I cracked open my ‘Textbook of Medical Physiology’ by Guyton and Hall to see if they had any definitive answers to this discussion. They did.
Regarding the myofibrils (actin and myosin):
‘Virtually all muscle hypertrophy results from increase in the number of actin and myosin filaments in each muscle fiber, thus causing enlargement of the individual muscle fibers, which is called simply fiber hypertrophy.’
‘In turn, some of the myofibrils themselves have been observed to split within each muscle fiber to form new myofibrils, but how important this is in usual muscle hypertrophy is still unknown.’
Regarding hyperplasia:
‘Under rare conditions of extreme muscle force generation, the actual numbers of muscle fibers have been observed to increase, but by only a few percentage points, in addition to the fiber hypertrophy process. This increase in fiber numbers is called fiber hyperplasia. When it does occur, the mechanism is linear splitting of previously enlarged fibers.’
Regarding adding sarcomeres at the end of the muscle fiber:
‘Another type of hypertrophy occurs when muscles are stretched to greater-than-normal length. This causes new sarcomeres to be added at the ends of the muscle fibers where they attach to the tendons. … Conversely, when a muscle remains shortened continually to less than its normal length, sarcomeres at the ends of the muscle fibers disappear approximately equally as rapidly.’
It does not say if hypertrophy will occur on only one side of the muscle during certain exercises. However, this may not increase the size only near the musculotendonous attachment because it is just an adaptation to ensure the proper sarcomere length for force production, by increasing the length of the myofibril. This seems to be the deep down scientific support for training through a full ROM. Basicly, someone who repeatedly uses a limited ROM is weak at the extremes of a full ROM because their sarcomeres are not at an optimal length.
As it seems, a number of you guys are on the right track of how hypertrophy occurs. Nothing was mentioned about different parts of the muscle fiber hypertrophying more than other parts, resulting from where the myofibrils split or where the nuclei are located.
[quote]Chris L wrote:
Regarding the myofibrils (actin and myosin):
‘Virtually all muscle hypertrophy results from increase in the number of actin and myosin filaments in each muscle fiber, thus causing enlargement of the individual muscle fibers, which is called simply fiber hypertrophy.’
Regarding adding sarcomeres at the end of the muscle fiber:
‘Another type of hypertrophy occurs when muscles are stretched to greater-than-normal length. This causes new sarcomeres to be added at the ends of the muscle fibers where they attach to the tendons. … Conversely, when a muscle remains shortened continually to less than its normal length, sarcomeres at the ends of the muscle fibers disappear approximately equally as rapidly.’
Chris
[/quote]
Thanks, that is building a really good picture for me right now, and also creating some questions.
A couple for now:
How many sarcomeres THICK would be a typical myofibril?
If stretching increases the number of sarcomeres long that a myofibril can be, wouldn’t this potentially create slack in the muscle in certain contracted positions? Wait a minute. I bet that the myofibril is different numbers of sarcomeres long from one end to another. In certain positions, some of the sarcomere “lines” are tight and others are still slack. I believe a lot of this was tested in rats, but if I remember correctly, rats more than doubled their leg muscle mass with stretching alone.
[quote]nptitim wrote:
A sarcomere has actin and myosin in it, I think the typical sarcomere is made up of 3 myosin with 8 actin. [/quote]
In researching, it doesn’t look like this is the case. Some diagrams show a sarcomere as being 4, 5, 6 or more actin/myosin units thick so I think the 3 and 8 is just textbook convention.
I think I have to conclude for now that a single sarcomere takes up the entire thickness of a myofibril.
Its interesting that I can not find a definitive answer to such a basic question online or in any textbook.
So what about this possibility. Under high force, myosin pieces can get completely dislodged from the actin sheath. Surely this must happen from time to time.
How do they get put back?
Could the free myosin pieces be a signal for hypertrophy?
They probably wouldn’t be - more likely they’d be digested and the amino acids reused to form a new myosin bit. It’d be a lot of work to move a whole protein around.
