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This information is included in yourillustrated notes. The first page shows you that skeletal muscle,in order to understand skeletal muscle we have to understand avariety of different levels of organization.
Starting at the top, we have the level oforganization that's represented by a whole muscle. This is alarge multicellular, of what we would call a gross anatomicalstructure. That is one that is big enough to be observed with thenaked eye and that muscle is composed of the next lower level oforganization which is the level of the muscle fiber. Andthe muscle fiber is a musclecell. As biologists it is good to know at what level ofthis organization are we dealing with a cell. And that's at thelevel of muscle fiber. An individual muscle may be composed ofanywhere from 100 to perhaps as many as 100,000 muscle fibers.The arrangement of the fibers within the muscle will differ indifferent muscles. In this particular muscle that's illustratedhere, the fibers are all arranged going longitudinally the entirelength of the muscle itself. But other arrangements are alsopossible.
The fiber itself functions as a unit,that is an individual muscle fiber is either relaxed or it iscontracting and what we call that contraction is a twitch.So a muscle fiber functions as a unit and it either twitches itor the whole fiber functions and twitches or else it just sitsthere and is relaxed. Individual vertebrate skeletal musclefibers can be of different diameters ranginganywhere from 5 to 100 microns in diameter. Thelarger the cross-sectional area of the fiber, the more force itcan generate.
Now a muscle cell has a very complexintracellular organization. That is within the cell itself, wefind the complex organization and one of the sub-units of thatorganization is the myofibril. You look atthe diagram of a muscle fiber right here, you can see that theway this diagram indicates, is the muscle fiber looks like abundle of little straws if you will and those little individualstraws are the myofibrils. They are separatedfrom one another by layers of a membranous structure called thesarcoplasmic reticulum. And the myofibrils are themselvesbundles of the muscle filaments. So this is kind of a confusingset of terms. I didn't make it up though so don't blame me. Wehave fiber, we have myofibril, we have filaments.
There are 2 types of filamentsfound in vertebrate striated skeletal muscle:
l) Thin filaments - They arecomposed of a protein called actin.
2) Thick filaments - They arecomposed of the other major protein called myosin.
The thin filaments as I said arecomposed of actin and they are really polymers of actin, the individualactin molecules are called G-actin. The G standing forglobular. In other words, they are little more or less sphericalstructures. And they are arranged together in strings thatform F-actin. The F standing for filamentous. And the thinfilaments are composed of 2 F-actins. If you look at thediagram you have in your illustrated notes, these little stringedbeads right here, each of those little beads is a single G-actin,you put G-actin together and it arranges itself by formingcovalent bonds into long filaments and a pair of those filamentswill wind around each other helically and that's what a thinfilament looks like. A thin filament is composed of 2F-actins, 2 long filamentous actins.
The myosin that makes up the thickfilaments also have a complex structure which isdiagrammed down here at the very bottom of the page. There is along rod-like tail on the myosin and then there is a head thatsticks out to the side, that's the myosin head. And located onthe myosin head are 2 separate chemically active sites.So this myosin head can actually be catalyzed to chemicalreactions or be involved in 2 different chemical reactions.One of these sites is the actin binding site. And thisone that the name implies can form a covalent bond with a G-actin.With one of those sub-units of the thin filament. The otherchemically active site on the myosin head is the ATPasesite. That tells you, the -ase ending on that word tellsyou that this is an enzyme and what it does is it splitsATP. It converts ATP into ADP an inorganicphosphate. Probably the most common chemical reactiontaking place in the cell.
We know that ATP is the energy currency of thecell. It's the molecule that cells use to store chemical energyin so that they can then be transferred to somewhere else in thecell and be used in some energy requiring chemical reaction. Andthe myosin head uses the energy in the ATP through a series ofsteps that I'll describe in a minute to cause the twitch, themovement of the muscle fiber.
The thick filament is composed of a very largenumber of these myosin molecules, with the tail and the head. Ifyou purify myosin, keep it at the proper pH and ionic strength sothat it maintains it's proper structure, it will spontaneouslyarrange itself into thick filaments that have a structure that'sdiagrammed 3-dimensionally up here. The tails tends to alignparallel to one another. The heads sticking out around the sideof the thick filaments. So that at a lower level magnification,what you see then are the thick filaments, these little bumpssticking out from the side, those are the myosin heads, and thethin filaments arranged between them.
