Kin 4433 Exercise phys final

what are the activation signals to the stem cells? basically, how do satellite cells target the injured fibres?

\-ex in situation where muscle is damaged from exercise and needs to be repaired

* so the SC are activated upon receiving signals that damage has ocurred

\-activation signals: the injured fibre releases chemicals including **myofibre cystolic proteins(like CK), localized protein factors embedded in ECM, and paracrine factors from inflammatory cells**

\-the satellite cells then migrate to the exact site of damage, encountering increasing concentration of chemical signs of damage

what does it mean by the term “ localized protein factors embedded in ECM”?

\-protein factors that leak out of site of damaged stay embedded in ECM, so they are localized to the site of damage

\-the ECM is more fluid like, but has lots of components, so things move around only a bit

\-this basically means that leakage is more gradual and stays close to the site of damage

\-these signals then draw inflammatory cells to the area (around in blood?)

* **these inflammatory cells move to area and then released paracrine factors that influence satellite cells**

where do stem cells come from when tehy go to injured muscle?

\-they can come from the same fibre that damage is, or they can come from another fibre nearby

what is chemotaxis?

a fundamental biological process in which a cell migrates following the direction of a spatial cue

\-in this case, this is when the chemical signals bring in satellite cells to go to the injured area on the fibre

how many satellite go to an area of damage?

\-after the signals, the satellite cells migrate over to exact area of damage

\-depending on amount of damage, you can have numerous satellite cells (ex in very hevay lifting or marathon)

\-when damage is more micro scale, there is lower satellite cell response

\-so response basically depends on level of damage

what happens in satellite cell response after they arrive at area of damage?

\-the inflammatory signals etc that attract the satellite cells to area of damage in **first place will also stimulate the proliferation of the satellite cell** so they divide several times

* this is important because we only have a limited number of these

what are the two pathways a satellite cell can take once it is at the damage fibre and

\-so this is after satellite cell has proliferated and in greater number


1. the satellite cell fuse and form new immature myotubes
2. or they fuse to existing fibres

\-the fibres begin to differentiate and express contractile protein to become more myofibre-like

\-after a few days, they are functional muscle fibres

explain process of satellite cells in making fibres bigger(hypertrophy)? what is the significance of the placement of nuclei?

\-satellite cells just fuse to injured fibre (or they make a new one themselves?) and then donate nuclei to injuredfibre

\-the nuclei, when in new fibre, goes right to the centre of the fibre

\-a nuclei requires a certain amount of space within a cell, so when you add a nucleus in a cell, the cell gets bigger to accomodate

* ex more protein synthesis for more contractile units to be placed = muscle gets stronger

**-this is bc the nuclei maintains the cell and direct more protein , collagen, mito to be made**

how can we tell if a fibre is newly developped from a satellite cell?

\-when satellite cell fuses to injured fibre or to other SC, it moves the nuclei into the centre of the fibre

\-so this is how we can tell if nuclei has come from satellite cell (bc other nuclei will be under the sarcolemma)

\-after damage is fixed, then these central nuclei will go t regular position by sarcolemma,

\-so position is a good marker of where we are in the recovery process

recap: changes to fibre type w certain training

\-with resistance training, both BB and PL processes stimulate growth of muscle fibres (Type 2, either 2a or 2x

\-with aeribic training, there is enlargment of type 1 (or transition type 1%

\-basically bc damage will be in those areas more with that type of training, so that is what is repaired and gets bigger

explain second way that satellite cells are used in muscle: making their own new fibres

\-aka hyperplasia

\-this is when, instead of binding to new fibres, they bind to injured fibres instead and start to make their own fibre

\-they do this because after exercise not all fibres are salvageable, so would just need to make a new one

\-we have a full recovery of these muscle fibres that are rejuvinated within a few days, so pretty fast process

how do drugs affect the satellite cell response?

\-certain drugs suppress the inflammatory response, so slow down the SC cycle

\-common NSAIDS like ibuprofen or aspirin slow down satellite cell activity and delay msucle regeneration in healthy, young people

\-so maybe not ideal ro take NSAIDs after hard/sore workout bc max recovery could be suppressed

explain graph of satellite cell response post exercise

\-this is looking at satellite cell repsonse to intense, damaging, eccentric exercise

\-**at 72 hours,** the % of satellite cells in muscle has increases the most, bc they have started to replicated an rejuvinate the muscle

* this coincides w the peak in soreness and when CK is going up (signal of damage)

how does soreness response change after training?

\-so look at response of last cue cards, then at 4 weeks past the damaging exercise, they do this exercise again and look at damage to muscle (white dots)

\-they found soreness is down significantly, and there is lack od CK released

\-shows physiologucally there is less damage

* clearly the muscle has adapted to avoid as much damage, even after 1 bout of exercise!

does fibre splitting happen?

