MSS: Muscle Structure and Adaptation Flashcards
What is muscle formed from?
somites (paired blocks of paraxial mesoderm on either side of the notochord)
Somitogenesis
formation of somites
How do somites form?
by paracrine signalling from the neural tube and notochord triggering a mesenchymal-to-epithelial transition of the paraxial mesoderm, forming a hollow ball of epithelial cells (epithelial somite)
What happens to the epithelial somite?
due to other paracrine signalling, epithelial somite further subdivided into 4 major cells which go on to form specific tissue types:
- sclerotome
- myotome
- syndetome
- dermomyotome
Sclerotome forms…
bone, ribs and cartilage
Myotome forms…
muscle precursors (which form muscle)
Syndetome forms…
tendons
*syndetome is in between myotome and sclerotome
Dermomyotome forms…
the new source of muscle cells that later populate the myotome and give rise to the dorsal dermis
myogenesis.
the process by which embryonic mesoderm cells of the myotome become muscle tissue
Steps of myogenesis
1) Paracrine factors signal for the mesodermal cells to produce myogenic regulatory factors Myf5 and MyoD, which commit those cells to a myogenic fate and become myoblasts
2) Myoblasts then differentiate and proliferate in presence of growth factors until they exit the cycle after expression of another myogenic regulatory factor Myogenin
3) Myogenin causes terminal differentiation of muscle fibres and differentiates the myoblasts into myotubes, and structural proteins start being expressed (e.g. myosin + actin)
4) Myotubes align and fuse together to form multinucleated muscle fibres.
Why is muscle development biphasic?
After the initial formation of large primary muscle fibres, smaller secondary muscle fibres then form on their surfaces which make up the bulk of the muscle
Satellite cells
third group of muscle cells (primary and secondary fibres, satellite cells)
- they are undifferentiated muscle precursors and are self-renewing.
- muscle stem cells which sit dormant on muscle fibres until they are activated in the case of regeneration and postnatal growth where they can start dividing and forming myotubes which then fuse to form the muscle fibres
Embryonic muscle fibre number
at the end of embryogenesis, the number of muscle fibres that you have are what you have for the rest of your life (muscle fibre number is therefore set from birth and this is genetically determined)
-HOWEVER can be manipulated during embryogenesis
What affects fibre number during embryogenesis?
Our muscle fibre number is set at determined; thus, it is genetically determined.
However, the fibre number can be affected by:
- temperature
- hormones
- nutrition
- innervation
These affect myogenic regulatory factor (MRF) expression e.g. My5, MyoD and Myogenin
Fibre number is increased by…
Fibres increase in mass by…
hyperplasia
hypertrophy (increasing muscle mass postnatally)
Postnatal Hypertrophy (increase in skeletal muscle mass)
After birth, an increase in muscle mass is due to an increase in fibre size (hypertrophy).
muscle fibres require more protein and therefore muscle stem cells (satellite cells) start dividing and making myotubes which fuse to make muscle fibres, producing more structural proteins
-increases cross-section and size of the muscle fibre
They maintain the cytoplasm: nuclei ratio in the muscle fibre.
Why are muscle fibres multinucleated?
This is done to supply the increased production of structural proteins for the proper functioning of the muscle fibre.
Also, the muscle fibres require a lot of mitochondria, and a lot of the genes needed for mitochondria production is found in the nuclei.
Variability between muscles
all sarcomere structure is the same, however, there is a lot of molecular variability between muscles due to multiple isoforms of myofibrillar proteins produced by alternative splicing or promoters
Give some examples of myofibrillar protein isoforms, and what differs between them.
MYOSIN isoforms:
- different chemo-mechanical transduction
- ATP hydrolysis
- more rapid/speed of contraction
TROPONIN and TROPOMYOSIN isoforms:
- sensitivity to Ca2+
TITIN isoforms:
- elastic properties
Which isoforms contribute to resistance to fatigue?
Myosin and Troponin isoforms
Types of Muscle Fibre
Type I Muscle Fibres (slow twitch)
Type II Muscle Fibre (fast twitch)
List the differences between Type I and Type II muscle fibres.
TYPE I:
- -produce slow maintained contraction which doesn’t easily fatigue (virtually inexhaustible)
- high mitochondria (aerobic metabolism and oxidative phosphorylation)
- more predominant in long-distance runners
- extensive blood supply and abundant myoglobin (Red muscle)
- Gastrocnemius (calf muscle) & Soleus (calf)
TYPE II:
- fatigue easily
- -fewer mitochondria and undergo mainly anaerobic respiration in a glycolytic nature (anaerobic metabolism)
- more predominant in sprinters
- poor vascularisation and lack of myoglobin (‘white muscle’)
- found in Lateral rectus (eye muscle) and Gastrocnemius (calf muscle)
Myosin Gene Cluster
there are 11 myosin heavy chain genes clustered on chromosome 17 which allow for the different isoforms which give the different properties to the muscle fibres.
main types of myosin in adults:
- Fast IIX fibres
- Fast IIA fibres
- Fast IIB fibres
- Slow muscle (Type-I/β) and heart
List some effects of training specific muscle fibre types.
Untrained individuals have a 1:1 ratio of fast (IIA and IIX) to slow (I) twitch fibres.
- long and middle distant runner have about 60-70% slow twitch
- sprinters have about 80% fast twitch
- trainees for sports that require the greatest aerobic and endurance capacities have slow muscle up to 90-95%
- trainees for sports that require greater anaerobic capacities (strength and power) have fast muscle around 60-80%
