the sliding filament theory of muscle contraction is a theory that proposes that our muscles contract when some filaments slide over each other and when i say it's a theory i don't mean it's a theory like my theory about who will win top chef this season i also have one of those but this theory is more like the theory of general relativity that einstein used to explain gravity or evolutionary theory from darwin i mean these are theories that have mountains of supporting evidence we're pretty sure this is how muscles contract but to understand this theory we need to understand a couple other things first we need to understand the concept of excitation contraction coupling contraction we know what that is when the muscles shorten excitation refers to your nervous system creating an excited signal to tell the muscles to contract but like how does this nervous system signal this action potential like zap a muscle and make it do stuff that's tricky to do so that's what we'll look at in this video in the next video we're going to look back at striations we've been talking about those since unit 1. oh they're just little lines on some types of muscle tissue but what are those because if we can really get deep down on striations and understand their form and function then combined with our understanding of excitation contraction coupling we'll have a good idea about how this sliding filament theory works so let's get started by looking at excitation contraction coupling this is taking place at the neuromuscular junction that's where a neuron meets a muscle at a junction so this is where the excited signal is going to somehow jump into the muscle tissue zap it and cause it to contract so let's start by looking at this junction at the top of my image i have a neuron reaching down towards at the bottom of the image a skeletal muscle fiber now there's a small space in between these called the synapse so much of what we'll discuss is what's going on at this synapse this all begins in skeletal muscle when you decide voluntarily to make the muscle move so you initiate an action potential an electrical signal an excitation that will travel from your central nervous system down towards the muscle so i've just indicated that as in a lightning bolt here in our image you know the axon has ion channels that open ions are flooding into the axon causing this action potential to occur we don't need to get into the details of action potentials right now but we want to understand that that signal will eventually reach the end of this axon at a structure called the axon terminal now inside this axon terminal are vesicles we talked about vesicles back in unit one there's a lot of different types but these vesicles are called synaptic vesicles they're full of neurotransmitters and what i want you to see here is that they will drift towards the bottom of the axon terminal fuse with the cell membrane of the neuron and dump their contents out into the synapse when we say contents in this case when you have a neuron talking to a muscle fiber the contents will be a neurotransmitter called acetylcholine acetylcholine is pretty cool it's the first neurotransmitter ever discovered now we're keeping it pretty basic here right but even just this is a pretty amazing feat we've taken this electrical signal and we've converted it into a chemical signal which is now drifting inside of our body across this tiny tiny tiny space so what happens next these neurotransmitters are floating out there and they will eventually reach receptors after crossing the space called the synaptic cleft so the neurotransmitters hit these receptors they're not going to enter the muscle fiber the neurotransmitters will just attach to the receptor for a period of time and then they'll detach and drift away but when they attach and if they attach in sufficient numbers what can happen is that this excited signal this action potential that's now turned into this chemical signal can trigger ion channels in the muscle fiber to open and ions start flooding in just like they did in the neuron and we trigger a new action potential a new wave of electrical activity traveling down the cell membrane of this cell so that's pretty good we took an electrical signal we turned it into a chemical signal and we re-initiated that electrical signal on the other side of the synaptic cleft but if we stop here all we're doing is zapping the superficial portion of the cell in other words this signal right now is just traveling down the cell membrane we need to get that signal deep into the skeletal muscle fiber we do that by allowing that action potential to travel down these tubes called t tubules and to get an idea of what this looks like if you took a muscle fiber you know they're like long cylinders and you imagine taking a pin and just poking little holes in the cell the hole would be coming in perpendicular to the cell membrane forming the shape of a t so that's what these t tubules are they're holes and passageways perpendicular to the cell membrane and the action potential can access the deep parts of the cell through the t-tubule and as we follow our image down we can see that the t tubule reaches some type of structure here which is called the terminal cisternae a cistern is like i don't know a spittoon do you know spittoons like old-timey jars that people would spit into because spittoon well cistern is also like an old-timey word for some storage receptacle and terminal means at the end right terminate the john kahn you terminated you know and so we're at the end we've reached this storage unit and the storage unit is part of a bigger structure called the sarcoplasmic reticulum this sounds very fancy all it really is it's the endoplasmic reticulum but we're in a muscle fiber and sarco means meat or flesh which is what muscle is so this just means the endoplasmic reticulum of the muscle cell but what the sarcoplasmic reticulum does is store loads and loads of calcium and as that action potential travels down the plasma membrane deep into the t tubule hits that terminal cysterine and the signal spreads into the sarcoplasmic reticulum calcium is released all throughout this muscle fiber calcium is released and that's really good for reasons we don't know yet but it's important for calcium to flood into the cell and once we understand what striations are we'll see that there is a calcium binding site in and amongst the structures that make striations and only when calcium can access that binding site can contractions happen so we'll do that in the next video but a final word here on this concept of crucial importance is this synaptic cleft the events that take place within the synapse are very important not just in neurons meeting up with muscle cells like if you've heard of serotonin and dopamine and ssris selective serotonin reuptake inhibitors like prozac those medicines and those neurotransmitters are all involved in regulating humans mood so lots of antidepressants are acting on these synapses and the behaviors of the neurotransmitters and the receptors and some enzymes and what happens in there and so that's true also with acetylcholine acetylcholine uh imbalances seem to be playing a role in parkinson's and alzheimer's and other pathologies also if you really understand this stuff you can get into some trouble with stuff like creating vx nerve agents and no better place to learn about those than the greatest movie of all time the rock starring sean connery and nicholas cage where they keep the world safe from vx nerve agent but this is real stuff and serious stuff if you recall the dictator of north korea kim jong-un's brother was killed when these two women ran up behind him and rubbed something on his face and they said it was for a game show and it ended up being vx nerve agent and that stuff messes with what happens in the synaptic cleft when that acetylcholine floods the synaptic cleft it's supposed to bind with receptors let the muscle contract and then it detaches and there's an enzyme that comes in and gets rid of the acetylcholine what happens if you don't get rid of it well that's what vx nerve agent does it messes with that enzyme you've got all this acetylcholine that's just now it's hanging out in the synaptic cleft so your muscles are contracting and contracting contracting again and again and again so you're spasming until all that shuts down all your muscles are fatigued including your diaphragm and then you affixiate due to what's going on at these synapses so just a few little real world i mean except for the movie the rock real world applications of just this simple concept interesting