\-we know that satellite cells can proliferate, fuse, and bind together to make a myocyte

\-could these same satellite cells go over to damaged fibre and just make a branch off the muscle

\-so causes kind of like a “split” in fibre, which is just a branched myotube

\-this is actually how we get the intercalated discs in cardiac muscle!

\-so basically is a branch of a new muscle fibre off an existing fibre

according to fibre splitting study, what are the 4 mechanisms of regeneration?

\-this study showed that hyperplasia can take place


1. lost fibres replaced by clusters of myotubes formed by satellite cells (looked like longitudinally split fibres)
2. viable fibre fragments fused w satellite cells
3. satellite cells of surviving fibres proliferated and fused to myotubes localized beneath the basal lamina
4. thin new fibres occured in the interstitium, this is the “splitting or branching”?

recap: the two ways that stem cell can help repair damage/ make muscle stronger

\-either SC will create a brand new fibre (hyperplasia and fibre “branching”), OR it can bind to injured fibre and work to repair that fibre (hypertrophy)

\-exercise elicits this microdamage and we see repair with stem cells, and this is what makes the muscle bigger

\-so both hyperplasia and hypertrophy exist to an extent

how is muscle fibre size related to the # of nuclei in that fibre?

\-Typically, larger muscle fibres are observed to have more myonuclei

\-satellite cells fuse to damaged fibres to provide their nuclei and permanently become a part of that fibre

myonuclei and concept of “space management”

\-think of myonuclei having a certain space around it that it manages, or a volume of sarcoplasm

* ex **it will continuously replace collagen fibres, contractile units, mito turnover** ; **basically just the overall maintenance of the cell in a specific area by a specific nuclei**
* basically each myonucleus responds to signals to change expression of genes (ex. provide mRNA to become translated)
* so if you add another nucleus, you enhance the size of the cell because now more nuclei are available to take care of that space

What are the suspected rules for using stem cells to change fibre type?

\-say you have injured fibre that sends signal for SC, does the stem cell come from the same fibre type? or different? or does it actuallt matter?

\-when building a new fibre type, are they all going to be the same SC that join together to make a similar type fiber?

\-does injured fibre send a particular signal to a certain type of SC?

\-then what happens in hypertrophy? if SC from another type binds to injured fibre? does it express the same type or different to the original fibre?

explain study looking at denervation on overloaded muscles studied in vivo: first part that doesnt have to do with denervation

\-this was in rats, so overloaded the muscle more than would typically be exposed to, and this constant strain caused a bigger muscle in 14 days

\-then they stained the nuclei, you can visually see that there are more nuclei in a single muscle fibre and the fibre itself is also bigger

* so nuclei and CSA of fibre increased according to graph

denervation and overloaded muscle rat study: explain the denervation portion of the study

\-so first showed with overload muscle that there was adaptation, so increase the CSA of a muscle fibre and the number of nuclei per mm fibre

\-then wanted to see if you remove neural input to the muscle, does the trained muscle lose its size? (bc normal denerving will cause muscle atrophy)

\-it does! when theu denerved over 14 days, the muscle got smaller (lost CSA)

\-despite this atrophy and loss of CSa below the start of training, the nuclei that are inside the fibre are still there and dont really change

* basically shows that the # of nuclei doesnt drop after increase in training

explain the basic model for the connection between muscle size and number of myonuclei with training and detraining

\-expanding on past study on nuclei # after denervation, the same thing basically happens with detraining: there is atrophy of muscle, but keep the nuclei

* this basically means if you start training again, you already have more nuclei, so gain back more quickly the strength/size of the fibre
* basically “muscle memory” that primes you to do this
* dont really know how long this priming lasts… mayb months?

explain study setup and results for the role of nerves in building and maintaining muscle

\-**if you change the stimulatory pattern of nerve to muscle, this could change the fibre type**

\-in study in rodents, had machine w low frequency of stimulation of muscle (1hz), so trying to mimick contractile frequency of Type 1 (doesnt fatigue)

\-results(in plantaris muscle):

* we see that fibre types change quite dramatically through this fibre stimulation
* type 2a changes rapidly and increases (5% to about 80% of muscle)
* we see quick drop in type 2b
* type 1 also increasing a bit here

\-this is basically just showing that with chronic stimulation, there is some sort of fibre type transition, and if you provide the right stimulation, fibre type CAN switch

what is the effect of radiation and how do we use it to look at muscle studies

\-Radiation kills off newly formed cells, so it impacts more immature cells

\-this is how it stops the progress of cancer

\-in muscle studies, you would look at stopping the effects of satellite cells

\-removing SC will let us look at their effect!

explain setup of satellite cell and radiation study (CONFUSED ON THIS?)

