MSS: Muscle Structure and Adaptation Flashcards
Describe the cellular origins of skeletal muscle.
The muscle forms from the somites, which are blocks of paraxial mesoderm.
Closest to the notochord, we have the sclerotome, which will differentiate into bone, ribs and cartilage. Following that, there is a layer of syndetome cells, which will become tendons. Then, a layer of myotome, which becomes the muscle precursors. Finally, on the outside, we have the dermotome, which becomes the dorsal dermis.
Describe myogenesis.
Paracrine factors induce the mesodermal cells with myogenic factors, such as Myf5 and MyoD. Once the cells have committed, they are known as myoblasts.
These myoblasts proliferate due to growth factors, and exit the cell cycle. They start to express myogenin, which is known as terminal differentiation.
They form myotubes as structural proteins start to be expressed. These myotubes align and fuse.
Then comes biphasic muscle development, where we first get a primary form of cells, then a secondary form of cells that are based off the architecture of the first cells. There is also a third group of myoblasts that doesn’t differentiate but sits closely to the muscle fibres. These are known as satellite cells, and are responsible for the regeneration and postnatal growth of muscle cells.
What is special about the embryonic fibre number?
Our muscle fibre number is set at determined; thus, it is genetically determined.
However, the fibre number can be affected by:
- temperature
- hormones
- nutrition
- innervation
Describe post-natal growth (hypertrophy).
After birth an increase in muscle mass is due to an increase in fibre size (hypertrophy).
Muscle Stem Cells (MuSCs) contribute to the muscle fibre. They’re called satellite cells; they are undifferentiated muscle precursors, and are self-renewing.
MuSCs proliferate and incorporate into the muscle fibres, where they contribute to the production of structural proteins, which causes an increase in muscle fibre size.
They are maintain the cytoplasm:nuclei ratio in the muscle fibre.
MuSCSs maintain the cytoplasm:nuclei ratio in the muscle fibre.
Why is this important?
This is done to supply the increased production of structural proteins in the growing muscle fibre.
Also, the muscle fibres have a lot of mitochondria, and a lot of the genes needed for mitochondria production is found in the nuclei.
How do we get fibre-type diversification?
Even though we have the same proteins between different fibre types, there is a lot of variation between the proteins, which gives the muscle fibres a distinct characterisation.
All the vertebrae’s sarcomere structure is the same. We simply have molecular variability in the proteins, depending on the function.
There are multiple isoforms of the myofibrillar proteins that have come about due to alternative splicing, or using different promoters to drive gene expression.
Give some examples of myofibrillar protein isoforms, and what differs between them.
MYOSIN isoforms:
- different chemo mechanical transduction
- ATP hydrolysis
- shortening velocity
TROPONIN and TROPOMYOSIN isoforms:
- sensitivity to Ca2+
TITIN isoforms:
- elastic properties
List the differences between Type I and Type II muscle fibres.
TYPE I:
- virtually inexhaustible
- high mitochondria (aerobic metabolism)
- oxidative phosphorylation
- extensive blood supply and abundant myoglobin
TYPE II:
- fatigue easily
- few mitochondria (anaerobic metabolism)
- glyoclytic
- poor vascularisation and lack myoglobin
List some effects of training specific muscle fibre types.
Untrained individuals have a 1:1 ratio of fast (IIA and IIX) to slow (I) twitch fibres.
- long and middle distant runner have about 60-70% slow twitch
- sprinters have about 80% fast twitch
- trainees for sports that require the greatest aerobic and endurance capacities have slow muscle up to 90-95%
- trainees for sports that require greater anaerobic capacities (strength and power) have fast muscle around 60-80%
How would a marathon runner’s muscles be adapted to their sport?
- Muscles are small but fatigue-resistant
- Muscles are dense and strong for their size, with a high oxidative capacity of the muscles
- They can work over very long periods of time
- They don’t contain explosive strength
How would a sprinter’s muscles be adapted to their sport?
- Muscles produce rapid, powerful contractions
- Muscles are easily fatigued at maximum effort
- Muscles have a low oxidative capacity via mitochondria
- Muscle can exert a high force per cross-sectional area of muscle
How would a power lifter’s muscles be adapted to their sport?
- Muscles are hypertrophied
- They are highly glycolytic
- They fatigue easily
- Have a high muscle to total body mass ratio
- Muscle size begins to interfere with locomotion
Thus, the power lifter is moving along the same path of adaptation as the sprinter, but more extreme.
Their power to weight ratio is moving to a point where they are less able to move their body through a distance, and hence would be less fast at running.
Describe the gender differences in skeletal muscle.
There are over 3000 genes that are different between male and female skeletal muscle.
TYPE I:
M - 36%, female - 44%
TYPE IIA:
M - 41%, F - 34%
Males have larger fibre cross-sectional areas (CSAs).
Describe testosterone, and how it contributes to muscles.
Testosterone is a primary male sex hormone required for the development of the male reproductive system. It also promotes secondary sexual characteristics, such as body hair, development of a deep voice, and increased muscle and bone mass.
Testosterone is a natural anabolic-androgenic-steroid (AAS).
It promotes the commitment of mesenchymal pluripotent cells into myogenic lineage, and inhibit adipogenesis (via an androgen receptor-mediate pathway).
It also stimulates:
- satellite cell replication
- muscle protein synthesis
- fibre hypertrophy
How does muscle repair differ with the type of injury?
In the case of a minor injury, we get the recruitment of satellite cells to the muscle fibre. The damaged muscle fibre necroses, and there is an inflammatory response. Macrophages and neutrophils will respond to the inflammation. There is an increase in satellite cell proliferation, which fuse with the muscle fibre and regenerate it.
Thus, this injury is reversible.
In the case of more severe injury, there is incomplete regeneration of muscle fibre, and so fibrotic tissue forms.
