Muscle Structure and Adaptation

Question 1

How does skeletal muscle form in embryonic development?

Answer 1

It is formed from blocks of paraxial mesoderm in the developing embryo.

The paracrine signalling from the notochord + neural tube triggers a mesenchymal epithelial transition.

Somite forms, and the hollow ball of epithelial cells undergo an epithelial to mesenchymal transition

Question 2

What does the sclerotome form?

Answer 2

Bone, ribs, cartilage

Question 3

What does the myotome form?

Answer 3

Muscle precursors

Question 4

What does the syndetome form?

Answer 4

Tendons

Question 5

What does the dermomyotome form?

Answer 5

Provides a source of new muscle cells to the developing embryo as well as the dorsal dermis

Question 6

What happens to mesoderm cells when paracrine factors are expressed?

Answer 6

These mesodermal cells become committed to a myogenic fate and form myoblasts

Question 7

What happens to the myoblasts after they differentiate?

Answer 7

They exit the cell cycle marked by the expression of myogenic factors, which cause terminal differentiation of the muscle fibres where they start forming tubes and structural proteins start being expressed

Question 8

What do myotubes form?

Answer 8

Align and fuse together to form a muscle fibre

Question 9

What are the two phases of muscle development?

Answer 9

Primary round: makes the architecture for the fibres

Secondary round: builds on the muscle fibres

Question 10

What are satellite cells?

Answer 10

Muscle stem cells which sit on the muscle fibres and sit dormant until they are activated in the case of muscle regeneration, where they start dividing, forming myotubes

Question 11

What can affect fibre number?

Answer 11

Generally speaking it is genetically determined. But it can also be affected by temperature, hormones, nutrition, and innervation

Question 12

What is hypertrophy?

Answer 12

An increase in muscle mass due to the increase in muscle fibre size (happens after birth)

Question 13

How does hypertrophy occur?

Answer 13

Satellite cells start dividing and fuse to the myofiber and start producing more structural proteins which increase its cross section and size

Question 14

Why does the muscle fibre maintain a cytoplasm:nuclei ratio?

Answer 14

We need nuclei along that long cell to produce the proteins required for the muscle fibre to function

Question 15

What is hyperplasia?

Answer 15

An increase in muscle mass by increasing fibre number

Question 16

What are the proposed mechanisms for hyperplasia?

Answer 16

1) The actual muscle fibre itself splits and there is hypertrophy which kicks in and increases muscle mass that way
2) Can occur through proliferation of the satellite cells which then start to use the architecture of the muscle fibres already there, and they start forming their own muscle fibres and fusing together

Question 17

What are the different isoforms of myofibrillar proteins?

Answer 17

There may be changes in amino acids in different muscle types for titin, and this will convey different elastic properties to the sarcomere

There could be changes in troponin and tropomyosin, which will create different sensitivity to the calcium released from the SR

There can be differences in myosin isoforms, which can convey more rapid or slower speed of contraction. These together can then also convey a resistance to fatigue

Question 18

What are Type I fibres?

Answer 18

• slow titch muscle fibres
• produce a slow, maintained contraction which does not easily fatigue
• aerobic respiration
• high mitochondrial count
• undergo oxidative phosphorylation
• extensive blood supply and abundant myoglobin

Question 19

What are Type II fibres?

Answer 19

• fast muscle
• fatigue very easily
• mainly anaerobic metabolism
• fewer mitochondria
• glycolytic in nature
• poor vascularisation and low levels of myoglobin

Question 20

Generally speaking, what do different types of people have in terms of type I and type II fibres?

Answer 20

untrained individuals: 50:50 ratio of fast and slow twitch muscle

long + middle distance: 60-70% slow muscle

sprinters: 80% fast twitch muscle

Question 21

What is the muscle profile of a powerlifter?

Answer 21

• muscles are hypertrophied
• highly glycolytic
• fatigue easily
• high muscle to body mass ratio
• muscle size interferes with locomotion

Question 22

What is the muscle profile of a marathon runner?

Answer 22

• muscles small but fatigue resistant
• work over long periods of time
• not explosive strength
• muscles dense and strong for their size, with high oxidative capacity

Question 23

What is the muscle profile of a sprinter?

Answer 23

• rapid powerful contraction
• easily fatigued at maximum effort
• low oxidative capacity via mitochondria
• high force

Question 24

What are the 3 main types of myosin?

Answer 24

2a

2x

2b

Question 25

What are the differences in muscle profiles between men and women?

Answer 25

- females have more slow muscle fibres - males have more fast muscle fibres - males have a larger fibre cross sectional area so will hypertrophy easier than women

Question 26

How is testosterone involved in muscle development?

Answer 26

Promotes muscle differentiation at the expense of fat cells, so is a natural anabolic androgenic steroid which will push cells down the myogenic lineage, but inhibit the formation of adipocytes Increases the contribution to the muscle fibre, and produces more structural proteins, increasing the muscle fibre size

Question 27

What are some risks of taking anabolic steroids?

Answer 27

- high blood pressure - cardiac problems - respiratory problems - liver disease

Question 28

What happens if muscle fibres cannot regenerate themselves?

Answer 28

you get fibrosis and scar tissue

Question 29

What is the normal process of a tear being repaired in muscle?

Answer 29

- necrosis - blood fills wound area - haemotoma - satellite cells divide and use haemotoma to build upon and fuse together - expression of MYF5 and myoD - muscle fibres self renew - vascularisation and innervation - homeostasis

Question 30

What are the 3 main phases of regeneration?

Answer 30

1) Degeneration/inflammation phase Myofibre rupture and necrosis, formation of hematoma, inflammatory response 2) Regeneration phase Phagocytosis of damaged tissue, SC activation and proliferation 3) Remodelling phase Maturation of regenerated myofibers, restoration of blood supply and innervation, recovery of muscle functional capacity and also fibrosis and scar tissue formation

Question 31

What is sarcopenia?

Answer 31

The decline of muscle mass with age

Question 32

What is a loss in muscle mass associated with?

Answer 32

A gain in fat mass. There are changes to mitochondria, and a reduced endocrine function due to reduced physical activity