\-setup:

* also used low frequency stimulation to train, so stimulated 1 leg, looked at fibre type % in that leg vs unstimulated
* did this another group, but went under radiation to kill off newly formed cells and satellite cells
* then did counting of fibres

\-results:

* with stimulation (no radiation) there was an increase in type 1 fibres and type 2a fibres
* when there was stimulation WITH RADIATION this change was not seen because the radiation effected the function of the satellite cells (more 2x?)

what biochem methods do we do to look at individual fibres on western blot?

\-can buy enzymes that break up collagen and connective tissue, so this makes it easier to seperate individual muscle cell from muscle belly

\-then we can analyze the individual fibre cells and do western blot to see what the level of myosin MCH in each fibre type

explain western blot study showing there is some hybrid fibres

\-Aside: this study did low frequenct chornic stimulation to try to change fibres to type 1?

**-did western blot on individual fibres that were labelled a specific type to see if there were any other type MCHs in there**

\-they found that there are some fibres with more than one MCH type = hybrid fibres

\-ex in type 2b, there is some overbanding in between the control band

\-so shows individual fibre has both types of MCH

explain how they figured out the type 1/2a or “C” hybrid fibre

\-took an individual fibre and through new staining, we can see that along the fibre, it changes fibre type ( in soleus)

\-type 2a was labeled with green antibody and type 1 was labelled w red antibody

\-found that portions of fibre were yellow, which means co-expression of two isoforms

\-so shows that the singel fibre is a type 2a on the left, but gradually changes to 1/2a hybrid fibre towards the right

How does fibre type change on the continuum of a singel fibre?

\-so some fibres are only one type throughout the whole continuum

\-but t**hen some fibres change their type halfway through, so go to type 1 fully to type 2a, but in between this there is “transition” where both MCH isoforms are expressed**

\-so basically evidence that fibre types are different along the fibre and need to consider this when doing a cross section

\-shows that hybrid fibres exist and its NOT just a mix of dif mysoin ATPases through whole muscle, but rather they have sections that are certain fibre types

\-dont know if this is result of training or if we are born with them?

what species haas the highest amount of hybrid fibres? why does this matter?

a study/review looked at # hybrid fibres throughout all the reports done in dif species and their genetic predisposition that makes the % of hybrid fibres in a muscle

\-rats have highest recorded at 39.5% compared to mouse, rabbit, dog, and human (24%)

\-what does this mean? basically just that we do a lot of muscle studies on rats, so harder to apply results from rats to humans bc they are key genetic differences here

explain study that looked at hybrid fibres in training with identical twins

\-found 8 pairs of identical twins that had different exercise habits, so one was untrained (UT) and other than trained (TT)

\-type 1 was much higher in TT over UT and type 2a higher in UT (?)

\-the total # hybrids was much higher in the untrained group

* hybrids are 1/2a and 1/2a/2b

what does the twin study tell us about hybrid fibres and training?

\-so UT group had many more hybrid fibres

\-this suggest that changes in fibre type are occuring through the use of these hybrids

* ex hybrid fibres start to transition to type 1 in training

\-so if hybrid has regions of fast and slow, t**he % of one fibre thats a specific type may just increase as the fibre is repaired**

\-**so hybrids are kind of in reserve for transitions to occur**

how is exercise intensity and muscle damage linked to the inflammatory response

\-the type of exercise intensity (load, speed, and contraction) will influence the amount of muscle damage

* ex heavy, fast, eccentric gives more damage than light, slow, concentric

\-exercise modes that induce greater damage also induce an inflammatory response to aid in muscle regeneration

\-so amount of damage determines the inflammatory response

what is the process of severe muscle injury?

\-severe muscle damage can result from excessive exercise intensity, physical trauma (impact or tearing), surgery…

\-within a few weeks, muscle will return to (mostly) normal

\-if we look at muscle images taken form days to weeks, we see intact fibres, then pretty intense damage with white (neutrophils), and then recovery with many more nuclei (first in centre and then move to periphery)

\- the overall repair process involves tissue degeneration and regeneration

are immature cells in repair process still functional?

\-this is referring to when satellite cell is still repairing muscle and has central nuclei

\-immature cells are not fully functional in terms of max strength, but they are still beneficial

what are myoblasts?

