okay so this is the second video on um muscle physiology and so we are going to start diving into how a muscle actually contracts so the first thing we're going to do is review the structure of a sarcomere from last time so last time when we looked at a sarcomere we were looking at this picture here all right so this is one myofibril and so then you can see the myofilaments those proteins and we have one sarcomere that's from z line to z line so now we're just going to isolate and look at that o
ne sarcomere okay so here we have one sarcomere all right so we have our z line would be here and then we have our second z line here so that's our sarcomere from here to here all right and we talked about last time the i band and the a band all right so uh remember the i band is going to be where there's only thin filament so that would start here and continue this way all right to the next point where those thick filaments start all right and the a band is where you have your thick filaments a
nd then there's some areas of overlapping thin filaments all right so that's our a band and then we would start another i band here and that would continue okay so we have those and then don't forget we also had our h zone which is this segment here in the middle where there's no overlapping thin filaments uh within that a band okay and so what's gonna happen when a muscle contracts all right this is called the sliding filament theory because these proteins these actin and the myosin are filamen
ts right thick and thin filaments and essentially they're going to slide past each other okay so the i'll kind of draw it on here right so the actin are going to be sliding right towards that h zone in the middle okay so the actin are the ones that are going to be moving the myosin stays still and actually they're responsible for pulling on the actin okay and so what's going to happen is these little i'm going to zoom in a little bit all right so for our myosin we have our little golf club kind
of myosin with those myosin heads right here and what's gonna happen is that myosin head is gonna bend upwards and grab a hold of the actin okay and so when it does that remember it's going to break down atp and pivot and as it pivots it pulls that actin forward okay so that's how that actin is being moved towards the h zone and all of these maya has sin heads would be doing that in their own time so they're all going to be grabbing and pulling and moving that acting forward so from each side of
the h zone the actin would be moving towards the middle that's the sliding filament theory all right the filaments are sliding past each other and that is essentially how muscles contract all right so two terms associated with that are the cross bridge right so this is when the uh myosin grabs a hold of actin all right so myosin binds to actin they form a cross bridge right so the crossbridge is just the myosin bound to the actin when let's try that again myosin binds actin all right so that's
the first thing it does and then remember it's gonna break down atp and that will cause it to pivot all right so myosin that is called a power stroke all right so myosin pivots after breaking down atp okay so those two events happen as part of that sliding filament theory all right okay so that's in general what's happening on the very very small protein level for muscle contraction all right so that's one piece and so let's talk about um what signal causes this contraction to occur all right so
we said that's an electrical signal before because these cells are excitable muscle cells are excitable so let's talk about that signal it's called an action potential okay and so to understand an action potential we need to understand the idea that excitable cells really all cells if a cell is alive it has a membrane potential and all that means is that the cell's membrane has a different charge on the inside compared to the outside all right so if we're looking at this is our cell membrane ri
ght our extracellular fluid that's on the outside cellular fluid here all right we have our intracellular fluid on the inside of the cell here all right so the inside is going to be slightly more negative than the outside okay and the outside would have a slight positive charge okay and so then with an action potential uh what happens is we change the charge all right uh and so it's a change in charge across the membrane so with this one right this is just an example of our membrane potential so
our whole membrane just has this positive charge on the outside negative charge on the inside okay so let's talk about what happens when you have an action potential all right so on a graph if we have on our y-axis here voltage all right generally that's measured in millivolts okay and then across the bottom we have time these would be milliseconds okay potentially even smaller all right um what we're going to see is that if we start at kind of a negative uh voltage all right so i'll just make
up kind of it's usually maybe around like minus 70 or so we'll get into the details in the nitty gritty with our nervous system chapter this is just to understand an action potential okay and so all this negative means is that the inside of the cell is minus 70 millivolts uh more negative than the outside of the cell okay and so in an action potential we're going to change that all right so that membrane potential the charge of the membrane is going to slowly get more and more and more positive