Now obviously there are some other things inhere. There are some structural proteins that produce andmaintain a very complex 3 dimensional arrangement of thesethick filaments and thin filaments. The reason that this muscleis called striated muscle, striated means striped, is becausewhen we look at a thin section of vertebrate skeletal muscle,under high magnification we can see stripes. Alternate light anddark bands along the length of the muscle fiber.
The actin binding site on the myosin head canform a covalent bond to actin molecule and I'll expand upon thatin a second.
Question: The book refers to thesarcoplasmic reticulum as the sarcolemma, do you have anypreference? There's a difference between the sarcolemma andthe sarcoplasmic reticulum. The sarcolemma is the cell membranethat surrounds the entire outside of the cell. The sarcoplasmicreticulum is a membranous structure within the cell that'swrapped around these myofibrils, the bundles. I refer to thethick and thin filaments as the contractile machinery, all right,cause that's the molecules, the supermolecules within the cellthat are actually responsible for the contraction. And so themyofibrils are bundles of this contractile machinery and theyhave wrapped around them this internal membranous structure, thesarcoplasmic reticulum (SR).
Appearance Under Light Microscope:
Now on the next page of your illustrated notes,you have a very high magnification of what this striated businesslooks like. It's not a really great copy. When you look at theskeletal muscle in lab, you look through your microscope field,you're going to see individual fibers that are fairly large. Theyare going to take up a good portion of the microscope field. Thesewill be the individual parts of the individual fiber. Theycan be very long.
And you will see that these fibers aremultinucleate, that is, they have many nuclei. Now thatought to sound a little weird in itself. Cause I've told you thata muscle fiber is a single cell. How does a single cell endup with many nuclei? These individual musclefibers are formed by the fusion of a large number of embryonicmuscle-forming cells (myoblasts). Each of thoseindividual cells contribute to some nucleus.
INFORMATION FOR LAB TOMORROW: (If you'relucky and you cut your light intensity down a little bit and scanaround, you may be able to find some place where there are very,very faint lines running across the fiber. Those are theindividual striations. The individual stripes of which this typeof tissue is named.) And the diagram you have is very, very, muchhigher magnification than you can achieve with the microscopesthat we have in our lab. But the reason for looking at thishigher magnification is to explain the organization of the thickand thin filaments that produces these alternating light and darkbands. And so that's what I want to talk about right now.
A single dark, one of those individual darklines magnified greatly would be this big and it corresponds tothe place in the cell where the thick filaments represented bythese heavy bars, that's where the thick filaments are. The thickfilaments are lined up parallel to one another, all the wayacross the entire muscle fiber. Those dark areas are called the A-bands.Stands for Anisotropic. A-band refers to thetransmission of polarized light. So the dark bandon the fiber is the A-band.
And the alternating light stripes,light areas, are the I-bands and stands forIsotropic. So the alternating light and dark bands in amuscle fiber which remember at the level of magnification thatyou're going to see, are going to be very close together, verytiny, hard to see, are produced by the very regular arrangementof the thick filament and the thin filament. The A-bandscorresponding to the location of the thickfilaments and the I-bands corresponding to thespaces between the thick filaments which includes the thinfilaments. At very high magnification, there is aline that can be observed with in the I-bandand that line is called the Z-line. What itrepresents is the location of a structural proteinthat runs across the fiber and holds the thin filaments in theirlocation. Maintains their position within the cell.
If the muscle was stretched before it wasfixed and prepared for thin sectioning, then therewill be a light area in the middle of the dark band and that'scalled the H-zone. That stands for the German word hellewhich means bright. So if the muscle has been lengthened greatlybefore it was frozen and sectioned, then you're going to havethese H-zones in the middle of the A-bands andthese observations about the light and dark bands and thepresence of the Z-lines and the H-zones, all of that wasunderstood a long time before anybody had a good idea of whatcaused it. But eventually additional studies lead to thedevelopment of the understanding that the arrangement of thethick filaments, which are composed of myosin, and the thinfilaments, which are composed of the filamentous actin, producethese alternating light and dark bands. And that very regulararrangement of the thick and thin filaments helps us understandhow these dark and light bands move and change their dimensionswhen the muscle changes it's length. In other words, I told youthat those H-bands only appear if the muscle is stretched, whenit is fixed. When the muscle has been contracted down so thatit's very short, when it is fixed, we see the dimensions of someof these things change and some of them do not.
