in the last part of the video pertaining to skeletal muscle contraction we have to talk about the neuromuscular Junction now what exactly is the neuromuscular Junction well let's break it down neuro basically means it has something to do with well neurons and muscular in this case means muscles and Junction by definition is a point where two or more things are joined so it is just the meeting point between a neuron and a muscle and in this case the muscle that I'm talking about is the skeletal muscle so the reason why we have to talk about the neuromuscular Junction is because skeletal muscles cannot contract randomly if they do you may have some muscular some muscle issues but under normal circumstances what needs to happen is the Moto neuron remember Moto neurons remember how it looks like it has that very large cell body quite long axon and then it also has the uh terminal branches with the synaptic knob over there so what happens is the electrical impulses will move in a particular direction from the cell body towards the terminal branches and what will happen is something will happen in that particular area which I've highlighted so the part we have just basically highlighted in yellow that's called the neuromuscular Junction because that is the meeting point point between the neuron and the skeletal muscle so what is supposed to happen is usually the Moto neuron sends the electrical impulse or the action potential along itself and then at the neuromuscular Junction there will be the transmission of acetyl choline and when there is the transmission of acetyl choline if you remember acetyl choline is just the neurotransmitters um then the skeletal muscles will contract so the Moto neuron stimulates the skeletal muscles to contract using acety choline as the neurotransmitter now immediately some students may ask wait a second when you're talking about acety choline shouldn't we use the word synapse or syapse synapses are the Junctions between two neurons so there's a neuron on the left and there's a neuron on the right in in my example but a neuromuscular Junction is between one neuron and one muscle so that's the difference that's why we do not use the word syapse here but the good news is the principles are still very much the same so if you understand how synapses work this will not be a particularly difficult topic for you so I would recommend that you go back and study uh go back to the previous video on synapsis just do a little bit of refresher on the transmission of impulses along the coleric sinapse and then come back to this because a lot of things are going to repeat itself so the question over here that we have to ask ourselves is how does the motor neuron stimulate the skeletal muscles to contract using acety choline so before we go through that in detail there are a few things that I have to talk about the first very important thing that we have to cover is when you look at the skeletal muscle fiber just a little bit of revision this is a generalized skeletal muscle fiber fine you have the fibril sarcoplasmic reticulum T tubules and the Sara together with the sarcoplasm now then after that I also Drew out I also gave a bit of revision and I told you that the sarola is a cell surface membrane which is directly connected to the T tubule which is infoldings of the cell surface membrane and then the t tubule is connected to the sarcoplasmic reticulum and inside the sarcoplasmic reticulum I told you in the syst or the container the space within the ooplasmic reticulum it contains calcium ions and I told you that these calcium ions are going to be very important then I also told you that in the contraction of muscles of the skeletal muscles I said that the first step in initiating the contraction of the skeletal muscle is when the calcium ions SP to troponin so right off the bat you must be like thinking ah there there is a connection then the calcium ions in the cop plasmic reticulum has something to do with the contraction of the muscle fiber so how are they connected to each other this is what the video is all about so let's go through them in detail so what I'm going to do here is I'm going to draw out a slightly different kind of structure I'm going to turn like instead of using it from a left to right structure I'm going to do it from a top to bottom kind of approach because it's easier this way for me to visualize the skeletal muscle contraction uh so you have the motor neuron and then you have the highlighted area which I've put in like a green color box that's the neuromuscular Junction together and it's connected to a skeletal muscle now so you have the terminal Branch all right then you have the calcium ion Channel this is all we have studied this in the video on synapsis and of course for the motor neuron they also use acety Coline as a neurotransmitter to cause muscle contraction now what I'm drawing is I'm drawing out this skeletal muscle I say cell but it's not supposed to be a cell right it it because it doesn't behave like a cell because it's too long it has many nuclei so but yeah so I'm throwing up the skeletal muscle over here and they are made out of the CMA the cell surface membrane a bit of good news here is they do have the AC receptor I mean obviously because if a is going to stimulate the muscle the muscle cells or the the citium would require um AC receptors with sodium ion channels and of course what do you call that part which is folded inwards that's the T tubule same thing and then the t tubu is connected to the SOP plasmic reticulum which contains calcium ions fine and then the blue color area which I've just highlighted uh just coloring it in that's the