correspondence that we may have. As I mentioned, the title of this presentation is Sarcomeres and Sliding Filament Theory Part three. So it is the final part of a three part suite of presentations, all about sarcomeres and the sliding filament theory. In this part three, the topic will be about control of skeletal muscle force and the role of calcium. So by way of an overview of the content of this presentation, firstly, we'll be considering calcium ions and the regulation of cross bridge formation, and then we'll be focussing on t-tubules, the sarcoplasmic reticulum and calcium ion movement. So from the previous two presentations, we learnt about the nature of the sarcomere, and we learnt that the sarcomere was the site of force production by virtue of the power stroke and the interaction between the myosin thick filament heads and the actin thin filaments. We also saw that sarcomeres have the characteristic that over a limited range of absolute length, they are capable of generating maximum force. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 And when they are positioned in an overly stretched or overly shortened position, the force drops off. So this partly reinforced the sliding filament theory, but also has bearing on the idea that if the behaviour of sarcomere has an optimum with respect to absolute length, then if you line a bunch of sarcomeres up in series to make muscle fibres and whole muscles, then whole muscles may have the same behaviour in terms of having. Lengths where they are capable of generating their maximum amount of force. Now if we remember that skeletal muscles typically attach from one bone to another, crossing at least one joint, that means that when we change joint angle, we change the distance between the attachment sites of the muscle, site of origin and site of insertion, which potentially means we change the muscle length. So irrespective of things like training status and nutritional status, things like geometry and physics, of the positioning of muscles relative to the bones that they attach to may have a bearing on the force that the muscle can produce, so therefore posture has an effect on muscle force production. And so we generally have sometimes a perception that certain postures we adopt allow our muscles to generate greater forces than others. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 And this is essentially a manifestation of this phenomenon that takes place essentially at the sarcomere level. So force lengths properties of sarcomeres provide a platform for thinking about how muscle force is influenced by geometry, joint angle and therefore posture. So what I want to do now is to consider how sliding filament mechanism is enabled to proceed in a controlled way. In other words, how do we control whether our skeletal muscles are turned on or turned off or in fact modulated somewhere between? So we think back to when we first introduced the idea of of the sliding filament hypothesis initially and further in sliding filament theory and one of the main sort of initial statements was the fact that the thick filament myosin heads by into the thin filament and that resulted essentially in a cross bridge. So thick filament myosin heads bind to thin filament act and cross bridge formed. And in the formation of cross bridges, the key ion that's important in the control of cross bridge formation, whether it's allowed or not, is calcium. So calcium ions are key. So here in this slide, we have three illustrations that illustrate the actin, the tropomyosin, the troponin so the three sort of proteins we introduced as part of the make up of a sarcomere, and we see the influence of calcium. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 So in this first diagram, the actin strand or the thin filament is represented by these circles that are linked together. So we have a linear type of protein with a gentle helix or twisting about its length. We also have another protein shown again and twisted about its length in close proximity to the actin and that's tropomyosin And then finally, we have this troponin component, so a troponin portion you can see in this top diagram in close proximity but not bound to calcium. What you can see is that the troponin protein component has a very unique shape that will attract and bind to calcium. So cross bridge formation is blocked until calcium binds to troponin and displaces the tropomyosin. So here you can see the calcium is bound to the troponin, and the troponin therefore, once it's bound to calcium, displaces this portion of tropomyosin away from the actin thin filament. So in this diagram, you can see its departing from direct contact here and now it's away from the actin. And that departure away from the actin changes the local environment of the actin protein thin filament such that the myosin head that globular portion of the myosin that sticks out from the linear heavy chains, that myosin head can now bind to the actin. And so now thick filament myosin head can bind to the thin filament actin. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 And this binding and connexion is what we know as the cross bridge. So the binding of calcium to troponin displaces a portion of tropomyosin from the actin, and that allows cross bridge formation. The binding of the thick filament myosin head to the thin filament actin. The next thing we want to do is consider when this takes place and how this takes place in a controlled way. So this slide has a relatively complex diagram and some text that goes along with it. So we'll take a look at the diagram first and what it's meant to show as a descending motor axon and the nerve terminal end of it. The terminal end is where the vesicles filled with acetylcholine are. And when the nerve is stimulated and the nervous stimulation reaches the end or nerve terminal, then the acetylcholine vesicles fuse with the membrane of the nerve cell and release acetylcholine into this cleft. The space between the nerve terminal ending and the muscle cell membrane acetylcholine binds to receptors in this membrane in the muscle cell membrane, and that opens up sodium channels. That influx of sodium into the muscle cell changes the relative charge inside the cell to positive. And if a certain threshold voltage is achieved, then we generate an action potential. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 So this process is called depolarisation, changing the voltage within the cell to positive. The action potential, that voltage threshold signal is propagated along the muscle membrane and down into the t- tubules, which are invaginations or parts where the membrane dives deep into the depth of the muscle. So remember that the muscle cells packed full of these myofibrils and to affect all of the myofibrils, the t-tubules have to sort of protrude deeply into the depths of the muscle fibre. So when the action potential signal reaches into the t-tubule, it will encounter lots and lots of these voltage sensitive receptors, and these voltage sensitive receptors are attached to a much bigger protein complex which involves a portion that's within the membrane, that traverses the cell itself, and then attaches to the membrane of the sarcoplasmic reticulum, where it is attached to a calcium channel. So when the action potential arrives at the t-tubule, this voltage sensitive component changes its shape and literally pulls upon the calcium channel to open the calcium channel. Calcium is stored in relatively high concentration in the sarcoplasmic reticulum. So when this channel opens, it naturally flows from high concentration to lower concentration in and around the region of the sarcomeres, which basically take up all of the rest of this space. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 So the action potential is generated at the neuromuscular junction that is essentially right here by sodium influx, so the flow of sodium into the sarcoplasm the plasma, over the plasma membrane of the muscle cell, and the key receptors at the neuromuscular junction are the acetylcholine receptor sensitive sodium channels. So the channels in the sarcoplasmic reticulum here are part of a large protein complex that has part of its portion in the membrane of the t-tubule. So the voltage sensitive dihydro pyridine receptors, that's this portion of this big protein complex, once it senses a change in the voltage, a depolarisation, that whole complex changes and literally distorts and pulls on parts of the calcium channel, literally opening it so the calcium can flow out of the sarcoplasmic reticulum and into the area around the sarcomeres which are all sort of around these organelles. So the voltage sensitive portion is called the dihydro pyridine receptor, and then the calcium channel is called the Ryanodine Calcium Release Channel, and the two are linked together to make one massive protein complex. Calcium flows from higher concentration to lower concentration naturally, so if calcium is concentrated in sarcoplasmic reticulum, once these channels open, it will naturally flow. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 So that is how you get the availability of calcium around the sarcomeres to allow for cross bridge formation. So high concentration of calcium within the sarcomeres, having left the sarcoplasmic reticulum now allows for continued cross bridge formation. And as long as you've got ATP available, the power stroke can proceed. Muscle fibre and whole muscle tension can increase, so the power stroke, remember, was that portion of the cyclic process where the myosin head distorted and changed shape and literally pulls upon its attachment with the actin pulling the actin proteins in towards the centre of the sarcomeres. So shortening the sarcomeres by a little tiny bit. So you can imagine if this attachment, detachment attachment, detachment of the myosin head and that pulling effect each time. If that occurred hundreds and hundreds and thousands of times, you can slowly get a very distinct shortening of the sarcomere. And I guess slowly isn't necessarily the word, because sometimes these contractions can be very fast. Sometimes they can be quite slow on a molecular level, the hydrolysis of the ATP to release the energy is relatively quick. So molecularly speaking, it's a relatively fast reaction, but you can see there's quite a few steps involved, so that still requires a little bit of time. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 Suffice to say, we can get on demand contraction so long as we're able to control the release of calcium and we have the availability of ATP. Now, how do we get the muscles to relax or how do we basically turn off muscle contraction? So the first thing is that muscle relaxation begins with the ending of neural stimulation. So you have to stop the the nervous stimulation of the appropriate motor units. So you've got to end the neural stimulation. And when you ended neural stimulation, ideally the action potential will stop. So once the action potential stops, that depolarisation voltage will no longer be there, so the voltage sensitive DHP receptors will return to the relaxed state. When the sarcolemma is re polarised and therefore the calcium channels will close. So no more calcium will flow out of those open channels. So with the change in the voltage in the surrounding area relative to those DHP receptors, if they return to the resting conditions, so too will the DHP receptors essentially return to their resting size and shape. And they will close the calcium channels. Now, the challenge remains in that the calcium ions are still now surplus to requirements in and around the sarcomeres. So the extra calcium has to be pumped away from the sarcomeres