okay and the reason that occurs this is important to understand for muscle contraction this is the really important part all right is if we have our normal membrane potential here negative and positive what happens is that channel proteins in the membrane open all right and they allow sodium ions remember there's a high concentration of sodium on the outside of the cell actually here let's write that in up here we have a little more space so we have a lot of sodium high sodium on the outside of
the cell and our high potassium on the inside of the cell that was from our sodium potassium pump that we talked about all right and so to change the charge of the membrane you open a channel protein in the membrane that allows sodium to rush into the cell okay sodium has a positive charge and so adding all those positive charges to the inside of the cell is going to make this charge more positive okay so now we've changed the charge all right and made it positive on the inside negative on the o
utside because we've moved all of our positive sodiums from the outside now they're in here okay and so that's what's happening on our green part of our action potential so that cell membrane becomes more positive the charge the voltage becomes more and more and more positive all right after that and this is not as important for muscle contraction right we'll talk about this a lot more with the nervous system but after that the membrane potential will come back down and that's just because anoth
er channel protein for potassium will open that allow potassium to go out so now you have a positive charge leaving and that'll reset us back to our positive on the outside negative on the inside okay but essentially an action potential that electrical signal is occurring because you're allowing ions to move across the membrane and change the charge of the membrane okay so that's our action potential a change in charge of the membrane okay so normally the membrane is negative on the inside posit
ive on the outside during an action potential it becomes positive on the inside and negative on the outside all right and that's because in this case we have sodium moving into the cell making it positive all right then our last piece of the puzzle is going to be where does this elec electrical signal come from okay and so it is going to come from the nervous system all right and so we have a neuromuscular junction all right and so what happens here at this neuromuscular junction is you have a m
otor neuron all right is a specific type of neuron that communicates with skeletal muscle it tells skeletal muscle when to contract okay so skeletal muscle cells are not self-excitable meaning they can't start their own action potentials so they have to be told by a motor neuron right part of the nervous system this information would be coming from the brain to that motor neuron and then that motor neuron would send that information to the skeletal muscle okay telling the muscle to contract alri
ght so the point where that information goes from the neuron to the muscle is the neuromuscular junction that's where the neuron beats the muscle okay so neuromuscular and a junction where the two things meet all right and so the part of the neuron that interacts with the muscle is called the axon terminal all right so on this picture this yellow kind of bulge here right so this would be the neuron and it ends in this axon terminal which kind of has like a bulb shape okay that's the axon termina
l it's part of the it's like the ending part of the motor neuron okay all right so the way that this um action axon terminal takes an action potential and sends it to the muscle we're going to see is going to be through the neurotransmitter acetylcholine all right so let's just talk through the process of how this happens so i'm going to zoom in all right so we have an action potential it's moving through this neuron right and it's going to be sent to the muscle to tell it to contract so we have
that change in charge positive charge on the inside negative on the outside that's our action potential what the action potential is going to do first is it's going to reach this channel protein this channel protein is called and we're going to see this a couple times a voltage gated channel and so this particular one is a calcium channel all right so this is a voltage-gated calcium channel all right and so what the voltage-gated part means and we're going to see it over and over is that this i
s a channel through the membrane just a tube through the membrane and it's opened by a change in charge like an action potential okay so voltage-gated channels are opened by a change in charge okay all right so this in particular is a voltage-gated calcium channel all right so what it's going to do is we're going to have so you know calcium is going to be on the outside of this neuron all right and it's going to open and allow that calcium to move in when that calcium enters one thing calcium do
es is it stimulates the release of secretory vesicles okay so it's going to go and tell these so we have these vesicles that are full of the neurotransmitter acetylcholine all right so this is a neurotransmitter uh in the axon terminal okay and so it's going to be located in vesicles all right just those little bubbles okay so that's what these little green dots inside of here those are that would be acetylcholine okay so that acetylcholine in the vesicles right and you're gonna have calcium com
ing in and it's gonna trigger those vesicles to i think black is a little more visible all right to move towards the membrane right for exocytosis okay so calcium triggers exocytosis of these vesicles containing acetylcholine all right and so all exocytosis is it just means that this vesicle moves closer and closer to the membrane until it fuses with it like this all right and then that releases the contents in this case the acetylcholine out of that axon terminal so that's exocytosis okay and s
