Muscle Structure + Adaptation Flashcards
(40 cards)
What are the origins of skeletal muscle?
Where does muscle form from?
Muscle forms from somites (paraxial mesoderm)
Paracrine signalling from notochord and neural tube triggers mesenchymal epithelial transition
The somite forms resulting in a hollow ball of epithelial cells, further paracrine signalling allows formation of four different groups of cells
- 1) Sclerotome (bone, ribs and cartilage)
- 2) Myotome (muscle precursors)
- 3) Dermomyotome (myotome and dorsal dermis)
- 4) Syndetome (tendons)
What are the paracrine factors which induce myogenic commitment to myoblasts?
Myf5 and MyoD
Briefly summarise myogenesis
- Muscle cells start as mesodermal cells
- Paracrine factors induce Myf5 and MyoD
- Expression of paracrine factors cause them to become commited to a myogenic fate = MYOBLAST
- MYOBLAST differentiate, increase in number and exit cell cycle
- Structural proteins expressed and myotubes form
- Myotubes align + fuse to form a muscle fibre
What is biphasic muscle development?
initial primary round of muscle differentiation which then allows other secondary muscle fibres to form from that
Satellite cells
Muscle stem cells
- Sit dormant on muscle fibres until they are activated allowing regeneration and postnatal growth
What is the embryonic fibre number?
Fibre number generally set at birth
- genetically determined
How can the embryonic fibre number be affected?
- Temperature
- Hormones
- Nutrition
- Innervation
These have an effect on myogenic regulatory factors (MRF) in expression duration
What is hypertrophy?
Increase in fibre size
- Muscle stem cells (MuSCs) called satellite cells. Undifferentiated muscle precursors, self-renewing
- MuSCs proliferate and incorporated into muscle fibres. Return to quiescence when not needed
Why is it important to maintain the cytoplasm to nuclei ratio?
Muscle fibres are multinucleated
Nuclei along cell are required to produce proteins required for the muscle to function, (the muscle fibre requires many mitochondria and structural proteins along the length of the muscle cell)
What is hyperplasia?
Increase in cell/fibre number after birth
In what way is muscle mass increase during exercise?
Hyperplasia or hypertrophy?
Muscle mass is increased through hypertrophy and not hyperplasia - however instances where muscle fibre number is increased through hyperplasia
What supports the theory for hyperplasia?
AVIAN STRETCH MODEL
- Weight on wings
- Development of muscle on their backs
- Count number of muscle fibres in quail
- Increase in fibre number
CAT WEIGHT LIFTING MODEL
- Cat wrist flexion
- Increase in fibre number in paw required to lift weight
What are the proposed mechanisms for hyperplasia occuring?
Fibre splitting = muscle itself splits and hypertrophy occurs to increase muscle mass
Proliferation of Satellite Cells = form their own muscle fibres
Evidence for this is lacking as difficult to remove muscle from humans and count fibres = ethically doesnt seem accurate
What is the diversity in the muscle fibre type?
- Vertebrate sarcomere structure the same
- But molecular variabillity depending on function e.g heart, smooth and cardiac muscle
- This can be due to:
- Multiple isoforms of myofibrillar proteins
- Titin isoforms
- Troponin and Tropomyosin isoforms
- Myosin isoforms
- This can be due to:
- But molecular variabillity depending on function e.g heart, smooth and cardiac muscle
What is the purpose of the different isoforms?
- Titin = elastic properties
- Troponin and tropomyosin Isoforms = determine sensitivity to Ca2+
- Myosin isoforms = different chemo mechanical transduction, ATP hydrolysis, shortening velocity leading to resistance to fatigue
What are the two different types of muscle fibres?
- Slow twitch muscle fibre
- Fast twitch muscle fibre
What are the properties of slow twitch muscle fibres?
- Virtually inexhaustible
- High number of mitochondria - aerobic respiration
- Extensive blood supply and abundant myoglobin (dark in colour)
- MYOGLOBIN = acts as an oxygen storage unit providing oxygen to the working muscles
What are the properties of fast twitch muscle fibres?
- Fatigue easily
- Fewer mitochondria - mainly undergo anearobic metabolism
- Glycolytic in nature (follows from anearobic resp)
- Poor vascularisation + lack of myoglobin (hence white colour due to lack of myoglobin)
What parts of the chicken have slow and fast twitch muscle?
- Dark meat (contains myoglobin) e.g chicken legs contains slow twitch fibres
- Used for walking and standing which they do most of the time + doesnt use much energy
- White meat (poor vascularization + lack of myoglobin) e.g wings and breast
- Doesnt use much energy, brief bursts of flight, requires lots of energy and muscle tires fast
Describe the split in fibre type due to affect of training?
- Untrained individuals 50:50 ratio of fast (IIA and IIX) to slow (I) twitch fibres
- Long and middle distant runners: 60-70% slow
- Sprinters: 80% fast twitch
- Sports requiring greatest aerobic and endurance capacities: slow muscle up to 90-95%.
- Sports with greater anaerobic capacities (strength and power) have fast muscle from 60-80%
What type of muscles does a marathon runner have?
Adapted to her environment
- Muscles small but fatigue resistant
- Muscle dense and strong for their size High oxidative capacity of muscles
- Work over very long periods of time
- Not explosive strength (difficult for marathon runners to have the energy to perform the last high explosive strength at the end)
What type of muscle does a sprinter have?
Muscles adapted for explosive release of force:
- Rapid powerful contractions
- Easily fatigued at maximum effort
- Low oxidative capacity via mitochondria
- High force per cross-sectional area of muscle (more hypertrophic)
What type of muscles does a powerlifter have?
Muscle adapted for immense strength:
- Muscles are hypertrophied even more than sprinter
- Higher glycolytic content
- Fatigue easily
- High muscle to total body mass ratio
- Muscle size beginning to interfere with locomotion
Describe the myosin gene cluster
Myosin sits on the chromosome in a cluster of myosin genes this allows for the different isoforms which give the different properties to the muscle fibres