\-remember that muscle damage must be fixed to allow continued contractile function

\-the satellite cells become activated (now termed myoblasts), they proliferate, find regiond of damage, fuse to either damaged fibres or each other, then differentiate to become more muscle fibre-like

\-**so myoblasts are basically these immature muscle cells that have the central nucleus**

\-myoblast fusion provides opportunity to produce branched (split) fibres

what is this a picture of? identify the different components

\-this is skeletal muscle 5 days post very excessive eccentric heavy load, so more damage than usual

\-we see the degenerating cells that are kind of shrinking (dont have a central nuclei), and you can see inflammatory cells around them

\-all the small cells are immune cells

* ex macrophages engulf and eat up tissue
* neutrophils are white cells that mark the area and prep for macrophage to come over

\-also can see some new cells ebing built, and we can tell bc the nuclei are centrally located

what is the timeline of degeneration and regeneration stages in muscle repair

\-the degeneration begins after damage is suatained, so inflammatory response coming and establish area that needs to be regenerated

* is highest at beginning, then winds down

\-the regenerative phase is low at first, but then ramps up as degenerative phase winds down

\-we know that **inflammatory cells are present within a day after exercise damage and can last up to 10 days**

\-**generation of new myofibres and patching damaged fibres take place from 3-10 days**

what is the general role of inflammatory cells

\-monocytes/macrophages and neutrophils arrive at site of injury proliferate, and get rid of the damaged, unusable material via phagocytosis

* remember its the fragments of cells, organelles and proteins that signal this, and this is what is “gobbled up”, alsong with damaged fibres that are no salvageable, so make room to build new fibre

\-they may overcompensate and make a bit of extra damage by degenerating healthy tissue too

what happens to collagen in the degenerative phase of muscle repair

\-at 5 days post injury, the macrophages are swarming around badly damaged muscle fibres

\-but the collagen cytoskeleton is relatively intact even in severe damage

* this also includes **the endomysium and the basement membrane**, so basically acts as scaffold for new fibres to generate within and in between

what is the general role of neutrophils and when do their levels peak in the time course from degeneration to regeneration?

\-within a day post damage, neutrophils locate to the area

\-these are basically tiny cells from blood that kind of mark as site for inflammation

* they tend to overgeneralize the area of damage, so cause bigger damage than what is actually there

\-this signals the macrophages to then come in

\-so neutrophils peak at beginning, and macrophage then slowly rises and stays elevated

what are the 2 pools of macrophages that will help repair muscle damage?

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1\.monocytes are immature macrophages that are roaming around in blood and locate to damaged area (marked by neutrophils)

\-then the monocyte matures and differentiates into macrophage

* before this, could have proliferation of monocytes so that many differentiate to macrophages


2. quiescent macrophages in the epimysium and perimysium could become activated as well

\-so basically either locating to the damage or already existing in that muscle

what signals stimulate recruitment of macrophages?

\-CK release, broken bits of cells/proteins, paracrine factors all stimulate the recruitment of macrophages

\-ex cytokines starts signalling SC to start proliferating, but also for macrophages to strat replicating and coming to area

are satellite cells also signalled by macrophages?

\-macrophages undergo changes as the process of cleaning up damage continues

\-they release paracrine factors that stimulate satellite cells/mypblasts

\-so basically the macrophage provides **cues that guide activation, proliferation, and differentiation of satellite cells**

Step 1 in repair process: turn on macrophages

\-kind of recap: macrophages are attracted to damaged area in a number of ways:

* cellular debri and cytosolic proteins escaped from damaged fibres
* damaged fibres accelerate the expression and secretion of a host of cytokines too (myokines when in muscle)

\-these act as cues for inflammatory cells to migrate to site of damage, proliferate, and consume debri

what are cytokines?

\-**these are basically small proteins released from cells to provide paracrine and/or autocrine signalling**

\-cytokines can be used in other tissues other than muscle

\-so with training, its these cytokines that are released from muscle that go to other tissues (like bone) and trigger responses in other areas as well that leads to healthy changes

* so important in overall health benefits of exercise

\-**will be released by muscle to attract macrophages, but ALSO released by macrophages to attract SC**

how macrophage response a bit different when trained?

\-remember immature monocyte can come from blood and mature into macrophage, or macrophage in tissue will come over to damaged area

\-with training, you could leave some mature macrophages circulating in blood so they are ready to ho over to damage quickly when it happens again

\-so repair of muscle and use of satellite cells becomes efficient in training

Step 2 in repair process: gobble of broken muscle bits and turn on satellite cells

\-so macrophages at the damaged area will proliferate and then eat up/clean up the area and debri

* so they take up the contractile fibres, but the cytoskeleton stays intact, so provides new mold for new fibres to be built in

\-**while cleaning up, the macrophages express other cytokines received by satellite cells, driving their activation and proliferation (now the SC are termed myoblasts**)

what are the M1 macrophages

\-these are the first macrophages that are recruited to the damage and proliferate and cleanup

\-they release the cytokines that cause SC activation and proliferation

Step 3 in repair process: feel the DOMS (M2)

\-so M1 are in degredation and signal the satellite cells that go to either repair or build new fibres

* phagocytic cleanup and stimulate SC activation and proliferation

\-**the second set of macrophages (M2) determine the differentiation of the muscle cell (induce satellite cell fusion and differentiation**

* so basically matures the myoblast into a muscle fibre and turns these new or repaired fibres into functional units

\-so we feel the DOMS when M1 is peaked and there is a lot of muscle damage

\-make sure you know this timeline!

explain the repair timelime more in detail, and transition from degeneration to regeneration

\-in the degeneration phase, **there is inflammatory response with peak in neutrophils, M1 and peak in damage (usually within 1-2 days)**

\-then in regeneration phase, there is the M2 macrophage that raises in levels (**within 2-4 days)**

\-and between these two, we have the activation overlap of the satellite cells when they start building new muscle

what are the levels of M1 and M2 in inflammatory diseases? what role does exercise have?