because as long as they're still around, IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 you'll still potentially have cross bridge formation. So you need a reliable way and a very controllable way to pump the calcium or get the calcium out in a way from the sarcomeres when you want to or when it's required. So here on this slide, we have a summary of how that takes place, so it's a similar diagram to the previous slide in that it shows. A muscle fibre cell membrane, so sarcolemma and the t-tubule. As before, it just doesn't show this descending nerve terminal that it had on the other side. So you have to imagine that. Here, where I'm sweeping the arrow back and forth was the region of the neuromuscular junction. So the first comment at the very top here is all about sarcoplasm is repolarised by stopping the influx of sodium at the neuromuscular junction. And so this is stopped in the first instance by removing the nervous stimulation. So no more acetylcholine to bind to the sodium channels to open the sodium channels. At the same time, we have sodium being continually removed from the sarcoplasm so that's the inside of the cell of the muscle effectively. And that sodium is continually being removed by Sodium ATPase protein pumps. So these proteins are able to use ATP energy to pump sodium from the inside of the IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 cell back out into the space between the muscle cell and the nerve terminal ending. And so the whole idea is that these protein pumps, the sodium ATPase protein pumps are always working. But when you get the influx of acetylcholine, when the nerve is stimulated, you get an overwhelming amount of sodium flowing in. And that's why you get that depolarisation. But when that acetylcholine is no longer present, then you don't get that influx of sodium. You get a net departure of sodium outside the cell. So the cell returns to its resting condition of relatively negative inside, relatively positive outside. So that is the repolarization effect. So once the action potential is removed and the muscle is very polarised, then these DHP receptors return to their resting state and the whole complex returns to its resting state, such that the calcium release channel is now closed. So that literally closes off the gateway for the release of calcium from the sarcoplasmic reticulum into the region of the sarcomeres. Remember, most of the rest of the volume of the muscle cell is filled with sarcomeres. So as I mentioned now, the challenge, again comes in the fact that any calcium that is remaining around the region of sarcomeres has to be removed, and that is done by sarco/endoplasmic reticulum calcium ATPase protein pumps. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 So it's another ATPase, meaning that it's a protein pump that uses ATP energy in this case to pump or to move calcium ions from in and around the sarcomeres here back into the sarcoplasmic reticulum. And so that pump again is also going as well. And again, the case of stimulation of the muscle and these gates opening up means there's an overwhelming loss of calcium from the sarcoplasmic reticulum. But once these channels are gates are closed, then there's a net uptake of calcium back into the sarcoplasmic reticulum. So the final remaining challenge, I guess, in this sort of sequence of steps is the fact that you need to or the muscle cell needs to concentrate a high amount of positively charged ions, calcium ions in the relatively small organelles, the sarcoplasmic reticulum. And we know that positively charged or like charged particles will repel one another. So the challenge is how do we keep pumping in calcium into this very confined space? And we do this by having a special molecule called calsequestrin present in the sarcoplasmic reticulum that loosely binds to calcium ions. So the calsequestrin loosely holds or buffers the charge of the calcium ions and therefore allows for more and more calcium to be pumped into the sarcoplasmic reticulum. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 So this binding is strong enough to buffer the sort of positive charge, but it's not so strong that as soon as the calcium channels open again, the calcium will flow back out from high concentration inside to low concentration outside around the sarcomeres. So this calsequestrin loosely but sufficiently binds calcium to allow for continued pumping of calcium into the sarcoplasmic reticulum. So one maybe final comment, as you'll notice that there's ATP energy used for these protein pumps, partly at the neuromuscular junction for the sodium pumps, partly for the calcium pumping at the sarcoplasmic reticulum and partly at the power stroke phase. So the force production itself. So we can see that force production and muscle actually requires ATP at a number of different locations. So ATP energy is used partly to turn muscles on and off, as well as to generate the force itself. So by way of summary, we can consider the following points. Troponin and tropomyosin are regulatory proteins associated with the actin thin filaments. And that calcium binds to troponin, it displaces the tropomyosin and allows the binding of the thick filament myosin to form cross bridges. So it's the binding of calcium to the troponin that displaces tropomyosin and allows for binding of actin and myosin and cross bridge formation. Voltage sensitive receptors in the t-tubule are stimulated by action potentials and open calcium channels in the sarcoplasmic reticulum. Muscle relaxation requires removal of calcium ions back into the sarcoplasmic reticulum by calcium ATPase pumps. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3 And finally, calcium is concentrated in the sarcoplasmic reticulum by the effective buffering capacity of the calsequestrin molecules. IMS 1 TBL 4: Sarcomeres and Sliding Filament Theory Part 3