I ninja nerds in this video today we are going to talk about the structure of skeletal muscle all right so first off before we start even getting into this diagram I want to talk a little bit about muscle so what are some of the characteristics of muscles so what I want you guys to remember with muscles is that there are four characteristics that make muscle tissue a little bit different from other tissues so for example one of those four characteristics so if we were to go here in order one two three four the first thing about muscle that's really really special about is that it's excitable so it's one of the few tissue cells this in neural tissue that it's excitable so what do I mean by excitable let's say here I have just an actual muscle cell here's our muscle cell okay and then serving it is some going to be some type of motor neuron okay so here's our actual motor neuron this motor neuron is actually going to be responsible for stimulating this muscle cell how by releasing specific chemicals that we'll talk about more details in neuromuscular Junction they release out neurotransmitters for example in this case acetylcholine and what is this acetylcholine do it stimulates this muscle cell and what does this muscle cell in a response to that do it generates a change in membrane potential so it undergoes what's called an in play potential and an action potential right so that's one thing about muscles that they're very excitable so they can respond to some type of stimulus usually a neural stimulus and they change their membrane potential in response to that another thing about that is when their membrane potential changes and they develop is called an action potential that action potential can be propagated along the actual muscle cell membrane and eventually trigger this muscle cell to shorten what does that mean whenever it can shorten forcibly due to frequent or adequate amount of stimulation that's called contractility so muscles also have contractility so they're contractile so one thing about muscles is that they have the ability to be excited so they can be stimulated by some neural stimulus and respond to that they can develop a membrane potential that membrane potential can then cause this muscle cell to shorten forcibly and when it does that due to adequate stimulation it can contract so its contractile you know what else is really cool about muscle it has the ability to be stretched appropriately so you don't want to be able to stretch a muscle too much but it does have the ability to stretch beyond its normal resting length so for example if I have a muscle here let's say here's a muscle okay here's this muscle and this is at its resting length okay this is at rest what I'm going to do is I'm going to stretch this muscle so what would that do to this muscle it's going to allow for this muscle to be a little bit longer to be stretched beyond its normal capabilities so this ability to stretch the muscles so this is rest and this is stretched so the ability to stretch this muscle beyond its normal resting point allows for the muscle to be very distensible or extensible okay so we can use the word distance ability or extensibility okay so it's extensible just meaning stretchable okay another characteristics of muscle tissue is the fact that it's elastic though - okay so it's also elastic we'll talk about that when we talk about these connective tissue coverings so it has elasticity what is elasticity elasticity is for example we can say two different types of definitions one of them is whenever you try to stretch a muscle all right any type of tissue let's just any type of tissue for right now I stretch that when I stretch it it resists that one desire to want to stretch it always wants to recoil and go back to the small size possible so when you think about that me trying to stretch something and it's resisting at that elasticity that's also a one of the characteristics of muscle tissue okay so these are some of the muscle characteristics okay now we're going to go over a lot of these different function of these actual muscles for example we'll go over contraction with the neuromuscular Junction the excitation contraction coupling but just for right now what are four main functions of muscle if we were to just for right now go over four main functions of muscle just to overall concept of it the four major functions that encompass our muscles you know basically their function is going to be one is the ability to produce locomotion what does I mean you know what I want to be able to move my arm up it depends upon the muscles contracting pulling all my skeletal bones so because of that it's responsible for moving the skeleton or producing locomotion so I'm just going to put producing movement producing movement of our skeleton so local motion another thing for me to be able to while I'm standing to maintain this posture okay or stability my body position to maintain my posture against grab because gravity's trying to push me down in order for me to maintain my posture and stand up straight and have normal body position I have to have those muscles there that are stabilizing that okay so because of that we also need the muscles to help us with our posture and stabilization right so helps to maintain our posture another thing is it wraps around many different types of joints so because it's wrapping around joints joints are naturally stable on their own right because usually bones are connected with ligaments and other different types of structures right connective tissue structures but muscles help to stabilize those joints even more so another function of muscles is they stabilize a lot of our body's joints and another thing they help with temperature okay that's how they help to be able to produce or generate heat you can think about that I like to think about that just in general muscles are constantly