\-Some inflammatory diseases (ex heart disease) is when this type of coordinated response but its chronic

\-so on a daily basis M1 and M2 and in general less organized

\-but exercise does a good job at cooridnating this inflammatory response in a poistive way

\-basically makes it so that its not uncoupled to when it is needed, so no longer as chronic

\-maybe dont want to do exercise with huge damage though bc dont wan;t to increase the max amount/threshold of inflammatory response

how is the inflammatory response graded to the damage

\-the inflammatory response is graded to match the degree of damage of sustained (p**eaks are higher of all the inflammatory cells**)

\-length of time for each component of the response is longer, **so the peaks occur later => longer time course of responses**

\-can figure out the amount of inflammatory response by recording the amount of DOMS

recap: general function of neutrophils

\-we know that is floods the system or damaged area when there is injury

* it is continuous, so part of the initial degeneration phase and kind of fades away

\-they are inflammatory cells, so a marker of damage and kind of set up the paramter of where macrophage will come and clean up the damage

\-essentually, improving the location abilities of the neutrophil

are neutrophils good or bad? explain mice study and results

\-did study on mice missing the attachment protein that allows neutrophils to bind to, so now cant binds to tissue to show where the barrier of damage is

* so inflammatory response still occurs with damage, but no neutrophils come in

\-instead, the macrophage kind of eat up damaged tissue blindly without the governance of the area of damage

\-study did a fast rate eccentric contraction to damage and then **looked at force deficit and % injured fibres after damage**

explain neutrophil mice study results

\-the normal found had significantly more injured fibres than those w knockout ( lack of neutrophils) at 3d post injury

\-at 3d post injury, the normal/wild type also had significantly more deficit in contractile ability - then with time there is less and less deficit

\-reason: there is less marking of tissue without the neutrophils, but M1 is capacle enough to go into the area and identify damaged tissue, so dont technically need it

* so neutrophils kind of mark a bigger area of damage to make sure they emcompass everything, but this leads to more degeneration and more injured fibres/force deficit

in what situations would neutrophils be beneficial?

\-may be beneficial in situations like severe injury/surgery or something where there is a big area of damage to mark, but potentially marks off healthy tissue to make sure its getting all damaged fibres

\-ex in traumatic injury (like a cut or something), it doesnt matter on timr frame as much, so there is overexxageration to emcompass all damaged tissue => better to be safe and take a bit longer

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why would neutrophils not be as beneficial w damage due to exercise

\-without neutrophils, there is less overall damage because you are only repairing damaged tissue marked by macrophages

\-with exercise, the time frame to recover matters more

* so maybe not as efficient to have overexxageration of degeneration before the repair
* so macrophage identifying its own specific damage is better?

\-so macrophage response is essential, but neutrophil response seems to be dispensable

in healthy muscle H&E stain, why are there sometimes cracks?

\-when we do the stain, muscle gets dehydrated so shrinks a little bit, so there are some small cracks in between muscle fibres

\-but in in vivo muscle, there are NO spaces in between

what are myoblasts/myotubes? what do they need to do when they are first formed?

\-myoblasts/myotubes are the immature muscle cells - when they are formed, they need to build their internal cytoskeletal structure aka the actin myosin organization (new myofibrils)

* 1 myosin and 6 actin coordinated around it

\-also a lot of other proteins like creatine kinase, SR (bigger in type 2), mito in SS and IMF regions etc

\-as myofibrils are built and exanded, contractile proteins must be expressed and put in place => question is: what MHC gene is used?

explain differences in muscle at 10, 21, and 35 days post injury on an H&E stain

\-NOTE: this was severe damage, so lighter damage could be completely repaired by 35 days

\-10 days pots injury: internal structure of cell is growing, and there is still central nuclei present and in charge of “maintaining” that part of the cell

\-21 days post injury: now can see very clearly the different muscle cells, so stain is no longer as disorganized and fuzzy

* agents in cells that finely tune those structures to get more cleanness and organization
* the fibres are full sized and likely back to the original strength

\-35 days post injury: even more organized, still some central nuclei because fibres are still growing, now bigger than original size

\-when fully recovered: the myonuclei eventually work their way out to fibre periphery, so cell is in state of maintenance instead of building

what is something to consider with muscle damage studies and their applicability to damage in exercise

\-a lot of studies look at recovery of muscle after very severe trauma, so this is exaggerated response and takes a long time to recover

\-also want to see the kind of recovery with just micro damage from lighter exercise

how does having diabetes damage muscle?

