Muscle Structure and Adaptation: the genetic plan (S3)
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Do numbers of muscle fibres increase post-natally?
No, they are genetically determined. When we get stronger or larger, our muscle fibre numbers do NOT change. Growth involves increase in several muscle fibre parameters reflected in increased strength.
What is isometric stress?
When muscles tries to contract but we don’t let it - ie. the force generated by the muscle
What parameter increases to allow muscle to grow?
Fibre diameter not number
On which chromosome is the Myosin Heavy Chain Gene cluster found upon?
Chromosome 17
Which MHC isoform is able to split ATP the fastest? • 2B • 2A • 2X • 1 • Emb
Order goes from fastest to slowest 2B -> 2A -> 2X -> 1 -> Emb
Describe MCH2B (type 2B) containing muscle fibres
• Fastest • Gain energy solely from glycolysis • Do not contain myoglobin • White and largest/strongest fibres
Describe MCH2A (type 2a) containing muscle fibres
• Intermediate velocity • Fatigue resistance • May use glycolysis and Krebs cycle for energy
Describe MCH1 (type 1) containing muscle fibres
• Slowest contracting • Energy aerobically via Krebs and Oxidative phosphorylation • Red in appearance (high [myoglobin]) • Produce less power than 2B but are much more resistant to fatigue
Which type of muscle fibre is most resistant to fatigue?
Type 1 fibres (MHC1)
Which type of muscle fibre is the most efficient?
Type I fibres (MHC1)
There are 6 kinds of MHC in skeletal muscle: 2B, 2A, 2X, 1, embyronic and neonatal. Each muscle fibre in the adult will contain one of 4 of these, which 4?
2B, 2A, 2X or 1
What is staining for catalytic activity of MHC ATPase useful for?
Diagnosis of myopathy
Do the embryonic (EMB) and neonatal (NEO) correlate positively or negatively with contractile function?
Negatively - the more emb and neo, the less contractile the muscle
Do the IIX and IIB mature isoforms correlate positively or negatively with contractile function?
Positively - the more IIX and IIB the stronger contractile function.
How do muscle cells regenerate following intense exercise, what cells are involved?
Progenitor cells and satellite cells, which are activated upon injury and first proliferate then differentiate into myoblasts, which then fuse with the existing damaged muscle cell. Therefore regenerating and without increasing muscle cell number.
What is the difference between hypertrophy and hyperplasia?
Hypertrophy is increase in muscle fibre diameter/size Hyperplasia is increase in muscle fibre number
What is the principal growth agonist for skeletal muscle?
IGF-1, induces prolif and differentiation of satellite cells
Describe the autocrine action on muscle growth
Prolongued contraction of skeletal muscle releases large amounts of IGF-1, which induces regeneration of the muscle.
What 3 things does normal growth and devleopment of the limb require?
• Normal cell numbers • Normal locomotor elements eg. bone, joint, muscle • Normal blood supply and nerve supply
Describe the relationship between muscle fibres and nerves
One nerve contacts many muscle fibres, but one muscle fibre is supplied by only one nerve
Describe motor unit recruitment
It’s a measure of how many motor neurons are activitated in a particular muscle, and therefore measure of how many muscle fibres of that muscle are activated. The higher the recruitment the stronger the muscle will be
What happens in ageing muscle?
• muscle fibres die • muscles atrophy • muscles get weaker • nerves remodel (lose and rearrange connections with muscles) • muscles contract more slowly