Muscle Structure & Adaptation Flashcards
State the 2 different types of groups that skeletal muscles can be classified into
- Slow twitch fibres (Type I)
- Fast twitch fibres (Type II)
Outline the features of the Slow twitch fibres (Type I)
- Fatigue resistant
- Moderate max force
- Oxidative
- Many mitochondria
- Rich vascularisation
- Small diameter
Outline the features of the Fast twitch fibres (Type II)
- Fatigue rapidly
- High max force
- Glycolytic (IIx) or mixed (IIa)
- Fewer mitochondria
- Sparser vascularisation
- Larger diameter
State the visible features that allows you to differentiate between slow twitch and fast twitch fibres
- Slow twitch fibres have high myoglobin which appears as red muscle
- Fast twitch fibres have low myoglobin which appear as white muscle
What are the structural and metabolic differences between the 2 groups?
- When slow twitch fibres are stained for capillaries, the microscope shows a small cross sectional diameter muscle surrounded by more capillaries
- When fast Twitch fibres are stained for oxidative enzymes, the microscope shows a darker staining which shows higher capacity for oxidative metabolism
What do myofibril proteins exist as?
As multiple isoforms with different functional characteristics
Describe one form of these myofibrils
- Vertebrate sarcomere structure essentially the same
- But functional tuning occurs via isoforms of muscle proteins
- Variable Ca sensitivity (troponin, tropomyosin)
- Slow twitch fibres (type I) express type I myosin heavy chain
- Fast twitch fibres (Type II) express type II MHC
What does the fibres type do when it is present in a muscle?
Fibres type composition of muscles adapts to function
- Muscle vary in proportion of type I / type II fibres
- Lateral rectus (eye muscle) mainly type II fast twitch
- Gastrocnemius (calf muscle) mixed type I & II
- Soleus (Calf) more type I slow twitch
What are the listed effects of training specific fibres types?
- Long and middle runners: 60-70% have slow twitch fibres
- Sprinters: 80% have fast twitch fibres
Summarise what Slow twitch fibres (Type I) are
- Slow maintained contraction, fatigue resistant
- is Oxidative, has lots of mitochondria and vascularisation, myoglobin
Summarise what Fast twitch fibres (Type I) are
- Rapid powerful contractions that fatigue easily
- Glycolytic or moderate oxidative capacity, few mitochondria, lack myoglobin and poor vascularisation
Briefly summarise how myofibril proteins exist and the composition of muscle fibre types
- Myofibril proteins exist as multiple isoforms with different functional character
- Muscle fibre type composition be different in different muscles
- Muscle fibre types can adapt over time to the needs of the body
How is force generation in a muscle controlled?
Controlled at the level of the mortar unit
Describe the function and structure of the mortar unit
- The mortar unit consists of a mortar neuron and the set of muscle fibres within a muscle that it innervates
- Motor unit size ranges from 10 to many 100s of muscle fibres
- A muscle may be innervated by 10s to 100s of mortar MNs
- The size of the MN correlates with the size of the motor unit
- Muscle fibres of a motor unit are generally of the same type
What is a temporal summation in a muscle fibre?
- Fusion of individual twitches generates tetanus forces
- VD to describe graph
Compare and contrast the fast and slow motor units
- Fast motor units (A) need higher firing rates to generate tetanic forces than slow motor units
- Slow motor units are recruited first, followed by fast units followed by higher levels of force generation
What happens to the skeletal muscle tone when at rest?
- Most muscles at rest exhibit some low level of contractile activity
- Denervarion leads to complete relaxation (flaccid)
- Driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone)
Describe the process of myogenesis? (Part 1)
- Paracrine factors induce myogenic regulatory factors (MRFs) in the mesodermal precursor cells (myogenic commitment myoblasts
- Myoblast proliferate under influence of growth factors
- Cell cycle exit, myogenin expression – terminal differentiation
Describe the process of myogenesis? (Part 2)
- Structural proteins expressed and myotubes form from myoblasts
- Myotubes align and fuse, becoming multinucleated muscle fibres
- Satellite cells: regeneration and postnatal
Describe how postnatal muscle growth: hypertrophy occurs (Part 1)
- 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)
Describe how postnatal muscle growth: hypertrophy occurs (Part 2)
- 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. Maintain cytoplasm: nuclei ratio
- Satellite cells return to quiescence when not
Describe how Postnatal muscle growth: Hyperplasia occurs
- 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
Describe the fundamentals of the ageing muscle: sarcopenia
- Sarcopenia: reduction in muscle mass
- Part of ageing process
- Atrophy of muscle fibres
- May be due to disease or immobilisation
- Associated with decreased satellite cell number and recruitment
- Anabolic resistance – reduced protein synthesis in response to hormonal stimulation or resistance exercise
- Can be resisted