\-diabetes (in mice in this case) does affect skeletal muscle (type 1)

\-we normally see myopathy, so loss in muscle strength

\-there is a lack of ability to repair muscle, so continue to break down when there is damage and they never get back to that normal capacity

\-kind of same in diabetes as you see in aging?

explain study looking at fibre hypertrophy during recovery in normal and diabetic mice

\-elicited severe damage to mice and looked at fibre area

\-at 5 days post injury, there are much smaller fibre, so already degenerated and starting to rejuvinate and repair

\-at 10 days, start to get shape of fibre back and lay down cytoskeleton, so there is jump is size (but still lower than before damage)

\-at 21 days, size of muscle fibres in normal rats is bigger than fibre pre injury

* but the size in diabetic anumals is juts approaching their original size

\-at 35 days, you have bigger fibre than you had before!

* in diabetic, it takes wayyy longer to get to that pre exercise size and they dont really surpass the size either

compare light damage to severe damage: soreness and inflammatory response

\-soreness

* light damage: none - minor, so under 1 day
* severe damage: major, so 1-several days/weeks

\-inflammatory response

* light damage: minimal
* severe damage: major, required to clean up debri, “dead” fibres and stimulate greater myogenic response

compare light damage to severe damage: satellite cell response

\-light damage: minimal, there is probably some patching of damaged fibres

\-severe damage: major: large proliferative response, fuse to each other and damaged fibres

compare light damage to severe damage: time course of response

\-light damage: short, so functionally restored within a few days, the cellular activity continues for 2-10 days

\-severe damage: long, functionally restored in 5-10 days(so use muscle seriously again) or longer, cellular activity continues for weeks

compare light damage to severe damage: end result

\-light damage: patched fibres with more myonuclei, slightly larger

\-severe damage: some patched fibres, mainly fibres are newly made

what new MHC has been discovered that gives us more info onto what MHC is expressed after muscle damage

\-they discovered a new embryonic Myosin heavy chain, its very undifferentiated type of MHc, so can turn to any type of MHC and determine the ATPase and therefore the fibre type

\-even less differentiated then an neonatal MHC

when is embryonic MHC expressed? show staining evidence that shows this

-at 5 days post injury, they stained for embryonic MHC, collagen, and nuclei

* there were a lot of newly formed cells that are not functional yet, and they all have central nuclei and express immature myosin heavy chain form, which is the embryonic MHC from the satellite cells => can still differentiate into every type of muscle fibre

\-at 10 days post injury, the fibres are still not full sized and have central nuclei, but but the embryonic MHC no longer stains, so the cells express mature MHC isoforms now (type 1, 2a, or 2x)

So we know that embryonic MHC can differentiate into whatever MHC in muscle repair. What determines the direction of this change?

\-there are signals that determine if fibres are turning type 1 or 2a or 2b and these signals come from what type of training you do

\-so basically the **type of training controls the enviro of the cells and determines the way the ATPases transition too**

\-wouldnt change the whole muscle, but shows that training will help determine the fibre type of the muscle to some degree

explain study in cats that looked at reestablishing fibre type

\-looking at muscle recovery from 3-6 days post damage

\-had intact innervation, so nerves were still attached to the damaged muscles, or new fibres are regaining innervation

\-at 3 days post, there is no myosin type 1, 2a, 2b, or 2x, so shows they are all embryonic MHCs at this point

\-at 4d, there are type 2 fibres that start to appear and type 1 still not apparent (so embryonic MHCs have only transitioned to this?)

\-at 5d, we see the fibres start to turn more type 1

\-at 6d, the majority of fibres are now type 1 and a bit type 2a

\-summary: at day 0, initiatiation of embryonic MHC, then these fibres transition ot type 2 then keep progressing to type 1

same cat study: explain results when there was denervation instead

\-so basically the nerve was NOT intact with the muscle while it was regenerating

\-we see similar response at beginning, where at first there is expression of embryonic MHC and then then transition to type 2, but they DONT further transition to type 1, so kind of stay at fast glycolytic type 2x/b

* basically means without innervation, it stays at type 2 composition

\-this is in soleus, so mostly type 1 anyway, so nerve makes it go to type 1 over the tendency to stay at type 2

so what do the alpha Motor neuron do in establishing fibre type?

\-as alpha MN reestablish new NMJs, the fibre types are determined based on alpha MN type, so small MN makes type 1 myosin expression

\-so when there is innervation, the nerve basically “overruns” the type 2 to make it transition to type 1

* innervation acts as the stimuli to convert it to type.1

concept of nerve determining the fibre: how would this be if the MN was a type 2?

