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Musculo-Skeletal System > Muscle structure and adaptation > Flashcards

Muscle structure and adaptation Flashcards

1
Q

What is muscle formed from?

A

somites (paired blocks of paraxial mesoderm on either side of the notochord)

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2
Q

What is Somitogenesis?

A

formation of somites

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3
Q

How do somites form?

A

by paracrine signalling from the neural tube and notochord triggering a mesenchymal-to-epithelial transition of the paraxial mesoderm, forming a hollow ball of epithelial cells (epithelial somite)

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4
Q

What happens to the epithelial somite?

A

due to other paracrine signalling, epithelial somite further subdivided into 4 major cells which go on to form specific tissue types:

  • sclerotome
  • myotome
  • syndetome
  • dermomyotome
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5
Q

Sclerotome forms…

A

bone, ribs and cartilage

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6
Q

Myotome forms…

A

muscle precursors (which form muscle)

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7
Q

Syndetome forms…

A

tendons

*syndetome is in between myotome and sclerotome

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8
Q

Dermomyotome forms…

A

new source of muscle cells that later populate the myotome and give rise to the dorsal dermis

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9
Q

Describe the process of myogenesis.

A

1) Paracrine factors signal for the mesodermal cells to produce myogenic regulatory factors Myf5 and MyoD, which commit those cells to a myogenic fate and become myoblasts
2) Myoblasts then differentiate and proliferate in presence of growth factors until they exit the cycle after expression of another myogenic regulatory factor Myogenin
3) Myogenin causes terminal differentiation of muscle fibres and differentiates the myoblasts into myotubes, and structural proteins start being expressed (e.g. myosin + actin)
4) Myotubes align and fuse together to form multinucleated muscle fibres.

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10
Q

Why is muscle development biphasic?

A

After the initial formation of large primary muscle fibres, smaller secondary muscle fibres then form on their surfaces which make up the bulk of the muscle

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11
Q

What are Satellite cells?

A

third group of muscle cells (primary and secondary fibres, satellite cells)

-muscle stem cells which sit dormant on muscle fibres until they are activated in the case of regeneration and postnatal growth where they can start dividing and forming myotubes which then fuse to form the muscle fibres

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12
Q

What is the Embryonic muscle fibre number?

A

at the end of embryogenesis, the number of muscle fibres that you have are what you have for the rest of your life (muscle fibre number is therefore set from birth and this is genetically determined)

-HOWEVER can be manipulated during embryogenesis

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13
Q

What affects fibre number during embryogenesis?

A

Although fibre number is genetically determined, it can be affected by:

  • temperature
  • hormones
  • nutrition
  • innervation

These affect myogenic regulatory factor (MRF) expression e.g. My5, MyoD and Myogenin

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14
Q

Fibre number is increased by…

A

hyperplasia

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15
Q

Fibres increase in mass by…

A

hypertrophy (increasing muscle mass postnatally)

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16
Q

What is Postnatal Hypertrophy (increase in skeletal muscle mass)?

A

during growth, muscle fibres require more protein and therefore muscle stem cells (satellite cells) start dividing and making myotubes which fuse to make muscle fibres, producing more structural proteins
-increases cross-section and size of the muscle fibre

17
Q

Why are muscle fibres multinucleated?

A

because muscle fibre requires a lot of mitochondria and structural proteins along the length of the muscle fibre (very long cell), therefore many nuclei are needed to produce the structural proteins for the proper functioning of the muscle fibre

18
Q

What is Postnatal Hyperplasia (increase in muscle fibre number)?

A

Evidence it happens in animals:

Avian Stretch Model

  • weight put on a quale wing and there was muscle development on its back
  • quale was euthanised and muscle fibres counted, showing an increase in muscle fibre

Cat Weight Lifting Model

  • cats trained to flex paw and lift weight for the release of some food
  • as a result, an increase in fibre number in paw required to lift the weight

Lacking evidence it happens in humans, however 2 proposed mechanisms:
>muscle fibre splits and there is hypertrophy causing an increase in muscle mass
>proliferation of satellite cells which then form their own muscle fibres

19
Q

Describe the Variability between muscles.

A

all sarcomere structure is the same, however there is a lot of molecular variability between muscles due to multiple isoforms of myofibrillar proteins produced by alternative splicing or promoters

20
Q

Different myosin isoforms have…

A

different chemomechanical transduction and ATP hydrolysis, conveying a more rapid/speed of contraction

21
Q

What are the Different Titin isoforms?

A

convey different elastic properties to the sarcomere

22
Q

What are the Different Troponin and Tropomyosin isoforms?

A

different sensitivies to Ca2+ released from the sarcoplasmic reticulum

23
Q

Which isoforms contribute to resistance to fatigue?

A

Myosin and Troponin isoforms

24
Q

What are the different Types of Muscle Fibre?

A

Type I Muscle Fibres (slow twitch)

Type II Muscle Fibre (fast twitch)

25
Q

Give the features of Type I Muscle Fibres (slow twitch)?

A
  • produce slow maintained contraction which doesn’t easily fatigue
  • high mitochondrial content allowing for aerobic respiration and oxidative phosphorylation
  • extensive blood supply and abundant myoglobin
  • more predominant in long distance runners
26
Q

Give the features of Type II Muscle Fibres (fast twitch).

A
  • fatigue easily
  • fewer mitochondria and undergo mainly anaerobic respiration in a glycolytic nature
  • poor vascularisation and lack of myoglobin
  • more predominant in sprinters
27
Q

What is a Myosin Gene Cluster?

A

there are 11 myosin heavy chain genes clustered on chromosome 17 which allow for the different isoforms which give the different properties to the muscle fibres.

main types of myosin in adults:

  • Fast IIX fibres
  • Fast IIA fibres
  • Fast IIB fibres
  • Slow muscle (Type-I/β) and heart
28
Q

What is the Muscle difference between genders?

A

> 3000 genes different between males and female skeletons, with differences in myosin isoforms

females: more slow twitch fibres
males: more fast twitch fibres

males have a larger fibre cross-sectional area as male skeleton is more easily hypertrophied than women (mainly due to testosterone)

29
Q

What is the role of testosterone in muscle development?

A

promotes muscle differentiation at the expense of fat cells

  • commits mesenchymal pluripotent cells into the myogenic lineage and inhibits adipogenesis through an androgen receptor mediated pathway
  • this stimulates proliferation of satellite cells, increasing structural muscle protein synthesis and causes fibre hypertrophy
30
Q

What are the Adverse effects of synthetic anabolic steroids?

A

High blood pressure

Cardiac and Respiratory Problems

Liver Disease

31
Q

Describe the process of Muscle Regeneration.

A

DEGENERATION/INFLAMMATORY PHASE (first few days)

  • myofibre rupture and necrosis
  • formation of haematoma in injured area
  • inflammatory response where neutrophils start removing debris

REGENERATION PHASE (4/5 days post-injury)

  • satellite cells activated and proliferate, and increased expression of myogenic regulatory factors Myf5, MyoD and Myogenin
  • myotubes formed, formation of structural proteins and sarcomeres
  • phagocytosis of damaged tissue

REMODELLING PHASE

  • maturation of regenerated myofibres
  • restoration of blood supply and innervation
  • recovery of muscle function capacity
  • if damage too severe, there is fibrosis and scar tissue formation
32
Q

What is Sarcopenia?

A

age related loss of muscle mass and strength

-associated with decreased satellite cell number and recruitment

Musculo-Skeletal System (4 decks)

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