Muscle Structure and Adaption (MS system)

Question 1

Diversification of skeletal muscle fibre types

Answer 1

Skeletal muscle fibres can be classified into 2 main groups according to contraction speed:

• slow twitch fibres (type l)
• fast twitch fibres (type ll)

Question 2

Skeletal muscle fibre classification

Answer 2

Slow twitch fibres (type l):

• fatigue resistant
• moderate max force
• oxidative
• many mitochondria
• rich vascularisation
• small diameter
• high myoglobin (red muscle)

Fast twitch fibres (type ll):

• fatigue rapidly
• high max force
• glycolytic or mixed
• fewer mitochondria
• sparse vascularisation
• larger diameter
• low myoglobin (white muscle)

Question 3

Structural and metabolic differences

Answer 3

Stained for capillaries:
- small cross sectional diameter muscle fibres surrounded by more capillaries
Stained for oxidative enzymes:
- darker staining shows higher capacity for oxidative metabolism

Question 4

Myofibril proteins

Answer 4

• exist as multiple isoforms with different functional character
• vertebrae sacromere structure essentially the same, but functional tuning occurs via isoforms of muscle proteins:
• variable Ca sensitivity (troponin, tropomyosin)
• rate of ATP hydrolysis (myosin isoforms):
  slow twitch fibres (type l) express type l myosin heavy chain
  fast twitch fibres (type ll) express type ll MHC

Question 5

Fibre type composition of muscles adapts to function.

Answer 5

muscles vary in proportion of type l/type ll fibres:

• lateral rectus (eye muscle) mainly type ll fast twitch
• gastrocnemius (calf muscle) mixed type l and ll
• soleus (calf) more type l slow twitch

Question 6

Effects of training on fibre types

Answer 6

• long and middle distance runners: 60-70% slow

- sprinters: 80% fast twitch

Question 7

Summary of myofibril protein isoforms

Answer 7

• myofibril proteins exist as multiple isoforms with different functional character
• slow (type l): slow maintained contraction, fatigue-resistant (oxidative, lots of mitochondria and vascularisation, myoglobin)
• fast (type ll): rapid powerful contractions that fatigue easily (glycolytic or moderate oxidative capacity, few mitochondria, poor vascularisation, lack myoglobin)
• muscle fibre type composition can be different in different muscles and can adapt over time to needs of the body

Question 8

Motor unit

Answer 8

• force generation in muscle is controlled at the level of the motor unit
• the motor unit consists of a motor neurone and the set of muscle fibres within a muscle that it innervates
• motor unit size ranges from around 10 to many 100s of muscle fibres
• a muscle may be innervated by 10s to 100s of motors MNs
• size of MN correlates with size of motor unit
• muscle fibres of a motor unit are generally of the same type

Question 9

Temporal summation in muscle fibres

Answer 9

• fusion of individual twitches generates tetanus

Question 10

Summation in fast and slow motor units

Answer 10

• fast motor units (A) need higher firing rates to generate tetanic forces that slower motor units (B,C)
• slow motor units are recruited first, followed by fast units for higher levels of force generation

Question 11

Skeletal muscle tone

Answer 11

• most muscles at rest exhibit some low level of contractile activity
• desecration leads to complete relaxation (flaccid)
• driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone)

Question 12

Myogenesis

Answer 12

• paracrine factors induce myogenic regulatory factors (MRFs) in mesodermal precursor cells - myogenic commitment (myoblasts)
• myoblasts proliferate under influence of growth factors
• cell cycle exit, myogenin expression - terminal differentiation
• structural proteins expressed and myotubes form from myoblasts
• myotubes align and fuse, becoming multinucleated muscle fibres
• satellite cells: regeneration and postnatal growth (muscle stem cells)

Question 13

Postnatal muscle growth: hypertrophy

Answer 13

• after birth, increase in muscle mass due to increase in fibre size (hypertrophy)
• muscle stem cells called satellite cells: undifferentiated muscle precursors, self renewing)
• muscle fibre growth involves satellite cell proliferation and incorporation of nuclei into muscle fibres
• increased protein synthesis and muscle fibre size (hypertrophy)
• muscle fibres are multinucleated to maintain cytoplasm: nuclei ratio
• satellite cells return to quiescence when not needed

Question 14

Postnatal muscle growth: hyperplasia

Answer 14

• increase in muscle mass due to formation of new muscle fibres
• some evidence from animal models
• uncertain whether this happens (main mechanism is probably hypertrophy)

Question 15

Ageing muscle: Sarcopenia

Answer 15

• sarcopenia is reduction in muscle mass
• part of ageing process
• atrophy of muscle fibres
• may be due to disease or immobolisation
• associated with decrease satellite cell number and recruitment
• anabolic resistance - reduced protein synthesis in response to hormonal stimulation or resistance exercise
• can be resisted - importance of resistance exercise