o it's going to release it into this space here all right there's a space between the axon terminal and the muscle cell itself that is called the synaptic cleft okay so the synaptic cleft is the space between the axon terminal and the muscle okay so that would be if we kind of bracketed this right space that's our synaptic cleft all right so that acetylcholine is going to be released into the synaptic cleft it's going to diffuse out all right and what it's going to do is it's going to find acety
lcholine receptors right so the acetylcholine wants to locate a receptor for itself acetylcholine those are going to be located on the muscle okay and so there's a special part of the muscle you can see how it gets like all wavy all right and it has receptors on it for acetylcholine that area of the muscle that has these receptors sorry is called the motor end plate all right so this is the part of muscle uh let's say this part of the muscle cell membrane with acetylcholine receptors okay and so
these acetylcholine receptors are a special type of receptor called a chemically or you may see in your textbook i can't remember if it says chemically sometimes they'll say ligand gated it means the same thing all right gated sodium channel okay so we had before our voltage-gated calcium channels right so voltage-gated means that a change in charge causes the channel to open here we have a chemically gated channel and this just means that when some chemical binds to it in this case acetylcholi
ne that will cause the channel to open all right so this one opens when a specific chemical right they're usually specific for something this one's a specific for acetylcholine all right binds okay all right so we have our motor in plates here right that's on our muscle cell it has acetylcholine receptors all embedded in its membrane so that acetylcholine is going to be exocytosed from that synaptic uh from that axon terminal it'll diffuse into the synaptic cleft and it'll find a receptor when i
t binds to that receptor what's going to happen is it's going to open and sodium is going to rush into that muscle cell all right so that's going to happen at all of these receptors that are binding to acetylcholine okay when it does that remember an action potential is caused by the moving in of sodium into the cell changing the charge of the membrane right so now we're changing the charge and we have an action potential right in our muscle cell okay so the function we'll zoom out here all righ
t so the function of the neuromuscular junction is to take an electrical signal alright that's our action potential in the neuron we want to transfer it to our muscle cell but electrical signals can't just jump across a gap like the synaptic cleft so we have to transform it into a chemical signal all right that would be our acetylcholine and then that acetylcholine now initiates our action potential right our electrical signal in our muscle cell okay so that's the purpose of our neuromuscular ju
nction is to take the action potential that is in the motor neuron and transfer it to the muscle cell and so it's transferred by acetylcholine okay so this is the first step to muscle contraction right which is a series of many steps okay so what we're going to do now is we're going to take everything we've talked about so far and put it all together in one story for muscle contraction and so i think this will help to kind of assemble all the pieces and make sense of it and how they work togethe
r and if you can in detail discuss this process we're about to go through from start to finish then you should be pretty well prepared for this aspect of your exam alright so i'm going to be making all right so obviously this is hand drawn i hand drew it and scanned it in so we can fill it in but i'll make another kind of copy of it and post that online for you guys to even if it's just you know the scanned version of this um and i'll post that online for you guys to use to just practice this pr
ocess so if you can fill this in from memory all right and i've got it numbered for the different kind of steps right one two three four five and so on if you can go through all that then for the process of muscle contraction for your exam you should be really well prepared okay so let's start with what we just talked about all right and then move all the way through whoops the muscle actually contracting okay so we're gonna start with number one so for number one right an action potential is mo
ving down the membrane of our motor neuron okay it's going to reach a voltage-gated calcium channel all right and when that action potential reaches the voltage gated calcium channel it causes that channel to open calcium is going to move into the axon terminal all right that calcium and this would be our axon terminal here the moving in of that calcium triggers vesicles containing acetylcholine to be exocytosed all right so that's what's happening here our vesicle is moving towards the membrane
all right merges with the membrane that acetylcholine can then diffuse across our synaptic cleft yeah there we go cleft okay and what it'll do is it'll diffuse across and it will bind to an acetylcholine receptor all right so an ach receptor remember that is a chemically gated channel okay all right and those acetylcholine receptors are located on the motor end plate that's the area of the muscle cell that interacts with that axon terminal and it contains those acetylcholine receptors so this w