sarcoplasm also equivalent to the cytoplasm of the cell so but we use the word cop plasm over here and then there you go those are familiar things aren't they those are the myofibrils you know that they are the myofibrils because they have the light areas and the dark areas the light areas are the actin filaments and the dark areas contain the myosin filaments and we've also studied the how the contraction supposed to happen but remember just as a repetition for the filaments to start sliding or for the myosin heads to start pulling the actin filament calcium ions need to bind to the troponin on the actin filaments so where does the calcium ion come from the calcium ion has to come from the sarcoplasmic reticulum right so how does the Moto neuron stimulate the AC receptors which will then stimulate the sarcoplasmic reticulum which will then stimulate the contraction in the myofibril so look at the connections they're all connected to each other in a way not directly obviously but how does the impulse from the Moto neuron cause the entire muscles to contract that's what we have to know over here so this is where we're going to look at it in detail so the first thing that happens is for the motor neuron to cause the muscles to contract the Moto neuron has to have impulses or action potentials so a weave of depolarization is moving along the Moto neuron and remember when we studied the video on synapsis when it reaches the calcium ion Channel area the depolarization or the action potential will cause the calcium ion channels to open calcium ions rush in down the concentration gradient it causes the vesicles to move if you notice I'm actually speed running through this part of the video the reason is because we've done this in synapsis so I've explained it in synapsis so that's why I'm just kind of breezing through this part so the calcium ion channels open and the calcium ions Rush In which causes the vesicles with AC to move to the cell surface membrane and then therefore it causes the exocytosis of AC now you cannot say that the ac diffuses across the syapse because this is not a syapse the space here or the the the the the space between these two areas are not known as the syapse but it's known as the neuromuscular Junction so what happens is the ac diffuses across the neuromuscular Junction and binds to the receptor on the cemma and it will cause again same thing sodium ion channels open now remember I told you the CMA is connected to the T tubule which is connected to the sarcoplasmic reticulum so when the sodium ions rush into the muscle fiber obviously the sarola just like any other neurons maintaining a resting membrane potential the sarola will depolarize because when sodium ions go in the inside becomes more positive or of a higher voltage the outside will have a lower voltage so that's why we represent the inside as a positive symbol outside is negative so the membrane depolarizes when the membrane when the Sara depolarizes it causes the depolarization to continue down the T tubu because remember they are connected so depolarization of the sarola will cause the depolarization of the T tubule and when the T tubule depolarizes it causes the calcium ion channels I'm just drawing up the sarcoplasmic reticulum here and it causes the T tubules to uh it causes the when the T tubules depolarize the calcium ion channels in the SR or sarop plasmic reticulum will open and that's what causes the calcium ions to rush into the sarcoplasm so now you have calcium ions in the cop plasm which I've circled in a LD over there so what happens is where does the calcium ion go some of you will immediately make the connection that calcium ion will move towards the the propin on the actin filament it begins the uh contraction within the myofibril basically so what happens to the myofibril as you can see over here the distance of the saroma between the myofibrils will decrease the actin and myosin slides along each other and the muscle contraction takes place that is how everything is connected to each other so when the calcium ions bind to the troponin on the actin filament that's when it initiates the muscle contraction if a question asks you in the exam what exactly happens in the neuromuscular Junction then you just basically mention 0.1 until 8 for this so just be careful but if the question asks you about the sliding filament theory then you have to explain about what happens in the myosin head between the myosin head and the actin filament only usually questions will not like to combine the neuromuscular Junction and the sliding filament theory because if they do it's worth more than 15 marks and I mean I'm not saying never say never I'm not I'm not saying that it will never happen there is a possibility it might happen right where they where they first ask about what happens in the neuromuscular Junction then they may suddenly say oh in the next part of the question explain the sliding filament theory so it it might happen by the way so it's kind of good to know what's happening on both uh ends of the spectrum so when the when of course when the muscle contraction happens what happens the actin and myosin will slide along each other the distance of the sarcoma reduces the length the I mean and when the when the distance of the salomia reduces the entire muscle itself will contract and with that that's the final video on the human part of coordination and respon resp this was an extremely long and exhausting video to make because I had to make a lot of editing I had to redraw a lot of things because I was not happy with how I was teaching it um so I hope this was a more digestible version of this particular chapter