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肌浆网在控制肌细胞中钙离子浓度的作用. Sal Khan 创建




we know from the last video that if we have a high calcium ion concentration inside of the muscle cell then that those calcium ions will bond to the troponin proteins which will then change their shape in such a way that the tropomyosin will move to be moved out of the way and so then the myosin heads can crawl along the actin filaments and then we'll actually have muscle contraction so high calcium concentration or calcium ion concentration we have contraction low calcium ion concentration these troponin proteins go to their standard conformation and they pull or you can say they move the tropomyosin back in the way of the myosin heads and we have no contraction so this is contraction contraction muscle contraction and then low calcium concentration we could say relaxation so the next obvious question is how does the muscle regulate whether we have high calcium concentration and contraction or low calcium concentration and relaxation or even a better question is how does the nervous system do it how does the nervous system tell the muscle to contract to make its calcium concentration high and contract or to make it low again and relax and to understand that let's let's do a little bit of review of what we learned on the videos on neurons let me draw the terminal junction of a of an axon right here instead of having a synapse with a dendrite of another neuron it's gonna have a synapse with an actual muscle cell so this is its synapse with the actual muscle cell right here I could do it just like that and you'll see what this is in a second so this is a synapse with an actual muscle cell so let me label everything just so you don't get confused this is the axon we could call it the terminal end of an axon terminal axon this is the synapse synapse and realize off the screen synapse just a little terminology from the neuron videos the space was a synaptic cleft this is the pre synaptic knew this is I guess you could kind of view at the postsynaptic cell it's not a neuron in this case and then just so we have this is our membrane this is the plasma this is membrane membrane of muscle cell and I'm gonna do probably the next video or maybe a video after that I'll actually show you the anatomy a muscle cell this and this should all be a little abstract because we really want to understand how the calcium ion concentration is regulated membrane of muscle cell and this is called a sarcolemma sarcolemma so our comma looks or sarcolemma i think it's are called them however you want to say it so this is the membrane of the muscle cell and this right here you could imagine it's just a fold into the membrane of the muscle cell if I were to look at the surface of the muscle cell then it would look like a little bit of a hole or an indentation that goes into the cell but and here we did a cross-section so you can imagine it folding in but it's it's it's like if you put a if you if you poked it in with a with a needle or something this is what you would get you would get a fold in the membrane and this right here is called a t tube you'll t-tubules get to get our good to get a terminology out of the way and the t just stands for transverse it's going transverse to the surface of the membrane and over here and this is the really important thing in this video or the really important organelle in this video you have this organelle inside of the muscle cell called the sarcoplasmic reticulum this is the sarcoplasmic sarcoplasmic reticulum and it actually is very similar to an endo plasmic reticulum and kind of it's in and somewhat of what it is or maybe in how it's related to an endo plasmic reticulum but here its main function is storage well endoplasmic reticulum it's involved in protein development and you know it has ribosomes attached to it but this is a this is purely a storage a storage organelle and with the sarcoplasmic reticulum does is it has calcium ion pumps on its membrane it has calcium I pumps on its membrane and what these do is they they're their ATP ASE's which means that they use ATP to to fuel the pump so you have ATP come in the ATP attaches to it and maybe a calcium ion will attach to it I'll do the calcium ion in pink maybe a calcium ion will attach to it and when the ATP hydrolyzes into into adp into adp plus a phosphate group that changes the conformation of this protein and it pumps the calcium ion in so the calcium ions get pumped in so the net effect of all of these calcium ion pumps on the membrane of the council of the sarcoplasmic reticulum is in a resting muscle will have a very high concentration of calcium ions on the inside a very high concentration of calcium ions on the inside now i think you could probably guess where this is going when the muscle needs to contract these calcium ions get dumped out into the cytoplasm of the cell and then they're able to bond to the troponin then they're able to bond to the troponin right here and do everything we talked about in the last video so what we would we care about is just how does it know when to dump its calcium ions into the rest of the cell this is the inside of the cell inside the muscle cell inside the inside the cell and so this area is what the this is areas what the actin filaments and the myosin heads and all of the rest and the troponin and the tropomyosin they're all exposed to the environment that is over here so you know you can imagine I could just draw it here just to make it clear you know we have our actin filament right there I'm drawing it very abstract we'll see more of the structure in a future video maybe you have your myosin head right there and then you have your tropomyosin that's wrapped around and it's being nailed down by troponin by the troponin proteins just like that this is a very abstract drawing but think this will give you a sense of what's going on so let's say this neuron and we'll call this a motor neuron motor motor neuron it's signaling for a muscle contraction so first of all we know how signals travel across the neurons especially across axons with an action potential we could have a sodium channel right here it gets its voltage gated so you have a little bit of a positive voltage there that tells this voltage gated sodium channel to open up so it opens up it allows even more of the sodium to flow in that makes it a little bit more positive here so then that triggers the next voltage gated channel to open up and then that so it keeps traveling down the membrane of the axon and eventually when you get enough of a positive threshold voltage gated calcium channels open up and this is all a review so the calcium ions this is all a review of what we learned in the neuron videos so eventually when it gets positive enough