whenever we actually undergo say the respiration one of the byproducts of Seiler respiration is heat production also I like to think about if you're really really cold what does your body try to do to be able to compensate for that cold it shivers whenever you shiver the contractions are complete contraction so those quivering contractions help to be able to generate some heat okay so that's one of the functions also muscles so just a real quick recap just remember the general characteristics of muscle that's excitable so I can receive a neuro stimulus and it can respond by having a membrane potential if that membrane potential is adequate enough to produce an action potential the muscle can shorten forcibly which is called contractility on top of that it can be stretched beyond its normal resting length right and that's because it's extensible okay so you can think about sometimes eccentric contractions we'll talk about that later elasticity is it has the ability to be stretched right but I should actually be rephrase that it specifically elasticity is once you want to reduce stretch you don't want you want to resist the change in the stretch you wants to recoil and assume the small size possible and also that functions it likes to produce local motion maintain posture and body position against anti you know anti gravity effects helps to stabilize certain types of muscle joints think about the rotator cuff the rotator cuff is a very good stabilizer of the actual shoulder joint the supraspinatus infraspinatus teres minor and subscapularis if you remember sits okay and it's also responsible for being able to generate heat to maintain certain types of body temperature for example we're really cold we shiver to generate some heat okay so that's that now what I want to do is I want to zoom in on the actual overall macroscopic structure of the skeletal muscle and then we'll work our way down from macroscopic to microscopic okay so let's do that so there here we have a big old skeletal muscle so for example this is the femur so let's just say it's one of the actual quadriceps muscles right one of the actual muscles they're coming off of the actual femur and what this muscle is going to do what we're going to do is we're going to take this slice it in half and we're going to look at it in this view so for example this is the whole muscle belly all I'm doing is I'm cutting this piece and opening up so we can see what it looks like inside of it okay so I'm making this transverse cut and I'm flipping it over now when I do that you have to realize something there's actually a nice little connective tissue covering right over this muscle belly you see all this right here we actually peeled it actually you guys to see so we peeled this little part here back so if you guys can see right here this little connective tissue structure it's actually peeling or peeling it back from this actual muscle belly so this whole thing in here is the muscle belly surrounding this muscle belly is a little connective tissue covering right there this connective tissue covering I'm going to come from the top here because it's this whole thing here we're just peeling that piece back this is called the EPI mice um okay this is called the epimysium now the epimysium is a dense fibrous let's actually write that down that'd be good to know it's a dense fibrous irregular I'm going to put IR connective tissue okay so it's very very tough I also call you know your white fibrous tissue right so it's a very very tough connective tissue so that's going to be one of the connective tissues so this is actually going to be dense fibrous irregular connective tissue and again it's actually going to be actually connected right on to the actual large muscle belly all right that's one part and also we'll talk about this in a second and the dis actual epimysium in certain types of situations very not as common as tendons and we'll talk about that it can actually fuse to the periosteum of the bone and form like direct fleshy attachments with the bone we'll talk about that in a little bit though alright so now what I did is okay so we have the epimysium right the connective tissue covering a writer on this whole muscle belly a cus cus section of that muscle belly inside of that muscle bellow you see a whole bunch of bundles of muscle fibers so you see this bundle right here this is a bundle consisting of muscle fibers this is a bundle consisting of muscle fibers and this is a bundle bundle bundle bundle these bundles of muscle fibers are called fascicles okay so this bundle right here of muscle fibers and this bundle is called a fascicle so a fascicle is a bundle of muscle fibers okay now surrounding the fascicle is another connective tissue layer so for example we just blew up one of the FAFSA khals we blew up one of these fascicles and if you see there's a nice sort of black connective tissue covering around that fascicle right there we peeled that piece back again for you so you see a little connective tissue covering around this and we peeled a piece back around this fascicle this part right here this connective tissue is actually called the perimysium okay it's called the peri my see'em now the perimysium just like the epimysium they're actually continuous with one another and that's going to be in very important I'll explain why that's important in a second but the perimysium is a nice dense fibrous irregular connective tissue just like that be my seal okay so it's a nice dense fibrous irregular mapa i our connective tissue so really really cool stuff there all right and again what I said is that this actual perimysium the epimysium and this last one that we're going to talk about the endomysium all continues with one another all right so now this is the fafsa chol this is a fascicle fascicle fascicle fascicle fascicle fascicle fascicle the whole thing is a