Muscle Structure and Adaptation: the genetic plan (S3)

Question 1

Do numbers of muscle fibres increase post-natally?

Answer 1

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.

Question 2

What is isometric stress?

Answer 2

When muscles tries to contract but we don’t let it - ie. the force generated by the muscle

Question 3

What parameter increases to allow muscle to grow?

Answer 3

Fibre diameter not number

Question 4

On which chromosome is the Myosin Heavy Chain Gene cluster found upon?

Answer 4

Chromosome 17

Question 5

Which MHC isoform is able to split ATP the fastest?

• 2B
• 2A
• 2X
• 1
• Emb

Answer 5

Order goes from fastest to slowest

2B -> 2A -> 2X -> 1 -> Emb

Question 6

Describe MCH2B (type 2B) containing muscle fibres

Answer 6

• Fastest
• Gain energy solely from glycolysis
• Do not contain myoglobin
• White and largest/strongest fibres

Question 7

Describe MCH2A (type 2a) containing muscle fibres

Answer 7

• Intermediate velocity
• Fatigue resistance
• May use glycolysis and Krebs cycle for energy

Question 8

Describe MCH1 (type 1) containing muscle fibres

Answer 8

• 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

Question 9

Which type of muscle fibre is most resistant to fatigue?

Answer 9

Type 1 fibres (MHC1)

Question 10

Which type of muscle fibre is the most efficient?

Answer 10

Type I fibres (MHC1)

Question 11

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?

Answer 11

2B, 2A, 2X or 1

Question 12

What is staining for catalytic activity of MHC ATPase useful for?

Answer 12

Diagnosis of myopathy

Question 13

Do the embryonic (EMB) and neonatal (NEO) correlate positively or negatively with contractile function?

Answer 13

Negatively - the more emb and neo, the less contractile the muscle

Question 14

Do the IIX and IIB mature isoforms correlate positively or negatively with contractile function?

Answer 14

Positively - the more IIX and IIB the stronger contractile function.

Question 15

How do muscle cells regenerate following intense exercise, what cells are involved?

Answer 15

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.

Question 16

What is the difference between hypertrophy and hyperplasia?

Answer 16

Hypertrophy is increase in muscle fibre diameter/size

Hyperplasia is increase in muscle fibre number

Question 17

What is the principal growth agonist for skeletal muscle?

Answer 17

IGF-1, induces prolif and differentiation of satellite cells

Question 18

Describe the autocrine action on muscle growth

Answer 18

Prolongued contraction of skeletal muscle releases large amounts of IGF-1, which induces regeneration of the muscle.

Question 19

What 3 things does normal growth and devleopment of the limb require?

Answer 19

1. Normal cell numbers
2. Normal locomotor elements eg. bone, joint, muscle
3. Normal blood supply and nerve supply

Question 20

Describe the relationship between muscle fibres and nerves

Answer 20

One nerve contacts many muscle fibres, but one muscle fibre is supplied by only one nerve

Question 21

Describe motor unit recruitment

Answer 21

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

Question 22

What happens in ageing muscle?

Answer 22

• muscle fibres die
• muscles atrophy
• muscles get weaker
• nerves remodel (lose and rearrange connections with muscles)
• muscles contract more slowly