So at the top here you have a diagramrepresenting a thick and thin filaments in a stretched muscle andbelow that the arrangement of the filaments in a contractedmuscle. What you can see is that the length of the A-band is thesame. The width of the A-band which corresponds to thelength of the thick filaments, that doesn't changewhether the muscle gets shorter or longer. But the distancebetween the ends of the A-bands which really correspondsto the width of the I-bands, the light area, that getssmaller when the muscle contracts. And thedistances between Z-lines also gets smaller. Here's oneZ-line here, and here's another Z-line here and you can see thedistance between these 2 points is smaller in the contractingmuscle than it is in the relaxed or stretched muscle. And thiskind of information, these observations about changes in thewidth of the A-band or the absence of change in the A-band andthe decreased width of the I-band and the decreased distancesbetween the Z-lines lead to the understanding of how musclecontracts. What goes on in a muscle fiber when it shortens. Andvery extensive and detailed, and incredibly high-tech molecularbiological and biophysical measurements have illustrated themechanism of this contraction of the individual muscle fibers.
SLIDING FILAMENTTHEORY:
It's an extremely well established mechanismand to quickly summarize what happens in the contraction ofa single muscle fiber, if my left arm here represents athin filament and my right arm represents a thick filament and myhand represents the myosin heads, basically what happens is themyosin head forms a covalent bond to the actin and then themyosin head rotates and that rotating movement pulls the thickfilament and the thin filament past one another and the myosinhead releases and returns to it's original orientation and thenit attaches again, a little bit furtherdown the filamentous actin, and it repeats again. You can picturethis as an 8-man boat rowing down the river. Because you havehundreds of these little myosin heads sticking out and theyattach and they pull and they detach, and they attach and thepull, etc. Each one of these is a single cycle, the myosinhead is sometimes referred to as a cross-bridge becauseunder some conditions, if a muscle is fixed the thin filamentsand the thick filaments are attached to one another by the myosinheads so the myosin head is referred to as a cross-bridge when itis attached to the actin and this sequence of 4-steps iscalled a cross-bridge cycle. The sequence ofevents that is repeated over and over again, hundreds of times ina single twitch by a muscle fiber.
And the cross-bridge cycle has 4 steps that arediagrammed in your illustrated notes and they are represented bya chemical reaction right below the diagram.
We start in the upper right hand corner, that'sthe resting condition, that's what the chemical condition and thephysical condition of the thick filament and thin filament arewhen the muscle is relaxed and it's not active. In thatcondition, the actin represented by the letter A is separatedfrom the myosin, which is represented by the letter M, you'llnotice that the M has a little asterisk next to it and thatsymbolizes the fact that this is what's called a high energymyosin. It has obtained some energy that it's going to usein the movement phase and so it has a star on it. It still hasADP inorganic phosphate associated with it, that's what thelittle dots represent is that the ADP and PI are now physicallyattached to the myosin. But notice that ADP, not ATP, it hasalready given up it's energy and that's why the myosin head is inthe high energy state. So that's the starting condition, thefirst step in the cross-bridge cycle is the:
l) Binding step: Whathappens is the myosin head forms a covalent bond to anactin. And so if you compare the picture in the upperleft hand corner with the picture with the right hand corner, inthe upper right hand corner the myosin head is in contact withthe actin. Whereas in the left hand corner, it is not. And if youlook at the chemical statement immediately below that you haveA*M*ADP*Pi, in other words those little * represents bonds. Andthe change is that a covalent bond has been formed between theactin and myosin. That's the first step, the binding step.
2) Movement step: Where thehead of the myosin rotates a little distance. And thediagram in the bottom of the right hand corner compared with thediagram in the upper right hand corner, you can see the myosinhead has now been rotated a small distance. At the sametime that happens, 2 other things happen, one is that ADP andInorganic Phosphate are separated from the myosin head and theother thing if you look at the M in thebottom right hand corner you'll notice that it does not have it'slittle asterisk on it anymore, it is no longer a high energymyosin, it's no longer a high myosin because it had usedit's energy to produce that movement, that's the placewhere the energy is actually used, is in the movement and thatmakes sense.