muscle belly okay what I'm going to do now is I'm going to take out of this bicycle I'm going to pull one of these structures out this two because two that I pulled out this is a muscle fiber okay so this one right here is specifically called a muscle fiber now sometimes you might hear the word muscle fiber and muscle cell used interchangeably because they are the same thing a muscle fiber and a muscle cell is the same exact thing okay this muscle fiber so you can see that there's tons and tons of muscle fibers within this bundle of these structures bundle of muscle fibers in here it's called a fascicle right an individual structure here is a muscle fiber or a muscle cell now there is another connective tissue covering surrounding this actual muscle fiber the connective tissue but it's not as tough of a connective tissue there's a connective tissue that's black connective tissue we peeled a little bit of back it might be hard to see but there's another little connective tissue that we peel back surrounding this muscle fiber and that is called the endomysium endo mice you now the endomysium is an areolar connective tissue so it's not as tough it's not as a it's not as tough and actual resistant and resilient as compared to the dense fibrous irregular connective tissue because you know areolar connective tissue it has a little less collagen as compared to this dense fibrous irregular connective tissue okay we'll talk about this next thing in another video the next video where we discussed the neuromuscular Junction but the endomysium is covering this muscle fiber there's what's called the plasma membrane and the plasma membrane is actually covered by this endomysium so there is a plasma membrane it's here in red the muscle fiber is surrounded by a structure called the sarcolemma and we will talk about this in the neuromuscular Junction video but again don't get this confused with the endomysium the endomysium is rapping over the sarcolemma okay so this is actually the plasma membrane so this is your phospholipid bilayer okay so don't get that confused with the endomysium okay so the muscle fiber is surrounded let me actually make that clear it's surrounded by this sarcolemma surrounded by this sarcolemma alright so it's surrounded by the sarcolemma which is this plasma membrane which is again us phospholipid by layer okay now we have all of these actual connective tissue coverings right and again to recap surrounding the muscle belly is the epimysium which is dense fibrous irregular connective tissue in this whole muscle belly you have multiple fascicles and these fascicles are just bundles of muscle fibers each individual fascicle is covered by another connective tissue and that connective tissue is the same connective tissue as epimysium it's dense fibrous regular connective tissue but we call it perimysium okay because it's covering these fascicles then in each individual fascicle is many many many muscle fibers or muscle cells these muscle fibers each individual one is covered by another connective tissue which is an areola connective tissue called the endomysium but I'm surrounding this you know you have the endomysium surrounding this muscle fiber this muscle fiber cell has a membrane called the sarcolemma it's the plasma my phospholipid bilayer that's underneath the endomysium alright now I told you that all of these structures the endomysium the perimysium and the epimysium are all kin continuous with one another and what I mean by that is you know how we have this actual tendon here let's say this is the tendon so here's a tendon connecting the must what is a tendon by definition what it attendent so this right here is a tendon but a tendon is a rope like connective tissue rich and rich in collagen so what does that mean so what that means is that collagen is very resilient right so it's very resilient very tough so what this tendon is doing is it's connecting the muscle to the bone now here's what's really cool when this muscle fiber contracts so let's say we take this individual muscle fiber it contracts when it contracts it pulls on the endomysium what did I tell you all of these structures are continuous so this actual what this muscle fiber contracts what does it do it pulls on the endomysium it pulls on the perimysium it pulls on the epimysium and if that's the case then it's pulling on all these connective tissue sheets what is that going to do this can actually be connected with attendant and the contending is what's connecting the muscle to the bone so one more time if the muscle fibers contract they pull on the endomysium the perimysium and epimysium when they pull on that they pull on the tendon and if they pull on the tendon what is that going to do to the bone it's going to move it so that is the function of these actual connected two sheets so again one more time let's actually kind of follow it like a float pop pattern muscle contracts let's say that the muscle fiber contract what does that do to the connective tissue sheets all the endomysium epimysium and Paramecium the connective tissue I'm going to put CT sheets are pulled on okay when they're pulled on they're going to pull on a result the tendons so they're going to pull on tendons and when you pull on the tendons what is that going to do the bone it's going to pull on the bone and move the bone and what's that going to produce locomotion right so it's going to pull or move bone and obviously depending upon the type of muscle it also depends upon what's called insertion and origin what is what is meant by insertion and origin so let's say I take for example simple muscle so I take the master okay I say I take the masseter so here I have my masseter muscle now just to be really simplistic here there's two bones that the masseter is generally connected to let's say here's one bone unless they hear the other bone okay just a crude diagram