ould be our motor end plate here all right when that acetylcholine binds to the receptor that receptor will open sodium will move into our muscle cell and that will start an action potential in our muscle cell okay all right so from here that action potential is going to move down the sarcolemma all right of our muscle cell until it reaches these t tubules it will then move down the t tubules right towards the inside of the cell until it reaches our sarcoplasmic reticulum all right that's going
to be in the orange okay so let's label first our t tubules here okay and then our sarcoplasmic reticulum here okay all right and so when that change in charge that action potential reaches the sargoplasmic sarcoplasmic reticulum it triggers the calcium stored in the sarcoplasmic reticulum remember that's the job of the sarcoplasmic reticulum to store calcium it triggers all this calcium to be released okay there's kind of a more complicated mechanism involved that we're not going to worry about
for this class all right just know that this action potential triggers the release of that calcium all the steps in between you don't have to worry about okay so that calcium has been released and that calcium is now going to go to our sarcomere all right and it is going to bind to that troponin okay so let's label some stuff on our sarcomere all right so with our sarcomere this would be our thick filaments our myosin all right so this is our myosin okay then the blue would be our actin right t
hose little beads so this is the actin or the thin filaments that green line sitting on top of our beads is remember the tropomyosin that tropomyosin sits on the actin and covers up the binding sites on actin for myosin and then the little spots on top of that tropomyosin that are kind of orange that's going to be our troponin okay and then we have here our z line which marks the start of our sarcomere all right okay so let's continue our story so we have calcium right is going to be released fr
om the sarcoplasmic reticulum it's going to bind to troponin all right when calcium binds to troponin it causes the troponin to change shape that change in shape pulls the tropomyosin off of those binding sites on actin alright so that's triggered by calcium okay so once those binding sites are now exposed the myosin has the opportunity to interact with that accent actin because the binding sites are exposed so what it needs to do is it needs to hydrolyze or break down our atp into adenosine dip
hosphate and a free phosphate and that causes that myosin to kind of sit up all right so it's kind of in a i like to think of it as being now right i'll write this here in an energized state okay because it just broke down atp and used that energize or that energy to kind of sit up right so now it's in an energized state sometimes your book will say maybe it's like cocked right so it sits up i like energized because it kind of helps to relate it to the atp okay so you break down atp myosin enter
s that energized state where it's sitting up now because it's sitting up it's close enough to the actin to be able to interact with it so it binds to actin all right when it binds to actin that adp and phosphate are going to be released all right and this release of that adp and phosphate causes the power stroke okay so that kind of pivoting of that myosin head all right and also this here where we're in that energized state interacting with actin that's going to be our cross bridge that we ment
ioned before okay all right so then the last step in this process is that myosin needs to let go of actin so we can do it all over again so it can grab the next segment uh like the next bead on actin and move it forward okay and so to let go atp has to bind all right so myosin releases actin when atp binds all right okay and so this process then will repeat all right so it's going to release it and then we can start back over up here okay so as long as calcium is still present right that myosin
can break down the atp enter that energized state all right and then do the power stroke then release actin again and keep going over and over and over again as long as calcium is still present okay so i don't know why it's talking and writing i always end up adding like extra letters or leading leaving letters off so i apologize i notice it later on okay so our last kind of thing to talk about for this whole process is how do you stop muscle contraction all right so like i said this process rig
ht will keep going as long as calcium and atp are present okay and so your cells are always going to be making atp so the easiest thing is to get rid of that calcium okay and so what happens is the sarcoplasmic reticulum has a calcium pump all right so first let's go back to our axon terminal here all right so when the um so with this sorry so with the motor neuron all right it only very briefly sends an action potential and we'll look at some kind of graphs of those action potentials and you kn
ow what happens when you send them often but they're very quick they're milliseconds so you get acetylcholine building up and then you actually need an enzyme to get rid of that acetylcholine so it doesn't keep activating the muscle cell all right so there's an enzyme called acetyl choline esterase all right and so the the job of acetylcholine esterase is to break down acetylcholine all right that way when the motor neuron here is done sending action potentials you remove the acetylcholine from