3) Releasing step: When themyosin head releases the actin. You can see that if youcompare the separation between the myosin head and the actin inthe bottom left hand corner with the attachment that exists inthe bottom right hand corner. So that's the releasing step. Inorder for the myosin head to release the actin, it has to atthe same time, it has to bind an ATP molecule. So it'snot shown in the diagram but in the chemical statement you'll seethat now there is an M*ATP, the ATP molecule has come in and ithas attached to this ATPase site on a myosin head and that hasallowed the myosin to separate from the actin. And that's thethird step.
4) Priming step: This islike cocking the trigger of a gun. What happens is theenergy in the ATP gets transferred to the myosin head.The ATP separates into ADP and inorganic phosphate, that thirdgroup, high energy phosphate bond transfers it's energy to themyosin head. So the M in the bottom left hand corner does nothave an asterisk and in the upper left hand corner it does havean asterisk.
So those are the 4 steps in a singlecross-bridge cycle. And those 4 steps will be repeatedhundreds of times in a single twitch as the thin andthick filaments slide past one another. Now that's a very smallamount of movement. You can imagine, we're talking about myosinhead, we're talking about molecules that are just rotatingthrough a tiny angle so obviously the total movement produced byone single cross-bridge cycle is going to be something in thelevel of a molecules, couple of atoms of distance. But theimportant thing to recognize is that because a single musclefiber is composed of thousands of these units of organization,the thick and thin filaments, the alternating light and darkbands, that tiny molecular movement is multiplied by the numberof those sarcomeres, as they are called, the units oforganization and so you end up with an amount of movement that isa macroscopic amount of movement, in other words if you thinkabout, well here's the end of my bicep muscles right here, if Ipull that biceps muscle back I can measure a distance of the endof my biceps muscle of an inch or so and that inch of movement,that inch of shortening of those muscle fibers, is the result ofthousands of little molecular movements by these little headstwitching past one another and causing the thick and thinfilaments to slide past one another.
If you make a cross-section through here, whatyou see is a single thick filament, actually has heads stickingout in 6 directions like this and arranged around that will be anarray of 6 thin filaments and then over here is another thickfilament with 6 thin filaments arranged around it and over hereis another thick one so you get this really complex 3-dimensionalarray of thick and thin filaments and all of these thickfilaments are grabbing onto the thin filaments around them andthese things are then sliding past one another in order togenerate the force necessary for shortening.
Now muscle is a highly adaptable tissue in thatit can respond to exercises, if you increase the level ofexercise, you're going to get more muscle built up in some casesbut muscle fibers are also, there are 3 basically differenttypes of muscle fibers that are found in the bodies ofvertebrate animals.
l) Type I fibers - theyare relatively slow-twitching fibers. Inthat the speed with which they can do that shortening is on thelow end of the range of values. But these fibers are ones that canobtain all of the ATP that they need from oxidative metabolism.So they are called slow-oxidative fibers.
2) Type II fibers - theyare the fast fibers. They are fast-oxidative-glycolytic fibers and they are fast-glycolyticfibers. Now glycolysis, is just the break downof muscle glycogen and that can happen when there isn't enoughoxygen around for the mitochondria to work and it results in whatwe call anaerobic metabolism or results in the production oflactic acid. So if you go for a sprint up ahill, really fast, you're going to probablybegin to experience some discomfort in your musclestowards the top of the hill, that discomfort isdue to the accumulation of lactic acid produced by thesefast-glycolytic fibers. If on the otherhand you're doing a lot ofendurance running, your muscle fibers that are thefast-oxidative-glycolytic fibers, are the ones that can produceATP without generating lactic acid, those muscle fiberswill be the ones that will become trained and increase in theircross-sectional area and they will be ones that will beresponsible for the movement of your muscles in sprinting. So ifyou're engaged in some kind of athletic activity which involvedaerobic activity, you need to do aerobic training. If you areinvolved in sprinting, you need to do sprint activity cause these2 different kinds of fibers will respond differently and they areused differently by the nervous system.