then what I'm going to do is I'm going to link this muscle up between these two bones because that's really what's happening this muscle is being you know a connecting a connection between these bones and then what's happening is I'm having a tendinous connection let's show that here and this blue so let's say I have a tendon is connection here and attend in its connection right here let's say this bone that this massive muscle is connected to is the zygomatic bone so this one up here is the zygomatic bone and this bone down here is the mandible so let's say this is the mandible when the muscle is contracting again what is this muscle here called this muscle is called the we're just taking the masseter muscle as an example it's one of the muscles that helps them chewing what's called mastication what happens is we have to determine what's the insertion and what's the origin I'm going to write it down for right now and I'm going to explain why okay so if that's the case I'm going to explain to you it's going to make more sense after I get it all done the mandible is actually moving that's the function of the master so the masseter helps to pull the mandible up la demanda below i think about in this position and i'm going to elevate it that's the job of the master so it's moving the mandible up so that's the case this is the one that's moving nothing happened to my zygomatic bone I didn't move that bone it stayed still so it stayed still it's not moving if that's the case then what happens is when a muscle contracts the part of the bone that's not moving is referred to as the origin okay and then the part that's actually moving is referred to as the insertion what happens is when a muscle contract it moves from insertion to origin but again what is connecting this muscle to the bones the tendons but you know that's not all that's connecting them besides tendons you have other examples of these connective tissue sheets in your body for example on the top of the head first one that you like to think of or within the actual abdomen their sheets alright their sheets of actual connective tissue so a tendon imagine a tendon is like a rope it's a rope like connection between the muscles bones this would be an example of a tendon another one is like this imagine a sheet so imagine actual sheet this is an example of an apple neurosis so an aponeurosis is a sheet like connective tissue that's connecting muscle to bone a tendon is a cord okay our rope like connective tissue that's connecting muscle to bone all right so now again what do we cover here we've said all of these connective tissue sheaths are important because when a muscle fiber contracts it pulls on the connective tissue sheets which pull the tendons which pull or move the bone depending upon what the tendons are connected to determines the actual direction right so for example if we said the tendons are connected to the mandible they're also connected here to the zygomatic bone right what's going to happen well the mandible is not the part that's actually it's the part that's actually moving so whenever the muscle contract it moves that bone up that must be the insertion this point where the tendon is actually connecting is not moving okay that point is not moving that's fixed so that has to be the origin and insertion always moves towards the origin now another thing you know these connective tissue sheets they also contribute to elasticity what I mean by elasticity again elasticity is that ability to resist that change in the actual stretch or deformation and the reason why is because it's a dense fibrous irregular connective tissue they're not very good at allowing for stretch all right so now you know these connective tissue sheets you know it's also really important with these connective tissue sheets because it's dense fibrous irregular connective tissue those types of connective tissue you want to resist the stretch they constantly want to be able to pull the muscle and maintain its normal size they don't like being stretched so because of that they contribute to elasticity so another function of these connective tissue sheets you know again at endomysium perimysium epimysium these connective tissue sheets are contributing to elasticity I'm saying they're contributing to elasticity and that's really important another thing that actually happens is you know these actual dense fibrous irregular connective tissue is decently vascularized not superiorly vascularized but it's enough vascular ization so they do have blood vessels that are running through there and nerve fibers that are running through these connective tissue sheets those three functions I want you guys to remember for connective tissue sheaths one when a muscle contracts the muscles pull on the sheaths those pull on the tendons which pull or move the bone second thing I want you to know that they contribute to elasticity so they want to prevent the actual or resist the desire or the muscle to be stretched they want it to recoil I want to assume the smallest size possible third thing I want you guys to remember besides that is also that blood vessels and nerve fibers are running through these connective tissue she's alright last thing and we're going to go into the sarcomere another thing that's really important is understanding how muscles are connecting to bone because muscles can connect the bone in two ways okay let's let's go over that quick now okay so there's two ways in which the muscle connects so how can these muscles so muscle to bone connection two ways one way is a direct attachment okay the second way is indirect out of these this is less common this is not as common so this is less common and this is much much much more common many of the ways that we are actually connecting our bones to muscles if they're indirect connections we've already talked a little bit about them the