the synaptic cleft so you stop you know that start that action potential being transferred to the muscle cell okay so to stop muscle contraction that's the first step the action potential the muscle sorry the motor neuron has to stop in sending action potentials all right and this acetylcholine esterase will have to remove the acetylcholine from the synaptic cleft by breaking it down okay and so then at the same time you'll have this calcium pump here it'll be active and moving calcium from this
space out here back into the sarcoplasmic reticulum all right and so that will cause the troponin to release that tropomyosin so it can cover the binding sites again all right so moves calcium back into that sarcoplasmic reticulum the sr all right okay so that's the whole process i'm just going to briefly all right put this up on the screen so that's the whole process with all the blanks filled in and everything so when you're practicing you can make sure you haven't missed anything all right s
o make sure when you're doing it you go through each step and you talk about what's happening so that all the numbers what's happening at each of these steps all right okay so two more um little concepts to talk about uh before we wrap up this video so the first one i just think is really interesting and now you can kind of understand it now that we've talked about the process of muscle contraction and that is rigor mortis all right and so we know rigor mortis occurs after somebody has died afte
r a certain number of hours the body becomes very stiff and so what happens with rigor mortis is that calcium all right so the sarcoplasmic reticulum breaks down right as the body is decomposing and calcium is released from the sr okay so that sr breaks down the sarcoplasmic reticulum breaks down releasing all this calcium they go to the troponin cause that muscle to contract right so the body tenses up okay and then the next thing that happens is the body doesn't have atp right so the person ha
s passed away they're no longer making atp and we remember that the job of atp is to release myosin from actin okay so the reason the body is so stiff during rigor mortis is that atp is not present to release myosin from actin so it stays it contracts with that all that calcium and it stays contracted because it can't let go so not present to release myosin from actin okay so that's what's happening with rigor mortis all right okay and so then um let's just talk about motor units really quick si
nce we just talked about our motor neurons and our neuromuscular junction um and then we can talk about these a little more later all right so a motor unit is a motor neuron so this is just the definition right so it's a motor neuron and the muscle cells we say so the word is uh it innervates that's just a fancy way of saying what that neuron is talking to all right so innervates talks to communicates with all right and so what you can see in this picture here each motor neuron all right will in
nervate generally more than one muscle cell okay so muscle yeah muscle cells good i just want to make sure i included that all right so if we have this motor neuron here right and red it leaves the spinal cord so this is the spinal cord it will go out to the muscle and it actually will communicate it will innervate this one two three separate muscle cells all right and it varies from one motor neuron to another how many muscle cells they innervate so it can be a few or it can be a hundred or mor
e okay so you can see the purple one is only innervating too all right and so the different sizes of motor units all right kind of determines how much control you have over that muscle right so a small motor unit would allow for fine muscle control okay so this would be like in your fingers and your hands your eyes have very small motor units um and so your your brain your nervous system can activate just very small amounts of muscles to get this fine motor control versus a very large motor unit
all right so large motor units uh would give a little fine control right so this would be like your big muscles like your quadriceps and your thigh um or your biceps that kind of thing little fine control all right so they're not very good for fine delicate movement right like small motor units are okay and so once with a motor unit once that motor neuron sends an action potential all of the muscle cells that it innervates are going to contract all right so keep that in mind that's how this wor
ks right so if this this motor unit or this motor neuron sends out an action potential all three of these muscle cells will contract always okay all right and so in order for let's say let's say this is the biceps right and so we have um you're lifting maybe a sheet of paper all right you're holding a sheet of paper and you know somebody walks up to you and hands you the textbook for this class just plops it into your hands all right how do we go from just carrying that sheet of paper to now car
rying a heavy textbook all right what happens there is called recruitment okay so what happens with recruitment is that you increase the number of motor units that are active so now instead of just this motor unit being active you would activate this one as well as any other ones in that muscle that would help to increase that muscles capability of holding the weight all right so this would increase the number of active motor units okay all right good so then um we will pick up with the next vid
eo um and talk about some atp requirements as well as some other concepts associated with muscle activity um and that sort of thing
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