And another thing that's interesting aboutthose is that the glycolytic fibers store glycogen as theirmuscle energy store. That's the molecule that's going to be usedby the glycolytic fibers. The slow-oxidative and thefast-oxidative fibers are going to use fat as their primarysubstrate for the production of ATP and so the glycolytic fibersare going to store glycogen and the oxidative fibers are going tostore fat. And so when you had your turkey dinner, the breastmeat of turkeys or chickens, white meat, that tends to be drier,it has less fat in it and that's why it's kind of a drier meat,well that's composed almost exclusively of fast-glycolyticfibers. It does not store fat because it's not going to use fatfor it's production of ATP. Whereas the leg meat or the lean meatwhich is dark meat, is going to be much moister and juicierbecause it's going to be composed of the oxidative fibers thatare storing lipid as their metabolic fuel and so a muscle thatcan carry out prolonged aerobic activity, endurance kind ofmuscle is going to be composed of these red fibers that havelipid stored in them, muscles that are sprinting type of musclesthat have the capacity, have a very high power outlet for onlyshort periods of time are going to be the white muscles and theyare going to be drier because they are storing muscle glycogens,their primary fuel resource.
QUESTION: What causes muscle spasms? Wellcramps are kind of an uncontrolled contraction of the muscle andit may be triggered by irritation in the nerves. If the nervethat controls the muscle, fire up that will cause the muscle togo into a spasm. Or irritation in the muscle fibers themselvescan cause that as well. Fatigue is another phenomenon that's beenstudied an awful lot by exercise physiologists and is one wherethey don't really have a complete understanding of what causesfatigue. They know that if you extract a muscle with it's nerve,and you stimulate the nerve, you can cause the muscle tocontract, stimulate the nerve repeatedly long enough, eventually,the muscle will stop contracting and so that looks like you'veproduced fatigue in the muscle but if you come over and youstimulate the muscle directly it can still contract. That willsuggest that fatigue exists either in the nerve or in the neuralmuscular junction but then they can also demonstrate that fatiguedevelops in the muscle itself. You can stimulate that muscledirectly to the point in which it no longer responds and thenyou've established that there is fatigue in the muscle itself andthere's a lot of research going on right now trying to understandat what level within the muscle fibers is that fatiguedeveloping.
One of the interesting consequences of thesliding filament mechanism is that the force that a muscle cangenerate is the function of the length of the muscle. In otherwords, if you take a muscle out of an animal and you hook themuscle up to device, hook it up to a piece of equipment that canmeasure the force generated by that muscle and then you canchange the length of the muscle by just moving these mechanicaldevices further apart, you will develop a curve that relates theforce generated by the muscle to the length of the muscle whenit's being stimulated. And this curve looks something like that,in other words there is a length at which that muscle cangenerate a maximum amount of force and if you try to make thatmuscle contract from a much shorter length or a very much longerlength then it generates much less force. At very great lengthsout here where the muscle is stretched out very, very long, thereason that it can't generate any force if you get it out therelong enough or generate progressively less force the longer itgets, the reason is that there is no longer any overlap betweenthe thick and thin filaments. You've stretched this muscle outfar enough, if this is a thick filament here, there's no overlapbetween the thick and the thin filaments so the little myosinheads are sitting here wiggling out in space trying to grab holdof an actin but it can't get a hold of anything, if they can'tget a hold of anything then they can't generate any force.
In here where you're getting maximum forcegeneration then you have sort of an optimum amount of overlapbetween the thin and thick filaments. So you have all theselittle myosin heads are able to grab hold of some actin and theycan generate a maximum amount of force and then down here atagain very short lengths where the muscle can't generate anyforce, what's happened is the Z-lines have banged up against theends of the thick filaments and there's extensive overlap betweenthe actin and myosin, between the thick and thin filaments, butthe physical structural protein of the Z-line, they can't get anycloser together because they are physically banging up againstthe ends of the thick filaments and they are actually kind ofwrinkled up here at the ends, they are slightly compressed on theends. So there is this force length relationship for muscle andthat has an important consequences as to how the muscles andjoints and skeletons are designed. In other words, if you havedone weight lifting, curls, you know that if you start off with abar like this with your arms straight, then it's really difficultto get it moving and then it's pretty easy in the middle heresomewhere that's probably where your muscle has it's optimumlength for generating force and then getting it back up therethat last little couple of inches or so is hard again becauseyou're operating down there at the short length of force lengthcurb. That's a partial explanation for that phenomenon. The otherpart of it has to do with the mechanical advantage around thejoint itself. But the design of the system which is subject tonatural selection also has to take into effect this force lengthrelationship of the muscle that's a direct result of thearrangement of the thick and thin filaments.