\-if you have MN that was type 2, then embryonic MHC would transition to type 2 on its own and MN would make sure it stays as type 2, or maybe move more towards type 2b

what is the potential downside to MN influencing the fibre type of muscles in recovery?

\-this makes it a bit harder for muscles to change the type fully, bc you still have that innervation of nerves onto muscles that is a certain type

* so with a newly formed fibre, how its innervated will somewhat determine what fibre it ends up being

\-but we also know that training will push the firbe type to be in direction of past training

explain cross innervation experiment in cats: what are the implications of this study?

\-basically did a surgical switheroo in cats where the entire motor nerves were swapped between FDL(type 2) and soleus (type 1) muscles

\-basically showed that there was completely different twitch characteristics, so a slow type 1 muscle would now contract way faster => fibre type switching in each muscle with nothing to do with damage and regeneration

\-so its metabolic changes that will start changing the muscle to fit this new type of innervation

what is the role of fibroblasts in muscle repair?

\-fibroblasts are main cell type in connective tissue

\-they play an important role in muscle repair because they express lots of ECM components (mostly collagen) to form the new basal membrane (BM) and endomysium, and cytoskeleton in general

* remember how collagen network is important for forming these new fibres bs has holes that acts as new mold for new muscle cells

\-also helps guide motor neuron to newly formed fibres

In exercise, is ECM also stressed?

\-ECM will definitely still be stressed, bc needs to connect the contracting muscle to the bone to actually have an action

\-so connective tissue will be stretched, torn, and it needs to be reformed

expand on role of fibroblast and connective tissue on helping guide nerves

\-the endomysium and basement membrane is right around muscle fibre

\-this is where we are going to have nerves that are moving to the undifferetiated muscle

\-nerves move quite easily to reinnervate new structures, so will wiggle its way to any new cell to begin innervating it, so important for ECM to guide this new innervationfor muscle changes to occur

\-satellite cells and inflammatory cells also have to move through this region

explain balance in fibroblast and myoblast activity

\-there is usually an appropriate balance between myoblast proliferation/differentiation and fibroblast activity

\-this balance can be lost in certain disease states

what is muscle fibrosis? what happens in diabetes?

\-in T1 diabetes, muscle damage leads to an overabundance of collagen => then repeated damage/repair leads to muscle fibrosis (we already know they have inability to regenerate muscle to greater extend over its original state pre damage)

\-there is a hardening/calcifying of the ECM, so makes it harder for agents to come in to these newly formed muscles to change them to desired state

* so in recobery, there is enhancement of collagen depositiona nd fibres dont return to original state

\-then lots of fat starts to accumulate in this area too because too much ECM deposition allows for fat to develop where muscle fibres ought to be (more common in T1D) => also leads to signalling inability for insulin in muscle

* bc also they dont return to as big after damage, so emptier space is filled w fat

\-summary: in T1D there is more ECM, so makes it harder for fibres to regenerate, also makes nerves moving to innervate these fibres more difficult to navigate

What processes are necessary for gaining muscle mass?

\-to actually make larger fibres from training, we need to increase muscle protein synthesis

\-protein synthesis is important to allow newly formed muscle fibres from satellite cells to actually mature into functional fibres

\-we know that muscle mass increases largely as result of increases in protein synthesis, and these newly synthesized proteins are incorporated into existing intracellular structures to expand them (ex to fill up extra space when a satellite cell in in newly formed fibre)

what are the main factors that affect muscle mass

1. ability to repair damage


1. if have better ability to repair => more muscle mass
2. regulation of protein synthesis


1. obviously if you have more protein synthesis, then muscle mass can get bigger
3. regulation of gene transcription (mRNA)


1. there are many many levels of control on this transcription which then affect the amount of proteins made

\-note: there are many broad physiological and metabolic disturbances that impact muscle mass

what are the 4 factors that affect the 3 main factors to muscle mass?

\-recap: the three big ones are regulation of protein synthesis, ability to repair damage, and regulation of transcription mRNA


1. influence of alpha motor neuron (ex affects the type of muscle fibres type 1 or type 2 etc) in the ability to repair damage
2. homeones also influence the fibre types differently and hormones can augments protein synthesis or ability to repair after damage (ex steroids, even natural ones)
3. availability of amino acids will affect regulation of protein synthesis and ability to repair damage, bc need this to actually make protein
4. time under tension will determine the severity of damage, so then affects repair and protein synthesis and transcription, so muscle mass

what does it means when there is no net protein turnover?

\-there is usually a balance between rate of proteolysis (breakdown of proteins into amino acids) and the rate of protein synthesis (amino acid incorporation into new proteins)

\-no net means that there is a balance, so we make some protein and break some down at the same rate

\-to change this, we need a stimulus to alter this balance, ex add satellite cells to donate nuclei to stimulaye more protein synthesis

why do we breakdown proteins?