[quote]
2) Could the free myosin pieces be a signal for hypertrophy?[/quote]
Could be if it happens, although they’d somehow have to feed back into the genes that express the necessities for muscle growth. Not sure how they’d do it, unless they somehow produce an intermediary compound on their way to digestion that has that effect. An interesting possibility, though… wonder if anyone has/is doing any work in that avenue right now.
They probably wouldn’t be - more likely they’d be digested and the amino acids reused to form a new myosin bit. It’d be a lot of work to move a whole protein around.
Could the free myosin pieces be a signal for hypertrophy?
Could be if it happens, although they’d somehow have to feed back into the genes that express the necessities for muscle growth. Not sure how they’d do it, unless they somehow produce an intermediary compound on their way to digestion that has that effect. An interesting possibility, though… wonder if anyone has/is doing any work in that avenue right now.
-Dan[/quote]
One thing that kicked up this idea as a mechanism is that it would be caused by high force, but also could be affected by TUT. What I am visualizing is that in a contracting myofibril, although all sarcomeres are sliding at about the same rate, a given sarcomere may slide backwards due to diminishing ATP stores, and get pulled apart by the other adjacent sarcomeres. Since in this model, adjacent sarcomeres would be pulling each other apart, then it also makes sens as to why fast twitch fibers are easier to hypertrophy-because they can pull themselves apart.
Maybe the myosin acts as a cell marker for an immune response?
Maybe the myosin head breaks off? (ouch)
It doesn’t make sense that intact myosin would signal myosin synthesis because the more you have, the less you need, however, it could signal the actin-myosin assembly process.
Interesting thread. It is quite apparent we know very little really about muscle hypertrophy. I have always been quite interested in the role Titin may play in muscle contraction, elasticity and hypertrophy. It is the largest protein in the muscle cell and yet always barely rates a mention. If you really want to understand muscle you need to eventually leave the realm of undergrad textbooks and pretty internet animations and get into some hardcore reading. Here is an abstract that sheds some light on titin and some research trying to uncover its mysteries. Note its role relative to Myosin. Enjoy.
Self-association properties of the elastic region of the giant protein titin
Larissa Tskhovrebova, Ahmed Houmeida, Beatrix Thompson, Peter Knight and John Trinick
Titin is the largest known protein (~4 MDa). Single molecules span half striated muscle
sarcomeres. In the myosin filament region of the sarcomere we have proposed that titin acts as a
?protein-ruler? to regulate exact myosin filament assembly. The remainder of the molecule forms
an elastic connection between the end of the myosin filament and the Z-line. These connections
are the main route of mechanical connectivity through relaxed muscle and they give muscle its
passive elasticity. They also maintain myosin filaments midway between Z-lines during
sarcomere length variations; this ensures an even development of force between the two halves
of a myosin filament during active contraction.
Titin self-association
We have recently purified a 400 kDa proteolytic fragment of titin. N-terminal sequencing shows
that the fragment begins at Ig domain number I22 in the molecule, which is the region of the
molecule near the end of the thick filament in situ. We previously suggested that the six titin
molecules that emerge from the thick filament end self-aggregate in register over about 100 nm
to form distinctive structures called end-filaments with a 4 nm periodicity. In accord with this
prediction, the 400 kDa fragment forms aggregates in low salt that are very similar in appearance
to the native end-filaments.
Role of titin in muscle regulation
We have also suggested that the large charge in the part of the titin molecule that is elastic (the
PEVK region) may affect calcium diffusion in the muscle, and thus that titin may have a role in
controlling muscle contraction.
Publications
Tskhovrebova, L., and Trinick, J. (2002). Role of titin in muscle regulation. Biophysical Journal
82, 1946.
Tskhovrebova, L., and Trinick, J. (2002). Role of titin in vertebrate striated muscle. Proc. Roy.
Soc. Lond. B 357, 199-206.
Funding
