Muscle Structure and Adaptation Flashcards
Slow twitch fibres (type I)
Twitch contraction generates moderate peak force relatively slowly; resistant to fatigue with repeated stimulation.
- fatigue resistant
- moderate max force
- oxidative
- many mitochondria
- rich vascularisation
- small diameter
- high myoglobin (‘red muscle’)
Fast twitch fibres (type II)
Twitch contraction generates high peak force relatively quickly; fatigue easily.
- fatigue rapidly
- high max force
- glycolytic or mixed
- fewer mitochondria
- sparser vascularisation
- larger diameter
- low myoglobin (‘white muscle’)
Tetanus
When a motor unit fires action potentials at high frequency the individual twitches fuse and summate to generate a smooth contraction (temporal summation)
Oxidative
Muscle fibres powered by ATP generated via oxidative phosphorylation of nutrients in the mitochondria (type I fibres have high oxidative capacity; some type II have moderate)
Glycolytic
Muscle fibres powered by ATP generated via anaerobic breakdown of glucose to pyruvate through glycolysis (type II)
Motor pool
All of the motor neurons (10s - 100s) innervating a single muscle. Cell bodies in ventral horn of spinal cord, axons exit via ventral root and travels to target muscle in peripheral nerve. Muscle force can be increased by increasing the fraction of active MNs within its motor pool
Motor unit
A single motor neuron together with the subset of muscle fibres (10 - 100s) that it innervates within its target muscle. A single MN action potential will generate APs and twitch contractions in all the fibres it innervates. Increasing the number of active motor units will increase the force generated by the whole muscle (spatial summation)
Size principle
Refers to the orderly recruitment of motor units from a motor pool (small to large; type I first, then more type I and II as force generation increases).
Myoblasts
A cell commitment to a myogenic lineage, but not yet differentiated.
Myogenic Regulatory Factors
Transcription factor required for commitment and terminal differentiation of muscle cells.
Satellite cells
Divide and serve as a source of new myonuclei during postnatal growth. They contribute to the growth of the fibres and participate in the regeneration process.
Muscle Hypertrophy
Increase in muscle mass due to increase in fibre size
Muscle Hyperplasia
Increase in the number of muscle fibres.
Sarcopenia
Age-related loss of muscle mass
What’s the role of myoglobin?
In muscle cells, it facilitates the uptake of oxygen and makes oxygen more readily available.
= high oxidative metabolism capacity of slow twitch fibres is facilitated by high myoglobin, whereas fast twitch has less/low moderate myoglobin content
What is red and white muscle?
This is due to the fibre composition of muscles.
High myoglobin levels, together with the rich capillary supply in slow twitch fibres gives a reddish appearance to muscle with many type 1 fibres.
Lower myoglobin content results in a paler and whiter appearance, which is seen by muscles with many type 2 fibres, with lower vascularisation supply.
Myofibril proteins exist as multiple
isoforms with different functional
character
Vertebrate sarcomere 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 I) express type I myosin heavy chain
• Fast twitch fibres (type II) express type II MHC
Fibre type composition of muscles adapts
to function
Muscles vary in proportion of type II / type II fibres
•Lateral rectus (eye muscle) mainly type II fast twitch
•Gastrocnemius (calf muscle) mixed type I & II
•Soleus (calf) more type 1 slow twitch
running fast or slow twitch?
- Long and middle distance runners: 60-70% slow
* Sprinters: 80% fast twitch
Force generation in a
muscle is controlled at
the level of the motor
unit
- The motor unit consists of a motor 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 motor MNs
- Size of MN correlates with size of motor unit
- Muscle fibres of a motor unit are generally of the same type
Temporal summation in muscle fibres
diagram
•Fusion of individual twitches generates tetanus
Summation in fast and slow motor units
- Fast motor units(A) need higher firing rates to generate tetanic forces than slow motor units (B, C)
- Slow motor units are recruited first, followed by fast units for higher levels of force generation
Skeletal muscle tone
•Most muscles at rest exhibit some low level of contractile activity •Denervation leads to complete relaxation (flaccid) •Driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone)
Myogenesis
- Paracrine factors induce myogenic regulatory factors (MRFs) in mesodermal precursor cells – myogenic commitment (myoblasts)
- Myoblast 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)
Postnatal muscle growth:
hypertrophy
- 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. Maintain cytoplasm: nuclei ratio
- Satellite cells return to quiescence when not needed
Postnatal muscle growth:
hyperplasia
•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
Ageing muscle: sarcopenia
•Sarcopenia: reduction in muscle mass
•Part of ageing process
•Atrophy of muscle fibres
•May be due to disease or immobilization
•Associated with decreased 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
