Muscle Contraction and Dysfunction Flashcards
What are the two different types of skeletal muscle fibres?
Skeletal muscle fibres can be classified into two main groups according to contraction speed
- Slow twitch fibres (type I)- generate moderate force but do not fatigue - Fast twitch fibres (type II)- generate high force but fatigue quickly
How are the slow and fast-twitch fibres characterised?
Slow twitch fibres (type I): - Fatigue resistant - Moderate max force - Oxidative metabolism - Many mitochondria - Rich vascularisation - Small diameter- partly explain the force generated Fast twitch fibres (type II): - Fatigue rapidly - High max force - Glycolytic or mixed metabolism - Fewer mitochondria - Sparser vascularisation - Larger diameter
What is the myoglobin like in different types of twitch fibres?
Myoglobin in muscle cells facilitates the uptake of oxygen and makes it readily available
Slow twitch fibres (type I):
- High myoglobin (‘red muscle’)
Fast twitch fibres (type II):
- Low myoglobin (‘white muscle’)
Almost all mammalian muscles are formed of a mixture
Structural and metabolic differences
What type of muscle proteins does each of the twitch fibres have?
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 express type I myosin heavy chain
- Fast twitch fibres express type II MHC (which comes in various subtypes which are associated with the different glycolytic and oxidative capacities)
How do muscles differ in twitch fibre composition?
Muscles may vary in proportion of type I/ type II fibres:
- Lateral rectus (eye muscle) mainly type II fast twitch
- Gastrocnemius (calf muscle) mixed type I and II
- Soleus (calf) more type I slow twitch
What you find generally is that muscles you use constantly such as postural muscles tend to have more type 1 because they fatigue much slower
How is force generation in a muscle controlled?
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 muscle that it innervates
Motor unit size ranges from ~10 to many 100s of muscle fibres
A muscle m may be innervated by 10s to 100s of motor MNs
Size of MN correlates with size of motor unit, large motor neurons have larger motor units which are able to innervate larger/more muscle fibres
Muscle fibre of a motor unit are generally of the same type; slow or fast twitch
So we can control the level of force by the amount of motor units recruited and the frequency of action potentials
What is temporal summation and how does that help us control our muscles?
Temporal summation means that individual twitches can fuse together to generate a tetanus
If you have a second action potential before the tension has dropped back to zero then the twitch will partially sum with the decaying phase of the previous one so total tension will be greater
The calcium pulses return to baseline but the tension builds up in the sequence of action potentials- this is called an incomplete tetanus
As you increase action potential frequency further then the calcium pulses will begin to fuse, they wont have time to return to baseline, until eventually you have a smooth summed pulse of calcium with a summed summation of force- a tetanus force
The force generated by a motor unit can be controlled by the firing rate of its motor neuron
How does summation differ between the different twitch fibres?
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
Its easier to stimulate a smaller neuron than it is a larger neuron so they are recruited first
What is a skeletal muscle like at rest?
Most muscles at rest exhibit some low level of contractile activity sustained by low frequency firing
Denervation leads to complete relaxation (flaccid) e.g. if you cut the nerve the muscle becomes flaccid
Driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone)
Muscle spindles sense the length of a muscle
When a muscle contracts its length shortens and when the antagonist muscle contracts, the agonist muscle would tend to be stretched
What is myogenesis? How does it happen?
Myogenesis is the process of muscle development
Paracrine factors induce myogenic regulatory factors (MRFs) in mesodermal precursor cells snd so become destined for myogenic commitment (they become myoblasts)
Myoblast proliferate under influence of growth factors
Cell cycle exit, myogenin expression- terminal differentiation
They start to elongate
Structural proteins expressed, myotubes form from myoblasts
Myotubes align and fuse, becoming multinucleated muscle fibres
A portion of the cells undergoing myogenic commitment are held back and are called Satellite cells- regeneration and postnatal growth (muscle stem cells)
What happens to muscle development post-natally?
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 ration; as more nuclei are added more proteins are synthesised
Satellite cells return to quiescence when not needed
What is sarcopenia and how does it affect the elder population?
Sarcopenia- a reduction in muscle mass
Part of ageing process
Involved atrophy of muscle fibres
May be due to disease or forced immobilisation e.g. hospitalisation
Associated with decreased satellite cell number and recruitment
Anabolic resistance- there tends to be reduced protein synthesis in response to hormonal stimulation or resistance exercise
Can be resisted- importance of resistance exercise even shown in people in their 90s
