W7 - Muscle structure and adaptation Flashcards
How are skeletal muscle fibres diversified?
Skeletal muscle fibres can be classified into two main groups according
to contraction speed
*Slow twitch fibres (type I) - moderate force, but do not fatigue.
*Fast twitch fibres (type II) - high levels of maximum force, do fatigue.
What are the skeletal muscle fibre classifications?
Slow twitch fibres (type I)
*Fatigue resistant
*Moderate max force
*Oxidative
*Many mitochondria
*Rich vascularization
*Small diameter
Fast twitch fibres (type II)
*Fatigue rapidly
*High max force
*Glycolytic or mixed - can rely on anaerobic respiration
*Fewer mitochondria
*Sparser vascularization
*Larger diameter - more sarcomeres in parallel, therefore a greater maximum force can be generated.
Slow twitch fibres (type I)
*High myoglobin (‘red muscle’)
Fast twitch fibres (type II)
*Low myoglobin (‘white muscle’)
Myoglobin = version of haemoglobin for muscle cells - makes oxygen more readily available. High myoglobin content together with the rich capillary supply, gives muscles a reddish appearance. White muscle is a thing eg. breast muscle because of a lower myoglobin content.
What are the structural and metabolic differences?
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
Slow twitch fibres have a higher capacity for oxidative metabolism (full oxidation of glucose to CO2 and water. This happens in the mitochondria with oxidative phosphorylation. This requires ATP - slower than glycolysis, which makes a small amount of ATP quickly.
Fast twitch fibres have a variable capacity for oxidative vs glycolytic metabolism.
Slow twitch fibres are all characterised by a very high oxidative capacity.
How do 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 (Myosin Heavy Chains).
What are the 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
What are the effects of training on fibre type?
*Long and middle distance runners: 60-70% slow (endurance athletics)
*Sprinters: 80% fast twitch - small muscles, maybe because individual fibre diameter is smaller and also lacking the explosive strength capabilities.
In untrained individuals, there would be a roughly even mix of slow to fast fibres in most muscles.
Muscle hypertrophy is the general thickening of the muscle fibres.
Eg, Resistance training seems to engage pathways that stimulate protein synthesis. At least one of the pathways involve motor signalling, enabling muscle hypertrophy. If you have more actin and myosin filaments in parallel, you will have greater force generating capacity from each muscle fibre. Muscle fibres will be larger diameter, therefore, diffusion differences will be greater etc. This makes it effective for fast twitch phenotypes.
With prolonged activity, sustained calcium, we trigger signalling pathways that encourage mitochondrial growth and expression of phenotypes to slow twitch fibres.
How is force generation 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 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
A single motor neurone branches to innovate multiple muscle fibres that is a motor unit. The size of the motor unit correlates with the actual size of the motor neurone. The ones with smaller cell bodies and thinner axons tend to innovate a smaller number of muscle fibres with in their target muscle vise versa. Muscle fibres of a motor unit are generally of the same type - either slow twitch or fast twitch, not a mixture of both. The force generated at the level of the whole muscle is due to the summation of forces generated in the individual motor units.
We can control the level of force by recruiting motor units.
Second way to control is motor neurones do not just fire a single action potential and leave it at that. They fire trains of action potentials. Action potential firing rates/ action potential frequency then generates successive twitches, which can summate.
How is force generated in a muscle?
Starts with the action potential in a motor axon, triggering an action potential in the muscle fibres (few milliseconds - longer than neuronal), propagated along the sarcolemma down into the muscle fibre along the T-tubules. This then triggers a pulse of calcium from the sarcoplasmic reticulum (10-20 milliseconds - longer).
Following the calcium release, we have the sliding filament mechanism activated, which leads to sarcomere shortening and muscle fibre shortening.
What is the temporal summation in muscle fibres?
Fusion of individual twitches generates tetanus.
Fastest = single action potential = triggers contractile machinery = twitch
This means if we have a series of action potentials, then the twitch is not allowed to die. The decaying twitch gets built up more and more increasing the force with each new AP. This also means calcium pulses will begin to fuse as the AP frequency increases further. Incomplete tetanus.
This in essence means the force generated by motor units can be controlled by the firing rate of its motor neurone. Low firing rate = low force.
Moderate - high = stronger force.
What is 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
Motor unit = motor axon + individual muscle fibres within that muscle that it innervates.
This means they can stimulate single axon and generate a twitch that is the combined twitch of all the muscle fibres within that motor unit.
Spacial summation - eg. you have 100 motor units for a muscle - what percentage of those are active? If you stimulate just a single motor axon, then only the muscle fibres of that motor unit will contract. So you’ll have stimulated summation of the forced generated by the muscle fibres of the muscle motor units that are active.
This leads onto the size principle - this is when the motor units are recruited in an orderly fashion. Smaller first, followed by faster units at higher levels of activity. Easier to stimulate the smaller neurones.
What is skeletal muscle tone? What happens to muscles at rest?
*Most muscles at rest exhibit some low level of contractile activity
*Denervation leads to complete relaxation (flaccid) eg. if you cut the nerve to the muscle.
*Driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone) - they maintain a certain level of basal activity.
Muscle spindles are made from modified muscle fibres weakly contractile and are associated with sensory acts. They sense the length of the muscle. When the muscle contracts, the length shortens when the antagonist muscle contracts, the agonist would stretch.
Force generation is controlled by trains of individual action potential, firing at variable frequencies.
What is 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)
What is postnatal muscle growth (hypertrophy)?
After birth, the muscle fibres would more or less have been formed.
* 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 - these cells can be active periodically.
What is 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
Logically, one could increase muscle mass either by increasing the size of individual muscle fibres (hypertrophy) or by increasing the number of muscle fibres (hyperplasia).
How do ageing muscles work (sarcopenia)?
*Sarcopenia: reduction in muscle mass (peak muscle mass around late 20s/ early 30s, then it slowly declines)
*Part of ageing process - can be resisted or neglected.
*Atrophy of muscle fibres - degeneration/loss. There is an issue with zero gravity that increases this.
*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 - load bearing exercise.
There is a tendency for type 1 fibres to express type 2 phenotypes in aged muscle.
