[Music] hi everyone my name is andre and welcome to med school eu in today's lecture we are going to discuss the skeletal system so we are primarily going to talk about how skeletal muscle is innervated by the nervous system so we will discuss the anatomy and physiology of muscle innervation the first thing we must talk about is the skeletal muscle anatomy so we're going to go over striated muscle and the structure of it so that you get a good understanding of the anatomy before we learn the physiology of muscle contraction and the the first thing i wanted to discuss here is going to be the striations what depicts striations what makes up striations and why does a skeletal muscle look this way if we look under the light microscope well that is primarily because there are thick and thin filaments so these thick filaments are made up primarily of meiosin and the thin filaments are going to be made up primarily of actin and it is depicted in this diagram so if we were to take a side view of a muscle fiber this would be the muscle fiber if we take the side view the thick filaments are represented by the a band right here and the thin filaments so actin represented by i band now these lines that connect the actin so the thin filaments that holds the thin filaments together the two sides of the thin filaments it's called the z line and the z line is very important because it is the way we mark the sarco sarcomere and the sarcomere is the unit of is a contractile unit so here it would be labeled as sarcomere that's going from the z line to the next z line each z line represents a sarcomere so the distance between the two z lines is the distance and the length of the sarcomere now another line that would be important here as well is the m line and this line is the middle of the myosin connection so the thick filament is connected through the m line and the same thing is here so if we're looking at the different diagram depiction we've got m line right here right down in the middle of the thick filament these are obviously actin filaments and these are myosin filaments and the way that contraction happens is the thin filaments are going to slide over the thick filaments in this orientation and they will close in and make things shorter so let's discuss what actually gets smaller when a contraction occurs when we're talking about muscle contraction in essence what's going to happen is everything is going to shrink everything is going to contract and get shorter and so you will see these z lines are not going to be so far apart they will actually move closer together in this orientation and they will move closer together because the muscle is contracting and the actin is sliding against the myosin so myosin is going to stay there m line will stay constant the h band will be smaller because the thin filaments are going to move in more probably towards something like this over here so they're going to move all the way in closer to the m line so if we were to talk about like a summary of what occurs is we would basically have the m line is going to remain constant the a band will be constant the i band is going to shrink so it'll get smaller z lines will move closer to the m lines finally the h band is also going to shrink just like the eye band so this is just a basic summary of contraction of skeletal muscle and what happens to the sarcomere and sarcomere is basically just a point of the measurement of a single unit of a contractile muscle so if we were to take contractile unit that is able to contract the smallest one you would be able to detect is going to be the sarcomere and based on that depiction these will be the changes observed when you have a contraction of the muscle now if we just kind of zoom out of the muscle because of course it's not simply depicted by the muscle fiber but all of these muscle fibers will be composed together into a muscle fiber bundle and so there will be multiple muscle fibers that are covered under a single cell surface membrane the cell surface membrane is going to be called the sarcolemma now of course this would be the mitochondrion now if we're looking at the sarcolemma it's going to have these little holes sticking out that is going to continue on downwards inside of the muscle fiber or going around it now that's going to be the entrance to the t tubule and you will see that the t tubule is going to be very important in terms of muscle contraction and we're going to go over the physiology of that as well and also the uh this little mesh in between the t tubules that's going to be called the sarcoplasmic reticulum the so we're going to depict it as sr and so this muscle fiber if we were to make a full circle of it it would of course have multiple of these units and these units are going to be called myofibril so what we noted previously is that the sarcomeres in each myofibril will get shorter as the z discs are pulled closer together towards the m line now this diagram is going to show how this happens if we are to zoom in on the muscle contraction and it is known this entire mechanism is known as the sliding filament model and the sliding filament model is what's typically used to teach a muscle contraction and how it is depicted in its physiology so let's discuss that in greater detail first of all let's label a lot of this anatomy that we have to go through in order to provide a better understanding of the physiology so here we have the m line right and right through the middle we talked about this before now all of these little balls right here are going to be actin molecules so that's actin now this this one right here that's right around the act and it's wrapped all around it it's called troponin finally this long one is going to be called the tropomyosin and so the first thing that occurs right here is when a muscle's relaxed the tropomyosin and the troponin are sitting in a position in the actin filament that prevents meiosine from binding so as you can see if we're zooming in on this actin right here we could see that during a relaxation period when the muscle is not contracting the tropomyosin and the troponin are going to be in positions where it blocks the binding of the myosin head the troponin and the tropomyosin right here in the second stage are going to change shape and allow meiosin so this is the meiosin head they're going to allow this meiosis and head to bind with the actin and attach to it so as you can see here the attachment occurs all along the actins now if we're looking at the next stage of the sliding filament model and this comes into play to actually understanding what the sliding filament represents is because now the meiosin head is going to pull the actin to the left side so from here it's going to pull to the left from here it's going to pull to the right pulling the thin filaments towards the m line and the z lines are going to come closer together and this occurs as the meiosine head tilts and pulling the actin in those directions and finally the last thing that occurs part 4 is that atp is hydrolyzed so we're going to have atp hydrolysis and this atp hydrolysis causes the release of myosin heads that spring back and repeat the binding and the tilting process so this keeps going as a cycle and then it will begin right at the start each time and so atp a lot there's a lot of misconception here is that atp is used in order to cause the muscle contraction and that is inherently not true i mean indirectly it is true but what is atp used for the hydrolysis of atp is used to disconnect the myosin