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so when we're at a social gathering and we've determined that this is the most appropriate time to perform the worm that is our brain tell our muscles to contract well in this video we'll talk about the place where neurons and talk directly to muscles that's the neuro muscular Junction the junction of where motor neurons talk to muscle cells so that involves first the axon terminal this is the end of an axon which is the part of a neuron that casts a signal away and it looks like this it gets larger at the end right here and then it kind of tapers back off like that muscle cells sit a jayson to these axon terminals at the neuromuscular Junction and kind of look like a block but not exactly they have these in foldings that I'm drawing right here these in pouching x' and why does nature cause in pouching x' to occur what's the purpose of these guys what function do they serve well if you said that it serves to increase surface area you're absolutely right because with the increased surface area we're going to have extra space where we can have sodium channels present that will help us transmit a message into the muscle cell and so it's not just present on the outside but there are a bunch of sodium channels that are deep inside as well and in addition to sodium channels you definitely have calcium channels that are present as well they're situated very deep within your muscle cells to to make sure that the most deepest parts of your muscle cells will get an influx of calcium when it's the right time and to foreshadow a point we'll discuss later I'm going to draw another muscle cell right here just kind of chilling out on its own so now how does the axon send a message to the muscle cell well if you recall there's going to be a signal that's cast away from our motor neuron to this axon terminal and that signal is in the form of an influx of sodium ions so this is a depolarized membrane that propagates this signal to this axon terminal but it's not just sodium that's in fluxing you're also going to have some calcium that's running in as well and the calcium here is actually going to play the major role because in our axon terminal we have a bunch of vesicles that are sitting around in here these are just little pockets that are waiting for something to happen and in each of these pockets we have a message that's waiting to be released into the space between our axon terminal and the muscle cell this message is called a neuro transmitter neuro transmitter which is a very well named scientific term because all this is is a molecule that the neuron uses to transmit a message and so the neurotransmitter that we use in the neuromuscular Junction is called acid teal like from chemistry NASA teal choline NASA teal choline and often times you'll see it abbreviated as a CH so when there's an influx of calcium into the axon terminal what'll happen is that the calcium will actually bind to our vesicle there are proteins that are on it that'll grab onto the calcium and so when there's calcium attached to these proteins the vesicle will be drawn to this axon terminal membrane and actually fuse with it to become one continuous membrane as a result we release the acetylcholine into this space we call the synaptic cleft the synaptic cleft is just the space between our presynaptic membrane the membrane of our axon terminal as well as our postsynaptic membrane which is just the membrane of our muscle cell so this is our postsynaptic membrane all right and so we have a bunch of a Siddall choline that's released into the synaptic cleft and is ready to send a message on but let's take a minute here what just happened here with the membrane I mean the vesicle literally became one with the membrane of the axon terminal what would you call this if you have to give it a name well it looks like some compound within the cell exited the cell so I'll say EXO and it exited a cell so a site so site ptosis exocytosis and that's the process of molecules or substrates leaving a cell by vesicles fusing with membranes and so that's what we do when we want things to leave cells the exact opposite process where we have things enter cells by fusion of vesicles is called endocytosis endocytosis and these are very important terms to keep in mind great so now we've got a Sidel choline all over our synaptic cleft what is it going to do here well these sodium channels have receptors that sit on them that are called nicotinic acetylcholine receptors and as the name suggests acetylcholine can come and very snuggly sit here and send a message to this sodium channel that it's time to open and cause sodium to influx into our muscle cell and that's going to happen across the membrane causing a large amount of sodium to enter and once we've depolarize the membrane enough calcium will even start to enter and so in this way we'll have what's referred to as voltage-gated calcium release voltage-gated calcium release all right and so we have all this calcium that's entering the cell now what well this is just one cell contracting how does this make me able to to the worm or to kick a ball or to high-five my best bro well we can't just have calcium entering through the membrane there's another reservoir within our muscle cells that releases calcium for our disposal this reservoir kind of a large name and I'll write it out right here is called the Sarco plasmic Sarco plasmic reticulum r e t IC ul um sarcoplasmic reticulum this guy holds a whole bunch of calcium it's sitting in there waiting to be released waiting to do something so there are proteins that are attached to the membrane of the muscle cell and they're just waiting for enough calcium to be present here so that one of these calcium cations could bind this protein complex and then effectively cause calcium to be least from the sarcoplasmic reticulum this process is called calcium induced calcium release calcium induced calcium release and so now we have a bunch of calcium in here and this muscle cell will be contracting but it's still just one muscle cell what's the big deal well this muscle cell is attached to its neighbor right here actually there are proteins that link muscle cells together these proteins are called gap junctions gap junctions and they allow for cations to flow from one muscle cell into another so these muscle cells aren't really separate at all actually they're connected because of this gap Junction and actually continuous and there's a term that we use to drive home the fact that we can have our calcium cations move to this other muscle cell and to start a calcium induced calcium release process here and it's that our muscle cells are in a sin cesium a sin sition what does that mean well site just like we talked about here just means a cell and sin means that these muscle cells are in a synergy with each other when one contracts it causes its neighbor to contract as well and that's how we scale up because when muscle cells start contracting as neighbors you get the entire neighborhood contracting because you can then scale up and imagine that not just muscle cells but muscle fascicles will also be contracting to to produce a kick of a ball or the worm so this signal that started from our axon terminal here that began is just depolarization turns into an acetylcholine molecule that undergoes exocytosis to end up in the synaptic cleft where it binds a nicotinic acetylcholine receptor to cause sodium to enter a muscle cell and over time calcium to enter muscle cell which would then cause calcium induced calcium release which can then go to an adjacent muscle cell to cause a synergy of muscle contraction that spans from muscle cell to muscle cell from fascicle to fascicle and that's what happens at the neuromuscular Junction