direct connection and simple to make it the most simple as possible all it is is is this this connection from the epimysium to the bone so an epimer Xion is fusing with the bone specifically what structure of the bone so epimysium is dense fibrous irregular connective tissue there's another connective tissue that's actually clinging directly to the bone and this connective tissue that's cleaning directly to the bone is actually called periosteum so what happened is whatever the periosteum is fusing with the epimysium that is a direct fleshy attachment another thing though you know is that at the end of our bones usually usually you have a hyaline cartilage hyaline cartilage that's actually you know right here and then there's actually going to be this actual dense fibrous irregular connective tissue that's surrounding that so if that's the case what's that called that's called perichondrium so you have a periosteum or perichondrium any direct connection between the epimysium and the periosteum or the perichondrium is a direct connection okay that's a direct connection so let's write that one down so any direct connection which is less common is going to be epimysium fusing with the periosteum or it's fusing with the peri Condry 'um okay that is an example of a direct or fleshy attachment not a common the indirect ones are much much more common I'll give you two reasons why these indirect connections are mediated through tendons which we already talked about and aponeurosis which we already talked about okay I want you guys to think logically here about why tendons and aponeurosis more commonly tendons are more common for a connection between muscle to bone all right first reason tendons are much smaller okay so because tendons are a lot smaller they're going to conserve more space that's one reason why the indirect connections are better so one reason why indirect connections are better so let's say here we have indirect connections or attachments one reason why is it conserves space because tendons are much much smaller than these having these direct fleshy attachments that's one reason simple reason second reason a little bit different the other reason is actually because it's very resilient or tough okay so that's one reason it's very resilient what do I mean by that so you know these tendons are actually going to be undergoing a lot of rubbing between the bone all right so whenever the bones are actually moving right is whenever the bones are moving because of the skeletal muscles allowing for them to contract those tendons might actually have a little bit of friction against the bones what happens is is that that was fleshy connections those would completely get fricked up all right they would get destroyed in that kind of situation so they're not very good in that situation because tendons are a lot of collagen connective tissue so it's a lot of collagen what does that mean if there's a lot of collagen it's very very tough very very resilient and it's able to be able to allow for the bones to rub up against it and not actually break apart right so that's one reason so a good thing for indirect connections is that they're very small so they can serve a lot of space second reason is that they can resist a lot of abrasion and friction whenever the bones rub up against it okay that's that's the second reason so two reasons why is because it's very resilient because of the College of connective tissue and because of that it can undergo resistance against abrasion whenever the bones are rubbing up against it second reason is they're small in the conserve space alright so now what we're going to do is we're going to take and look here for a second this is our muscle fiber or muscle cell right that was covered with the endomysium and underneath the endomysium was the plasma membrane which is the sarcolemma now that muscle cell consists of thousands upon thousands of these little structures here what is this structure right here called this structure right here is called a myofibril okay a myofibril is consisting of tons and tons of proteins now a myofibril again like I said you can have hundreds to thousands of these myofibrils within one muscle cell and also muscle cells you'll notice that they have this blue structure around them we'll talk about that in the neuromuscular Junction video but that is going to be super super important going to write it down for right now it's called the Sarco plasmic reticulum and it's a nice factory of calcium and you'll see why whenever we talk about that with respect to the t tubules also you'll notice that these skeletal muscles are very cylindrical okay so each skeletal muscle fiber is very cylindrical and it's multinucleated okay so again remember this rascal muscles I want you to remember that they have a nice little filament asia's sarcoplasmic reticulum which is kind of like a derivative of the endoplasmic reticulum and it's a calcium storage Factory it's going to have a cylindrical shape and it's going to be striated I should write that down that's an important thing to write down so another thing about these actual muscle fibers or cells is that they're striated so what is meant by striated it means that it takes on like a striped appearance and that's what we're going to look at in this situation here this whole big beast that we're adding over here and you guys are probably wondering about this big beast over here is actually the functional and structural unit of the muscle cell it's actually called the sarcomere and that's what we're going to talk about in the next video alright so what we're going to do in the next video guys we're going to go over a little bit more detail on this actual specific myofibril consisting of this sarcomeric structure alright so I'll see you guys in the next video in part two where we talk about a little bit more about the actual sarcomere