close to these calcium ion channels they allow the calcium ions to flow in and the calcium ions flow in and they bond to those special proteins on near the synaptic membrane or the presynaptic membrane right there these are calcium ions they bond to proteins that were docking vesicles that we're docking in remember vesicles were just these membrane these membranes or these little yeah these membranes around around neurotransmitters so they are all containing neurotransmitters when the calcium binds to those proteins it allows it allows exocytosis to occur allows these the the membrane of the vesicles to merge with the membrane of the actual neuron and the contents get dumped out this is all review from the neuron videos you explain it a much more detail in those videos but you have all of these neurotransmitters get dumped out and we were talking about the synapse between a neuron and a muscle cell the neurotransmitter here is acetylcholine acetyl acetylcholine but just like what would happen at a dendrite the cetyl choline binds to receptors on the sarcolemma or the membrane of the muscle cell and that opens sodium channels on the muscle cell so the muscle cell also has a voltage gradient across this membrane just like a neuron does and then that allows so when this guy gets some acetylcholine it allows sodium it'll allow sodium to flow inside the muscle cell so you have a plus there and that causes an action potential in the muscle cells so then you have a little bit of a positive charge if it gets a high enough to a threshold level it'll trigger this voltage-gated channel right here which will allow more sodium to flow in then if it becomes and so it'll become a little bit positive over here and of course it also potassium to reverse it's just like what's going on in a neuron so eventually this action potential you have a sodium channel over here it gets a little bit positive when it gets enough positive then it opens up and allows even more sodium to flow in so you have this action potential and then that action potential so you have a sodium channel over here it goes down this t2 Buhl it goes down this t tubules so the information from the neuron you could imagine the action potential then turns into a kind of a chemical signal which triggers another action potential that goes down the T tubulin and this is the interesting part and actually this is an area of open research right now and I'll give you some leads if you want to if you want to read more about this research is that you have a protein complex that essentially bridges the sarcoplasmic reticulum to the T 2 Buell and I'll just draw it as a big as a big box right here so you have this protein complex right there and I'll actually show it to you know people believe I'll say it some words out here it involves the proteins try it in junked in junk tin Cal sequester in Cal sequester in and ryanodine Ryanne ryanodine that they're somehow involved in a protein complex here that bridges between the T 2 Buell and the sarcoplasmic reticulum but the big picture is what happens is when this action potential travels when we get positive enough right around here this this this this complex of proteins triggers the release of calcium and they think that the the ryanodine is actually what is the part that actually releases the calcium but we could just say that it maybe it's triggered right here when this when the action potential travels down let me switch another color I'm using this purple too much when the action potential gets far enough I use red right here when the action potential gets far enough so this environment gets a little positive with all the sodium ions flowing in this mystery box and you could do a web searches for these proteins people are still trying to understand exactly how this mystery box works it triggers an opening for all of these calcium's calcium ions to escape the sarcoplasmic reticulum so then all these calcium ions get dumped into the circle into the outside of the sarcoplasmic reticulum into just the inside of the cell into the cytoplasm of the cell now when that happens what were you gonna happen well the high calcium concentration the calcium ions bond to the troponin just like what we said at the beginning of video the calcium ions bound to the bond to the troponin move the tropomyosin out of the way and then the myosin using ATP like we learned two videos ago can start crawling up the actin it and at the same time it was once the signal once the signal disappears this thing shuts down and then these calcium ion pumps will reduce the calcium ion concentration again and then are cut and then our contraction will stop and the muscle will get relaxed again so the whole big thing here is that we have this container of calcium ions that when the muscles relaxed is is essentially taking the calcium ions out of the inside of the cell so the muscle is relaxed so that you can't have your myosin climb up the actin but then when it gets the signal it dumps it back in and then we actually have a muscle contraction because the tropomyosin gets moved out of the way by the troponin so I don't know that's pretty fascinating actually even fascinating that this is still not completely well understood you may be you know this isn't active if you want to become a a biological research this could be an interesting thing to try understand and you know what it's interesting just from a scientific point of view of how this actually functions but there's actually there's may be potential diseases that are are byproducts of malfunctioning proteins right here maybe these you can somehow make these things perform better or worse or who knows so there actually are positive impacts that you could have if you actually figured out what exactly is going on here when the action potential shows up to open up this calcium channel so now we have the big picture we know how a motor neuron can stimulate a contraction of a cell by allowing the sarcoplasmic reticulum to dump all of its all of its cat or or to dump up or to allow calcium ions to travel across this membrane into the cytoplasm of the cell and I was doing a little bit of reading before this video and these pumps are very efficient so once this signal goes away and this door is closed right here this can this sarcoplasmic reticulum can get back the ion concentration in about 30 milliseconds so that's why I we're so good at stopping contractions why I can you know I can punch and then pull back my arm and then have it relaxed all within split seconds because we can stop the contraction in in 30 milliseconds which is you know very the what's it's it's less than a thirtieth of a second so anyway I'll see you in the next video where we'll study the actual anatomy of a muscle cell in a little bit more detail