head with the actin so it's used to terminate each muscle contraction it is not used to activate it and what's used to activate it we are going to describe in the next slide because there's quite a bit of physiology involved with that and this leads us to the discussion about the neuromuscular junction and so here we have a lovely depiction of what's going on in terms of the neurons so this would be the neuron binding on to the sarcolemma so this is the muscle fiber cell membrane and the sarcolemma is going to have of course these bindings along with the neurons in order to receive impulses from the brain or the the spine in order to activate the skeletal muscle and how this occurs is very similar to what we saw with the connection between two neurons in our last lecture at the neuromuscular junction this is a neuromuscular so there's a muscle involved on the receiving end in this junction we have a little bit of further physiology to deal with so we're going to discuss that however the beginning of it is going to be exactly the same the signal is going to come in through the axon so the electrical stimuli there's going to be action potential that will be occurring throughout the neuron once it reaches its terminal ends it's going to get the calcium inside the cell so calcium two plus is going to enter inside the cell and is gonna cause a release of the neurotransmitter acetylcholine and so what it's gonna do is it's gonna go over and bind with the membranes here and it's going to release its acetylcholine that will be a ch so the acetylcholine will then be released all over the synaptic cleft and it binds to receptors in the motor and plate so this all of this right here is called the motor and plate and this motor and plate is the mechanism that triggers action potentials inside the muscle and we're going to talk about that so these little acetylcholines are going to bind to the receptors here and again these are ligand gated receptors we've discussed this previously because they are gated by chemicals so again if acetylcholine is released it is going to be activated and what it's going to do is the action potential is going to propagate along the sarcolemma and down the t tubules so how does it happen well there's going to be an influx of positive charges with n a plus and this occurs throughout the motor end plate and this continues to occur all over here because the signal is going to continue to travel this way and so the sodiums are going to continue to depolarize the membrane and they're going to continue to make the membrane more positive and that will continue to create action potentials all along the sarcolemma and eventually it will head down the t tubule as well so this is the t tubule here and this right here is the sr sarcoplasmic reticulum so once the signal heads down the t tubule what's going to happen is it's going to the action potential will trigger calcium release from the sarcoplasmic reticulum and so we're going to have this calcium released all over from the sarcoplasmic reticulum from both sides here because this the signal went down the t tubule and it causes the opening of the calcium channels and so now there's going to be all this influx of calcium two plus in essence the calcium two plus from the sarcoplasmic reticulum that is released due to the action potential will now travel to the troponin and the troponin so if we were to label let's say troponin would be right here the troponin would then change its shape because of the release of the calcium and now the meiosis and heads are going to bind and make a binding with the actin because the troponin will then cause tropomyosin to remove its blockage of the meiosis head and now it's going to be bonded it will then do its cross-bridge cycling so that the muscle fiber will contract and then again it's going to bring back and it's going to go back to its original shape because atp will cause the breakage of these bonds and what occurs next is that the tropomyosin blocks meiosis mining sites because atp was released and then what happens to all of these calcium channels is that they're going to open on this side of the sarcoplasmic reticulum and now all of the calcium will then be brought back into sarcoplasmic reticulum for another cycle of contraction and of course this occurs all over the entire muscle so the entire muscle is going to contract extremely quickly because of this movement of electrical stimuli the action potential and it's going to move and depolarize the membranes extremely fast almost simultaneously and so the contraction will be very coordinated because of that as type of stimulation but the important thing to note here is that the calcium is actively going to be transported back into the sarcoplasmic reticulum by these calcium transport pumps so the transport pumps are typically going to be closed but as soon as atp binds and forces the dismissal of myosin head from actin then it's going to open these calcium channels that the active channels to push the calcium back into cycloplasmic reticulum and basically be ready for the next contraction that's going to be the refractory period between one contraction and the next is this recycling of calcium and finally the last thing i wanted to discuss is going to be the way that atp is supplied for muscle contraction so we are going to have atp readily available inside the muscle ready to go as soon as we like and that's going to be due to creatine phosphate so that first contraction that first immediate contraction is going to happen due to the creatine phosphate that will provide the extra phosphate in order for the atp to be formed and provide the initial contraction or provide the disengagement of myosin from the actin because without it the contraction would not typically proceed uh the next major source so that's that's going to be uh one source another source is just straight atp that would be stored but this is going to be in very low amounts typically the first contraction the first couple of forceful contractions are going to be done by the creatine phosphate that will create atp and typically within about 30 seconds or so of activity is when the aerobic respiration will come in with the mitochondria is producing a whole lot of atp so that's going to be done by aerobic respiration and the final asset of atp is going to be made by lactate fermentation so if you would like to go ahead and review these two units we talked about in cellular respiration we talked about the lactate fermentation how it happens and why it happens as well as the aerobic respiration when the pyruvates actually enter into the mitochondria and they proceed to make a whole lot of atp and so typically if you are a long distance runner you would be relying on the aerobic respiration whereas if you're somebody like usain bolt you would be relying more on the creatine phosphate supply because the duration of your activity is about 10 seconds long and therefore you're not going to be using your aerobic respiration supplies because your the length of your activity will not be reached by that time however if you're running a marathon then of course aerobic respiration and lactate fermentation are the two main sources of atp for your muscles in order to continuously cycle through the muscle contraction this concludes our video and our unit on the muscle and the skeletal system and in the next video we are going to talk about more anatomy of the body and more particularly we will discuss the anatomy and the physiology of the eye [Music] you