\-we tag proteins for breakdown because they don’t last forever in cells, so need to be broken down at some point and replaced with new ones

\-we make proteins over time to “age” them or basically make sure they are degraded at the right time

\-this is so that cell doesn’t lose a function bc you have only old proteins with loss in function -the amino acids that are generated when proteins are broken down will make new proteins, but ALSO important fuel source in exercise

explain how amino acids are used as fuel source in exercise?

\-our body takes amino group off the amino acids and we have carbon skeleton

\-so simple to turn this into something used for fuel, so joins into aerobic metabolism

\-this is potentially another reason why we want to break proteins down

how do we get a positive net protein turnover?

\-To increase protein in cell, we can make more protein (turn on genes to transcribe then translate that protein), and we can also stop the degradation of protein

\-exercise can potentially start to breakdown protein so we can use it as fuel if diet is insufficent and we dont have enough AA available energy

\-so combining exercise and food (adequate protein) intake can stimulate a positive protein turnover rate by increasing protein synthesis and limiting proteolysis

explain protein synthesis and breakdown patterns of 12 hour period. What happens when we start training

\-muscle protein synthesis = MPS and muscle protein breakdown = MPB

\-normally, it is cyclical with making proteins or not during a 12 hour cycle

\-ex when we eat protein at meal, it raises protein synthesis, then drops down again and MPB increases

\-when we training, we alter the system so more protein is made and less is broken down ( dotted lines)

recap: what is the central dogma of gene expression

\-first DNA is transcribed to RNA, and then RNA is translated to protein

\-each of these processes is under very fine control of multiple signaling pathways

explain the time course of protein synthesis and gene expression responses

\-first, there is an existing pool of RNA within the cell; this is called the basal RNA, so this pool is for immediate translation of proteins

* so initial incline in proteins in still reliant on that pool

\-then t**here is a peak in RNA at about 12 hours post exercise**, which then is translated into proteins so we see newly synthesized proteins rise after

\-this is a generalization, so some genes will have RNA peak early or late and protein shows up accordingly

how does exercise change the activation of genes?

\-the process of translation (protein synthesis) gets “turned on” relatively quickly following exercise

\-the transcription of appropriate genes supplies the appropriate muscle-specific mRNA to the now highly activated translation machinery (ribosomes)

what types of genes are activated by exercise?

\-the list of genes activated in skeletal muscle post-exercise is in the 1000s

\-for ex. MHC, actin, other structural components (myofibrillar components)

* if exercise is intense, prolonged endurance, more mito genes are transcribed and not as much myofibrillar gene transcription

\-essentially what training is in this case is the repeated increases in expression of numerous related genes; this mediated long term adaptive process

how does signalling pathways play into the time course for protein synthesis and gene expression responses

\-there needs to be signalling pathways involved for DNA to RNA, and then also for RNA to protein

\-immediately post exercise, there is a robust change in signalling activity in a muscle cell

\-ex protein kinase A activity, which is activated by the adrenergic system (regulated by hormones?) => might have missed some info about protein kinase A to check other peoples notes

gives recap on the events that have to take place to increase muscle mass after exercise

\-several events must occur in coordinated fashion to successfully accumulate protein content


1. turn on protein synthesis machinery: ribosomes


1. has own signalling pathway associated with activating them too
2. translate mRNA to protein (whatever mRNA is already present in fibre)
3. express more of the right genes: make mRNA for particular MHC isoforms, actin, etc
4. reduce/limit the rate of protein breakdown (proteolysis)

\-coordinating this is the job of intracellular signalling pathways, so basically what fine tunes the increase in protein level response

explain actual process of signalling pathways receiving signal

\-receptor reads stimulus, carries signal to transcription factor, then transcribes gene, then RNA is translated to protein!

\-much more signalling occurs in all of these steps, ex sometimes protein needs to move to another part of cell to be activated, or signalling moved to other part of cell to pass it on

what is the actual stimulus that causes the activation of the signalling pathways that then causes RNA and then protein changes

\-stimulus that leads to increase mRNA etc are from alternation in metabolism, so enviro of cell changes

* so not always hormone binding to a certain receptor, but rather change in enviro around a transcription factor and this leads to increase in activity

\-**also some mechanical damage on muscle will activate some sensors too**

increase in metabolite to act as stimulus for signalling pathway: AMP

\-when there is elevation of AMP, it means muscle is in lower energy state (bc ATP has been used) and this happens during exercise

\-basically a signal to all of these pathways to start going to increase protein synthesis

\-AMP kinase is a metabolic sensor that will in turn stimulate many processes in cell when AMP is detected in high levels

* so help cell better itself for next exercise, will stimulate transcription factors that lead to increased mito content, mito biogenesis etc