Muscle structure and adaptation Flashcards
What does muscle form from?
Muscle forms from the somites - (paraxial mesoderm) on either side of the embryo. They consist of columnar epithelium but contain a cavity. Forms a highly formatted structure.
What structures are found within the somites?
- Sclerotome (bone, ribs, cartilage)
- Myotome (skeletal muscle precursors)
- Dermomyotome (myotome and dorsal dermis)
- Syndetome (tendons) (muscle to the bones)
Describe the process of myogenesis
How does skeletal muscle develop?
What are Satellite cells?
- During embryonic growth myoblasts develop from myogenic precursor cells which are of mesoderm origin.
- Paracrine factors induce Myf5 and MyoD (myogenic regulatory factors) (converts these mesodermal cells into myoblasts) = myogenic commitment (myoblasts)
- Myoblast proliferate (under the presence of Growth Factors) and form a pool of myoblasts
- They then exit the cell cycle and stop dividing and differentiate. We then get Myogenin expression= Terminal differentiation
- Structural proteins expressed and myotubes form. May being to express actin and myosin
- Myotubes align and fuse to form multinucleated muscle fibres. This process is called myogenesis.
- The skeletal muscle develops in biphasic way. Where the fibers form from 2 different populations: Primary (initial myoblast formation – provide framework for secondary fibres) and Secondary fibres (make up larger population and are formed from fetal myoblasts)
- Satellite cells: regeneration and postnal growth (muscle stem cells) (needed for the regeneration process)
Is myofiber number fixed?
What determines it?
Myofiber number is actually fixed at birth for most species – genetically determined. It can be affected by temperature, hormones, nutrition and innervation.
Affect Myogenic Regulatory Factor expression duration (time.)
Describe postnatal muscle growth: hypertrophy
Why must muscle maintain nuclei: cytoplasm ratio?
- After birth, increase in muscle mass due to increase in fibre size (hypertrophy). This is accompanied by the proliferative acitivity of satellite cells – giving new nuclei in muscle fibers
- Muscle Stem Cells (MuSCs) called Satellite cells. Undifferentiated muscle precursors, self-renewing.
- MuSCs proliferate and incorporated into muscle fibers. Return to quiescence when not needed.
- Muscle fibres are multinucleated. Maintain cytoplasm: nuclei ratio,why? The muscle cell is large and long – it has huge protein making requirements. They need lots of mitochondria as well produced from genes in nuclei.
What is postnatal hyperplasia?
After birth, an increase in muscle mass due to an increase in cell/fibre number (hyperplasia)
Has been debated whether or not this happens.
Evidence: Avian stretch model, Cat weight-lifting model - weight is applied to a bird and calculate muscle fibre number (16%) and 19-20% in cat weight lifting model
Proposed mechanisms: fibre splitting and SC activation.
Evidence this happens in humans is lacking.
Describe fibre-type diversification
- All vertebrate sarcomere structure the same
- Molecular variability depending on function
- Multiple isoforms of myofibrillar proteins: Alternative splicing or promoters
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Describe the properties of type 1 muscle fibres
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Describe the properties of muscle 2 fibres
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Describe the balance of type 1 to type 2 muscle fibres in people who train in different things
- Untrained individuals 50:50 ratio of fast (IIA and IIX) to slow (I) twitch fibres
- Long and middle distant runners: 60-70% slow
- Sprinters: 80% fast twitch
- Sports requiring greatest aerobic and endurance capacities: slow muscle up to 90-95%.
- Sports with greater anaerobic capacities (strength and power) have fast muscle from 60-80%
What properties does a marathon runner possess?
- Muscles small but fatigue resistant
- Muscle dense and strong for their size High oxidative capacity of muscles
- Work over very long periods of time
- Not explosive strength
What properties does a sprinter possess?
Muscles adapted for explosive release of force:
- Rapid powerful contractions
- Easily fatigued at maximum effort
- Low oxidative capacity via mitochondria
- High force per cross-sectional area of muscle
What properties does a powerlifter possess?
Muscle adapted for immense strength:
- Muscles are hypertrophied
- Highly glycolytic
- Fatigue easily
- High muscle to total body mass ratio
- Muscle size beginning to interfere with locomotion
Thus the Powerlifter is moving along the same path of adaptation as the sprinter but is more extreme.
His power to weight ratio is moving to a point where he is less able to move his body through a distance and hence would be less fast at running.
What are the effects of anabolic steroids?
- Irreversible adverse effects
- Stimulate male sexual characteritiscs in females
- Increase production of red blood cells
- Acne
- Hair loss
- Liver disease
- Heart disease
- Increased mood
- High blood pressure
- Weight loss
- Testicles can shrink
- Can be used for delayed puberty
- Hypogonadism
- Breast cancer
- Endometrioses
Describe PAX genes and regeneration
What do PAX3 and PAX7 code for?
Give a brief overview of muscle regeneration in mild and severe cases
- paired/homeodomain transcription factors Pax3 and Pax7
- Pax3 establishes MuSCs identity during embryonic development , expressed in the presomitic mesoderm, required for survival of the ventro-lateral dermomyotome, which gives rise to the hypaxial and limb musculature
- Pax7 establishes MuSCs during late foetal and perinatal growth
- Pax7 null mice are deficient in the number of MuSCs and fail to regenerate muscle after injury in adult mice
- Shows the different cells types involved in muscle regeneration
- Muscle stems cells become activated; they differentiate.
- The damage induces a fibre break and recruitment of satellite cells onto the intact side of the muscle fibre
- In severe injury you get complete myofibrillar damage followed by satellite cell proliferation and differentiation of the extracellular matrix
- So, in mild damage the satellite cells repair the damaged muscle however in severe cases these cells proliferative along the extracellular matrix
- It is a tightly regulated myogenic process with lots of key transcription factors. During homeostasis satellite cells are quiescence and express pax 7, pax 3. Myf5 and NOTCH
- These are highly active during the quiescence phase and upon damage they rapidly upregulate these factors. The severe then express myogenin that differentiate to form new muscle fibres
- During this process satellite cells replenish the pool of stem cells
- Macrophages, monocytes, mesenchymal stromal cells (mesoangioblasts), fibroblast are also needed for the regeneration of muscle tissue
