2. Muscular System Flashcards
What are the 4 determinants of muscle fiber phenotype for skeletal muscle?
- Cell lineage
- Nerve-evoked electrical activity
- Mechanical conditions
- Paracrine, autochrome, and hormones
How does cell lineage determine muscle fiber phenotype?
During embryonic development, all 3 muscles (skeletal cardiac smooth) are originated from the somatic mesoderm
Myoblasts (progenitor) originates from the somatic mesodermal germ layer and will migrate out to form phenotypic muscle fibers
Slide 3-4 sept 10
How is the date of myocytes determined once they leave the somites?
Coelomic graft model
Myoblasts start migrating from the lumbosacral somites into the chick hindlimb bud occur between stages 16 to 20
A section of graft hindlimb bud was removed between stages 16 to 18 then was placed into the coelomic cavity of host embryo and retrieved at stages 30 to 33
Slides 5-6
What happens when mammalian muscle are transported and made to regenerate in a different body location?
Some of the information determining fiber type is apparently derived from the muscle of origin rather than from the new position
What are satellite cells?
Can be found lying between the basal lamina and plasma membrane of muscle fibers
Aka myosatellite cells
Normally inactive but activated after muscle injury or in response to intensive physical exercise
Fuse among them selves to make new muscle fibers or fuss with damaged muscle fibers to repair damage (keeps same muscle type)
What can a single avian or rodent satellite cell transform into?
A single avian or rodent satellite cell can only transform into either fast or slow MyHC isoform, not both
Satellite cells derive from new myoblasts which that fused into forming multi-nucleated myotubules that express a MyHC type similar to the fiber from which the satellite cells were derived
What has satellite cell behaviour been noticed in humans?
Satellite cell from human is homogeneous in nature (single satellite cell has ability to transform itself into a muscle fiber with either fast or slow MyHC isoform
What is nerve-evoked electrical activity with muscle fibers?
External signals can change muscle phenotype in adult
When a nerve from a fast muscle is transplanted to a slow muscle (& vice versa) both the re-innervated muscles change phenotype according to the new nerve supply
What is the pattern of electrical activity evoked in fast and slow muscles?
In type I motor units, they seem to receive high amounts of impulses delivered in long low-frequency sequences
(Low frequency activity has been super imposed on normal activity leading to fast to slow transformation)
In type II motor units, they receive short bursts of high-frequency activity
What are the problems with in Vivo slow to fast transformation studies?
Exogenous activity (nerve stimulation studies) is superimposed on activity from the central nervous system (this limits pattern control) Since the external activity is always added, the effect of a reduced amount of activity can not be studied in innervated muscles as a control Slide 15 Sept 10
How does mechanical condition (stress) help determine muscle fiber phenotype?
Depolarization leads to shortening and/or mechanical tension in muscle
Believed contraction against a large resistance leads to larger muscles mass than contraction against a lower resistance
Use hind limb suspension study (rats lifted by rail so unload hind limbs leading to atrophy, initially also there is a halt in electrical activity indicated by an integrated electromyogram (EMG))
Changes in muscle usage will transform muscle phenotype
What happens in the hind limb suspension test when there is an iniatial halt in electrical activity indicated by the EMG?
The integrated EMG appears to gradually recover to normal levels within a few days where as muscle atrophy continues to progress
This could mean that the action potential activity is of relatively little importance
A more important role should be postulated for force generation per se
What are the 3 major conditions? regulating size of muscle?
The number of nuclei- increase nuclei, implies your satellite cells have fused with your muscle fibers, make it bigger, hypertrophic
The rate of protein synthesis for each nucleus
The rate of protein degradation
What is myostatin?
What happens when it’s disrupted?
Member of the trans-transforming growth factor β(TGF-β) superfamily
Stops your muscles from growing (don’t become monster) inhibitor of muscle growth
Disruption of myostatin gene leads to development of grossly enlarged muscles
Hyperplasia (increase number of fibers) and hypertrophy (increase fiber size) combine for enlargement
What is the insulin-like growth factor I (IGF-1)?
Works as local hormone that promotes hypertrophy in adult animals
In muscle tissue, IGF increases myotube diameter, suppressed proteolysis, stimulate protein synthesis, and induce a higher number of nuclei per length of myotube
Also increases DNA content in muscles
Slide 25 sept 10
