Lecture 32- Muscle plasticity: fibre types Flashcards
What changes can we make to muscles?
larger, smaller stronger, weaker faster, slower more resistant to fatigue repair better after injury
there are ___ myHC species indentified in mammalian muscles fibres (could be more)
10
limb muscles myHCs include
types 1, IIa, IIx, IId and IIb
even very slow muscles (e.g soleus), contain a portion of ___ muscle fibres
fast - and this can be changed
Is fibre composition fixed or malleable?
malleable - we can change it under certain circumstances
which elements other than myHC can be altered/
Sr density, capillary density, mitochondria, fibre diameter, contractile and regulatory proteins, fibre composition
every element of muscle can be altered
My HC isoforms differ in their functional properties. They are influenced by…
hormone levels, exercise, injury and ageing
MyHC inferred from functonal histochemical, electrophoretic and immunohistochemical analysis
What are Myosin light chains thought to influence?
speed of shortening and also stabilishing the myosin head during cross-bridge cycling
There are heaps of possible combinations of mHC and Light chains in human myosin molecules - think beyond just fast and slow
top endurance athletes typically have what amount of type I fibres in their leg muscles?
greater than 80%
clenbuterol will do what to fibre composition in rats?
slow fibre to fast fibre (type II)
plus hypertrophy
Cross-reinnervation experiments gave clues as to ….
muscle fibre plasticity - muscles properties can change if the nerve input is reversed.
true or False
The type of stimuli that is sent to muscle, dictates how quickly the muscle is contracted and whether its resistant to fatigue
true
low freq stimulation =
high freq stimulation =
low freq stimulation = slow-twitch characteristics
high freq stimulation = fast-twitch characteristics
the total number of impulses determines the fatiguabillity of the fibres
What is the S.A.I.D (specific adaptation to imposed demand) principles
every structural element of msucle can be altered if given the proper stimulus
fibre type distribution, MyHC composition, fibre diameter, capillary density etc…
biochemical adaptations e.g mitochondrial ezymes
generally there are two adaptive responses of skeletal muscle that can be considered:
- increased or decreased use
What are the skeletal muscle adaptions in increased use?
chronic electrical stimulation
chronic stretch
compensatory hypertrophy
intermittent electrical stimulations
exercise
What are examples of potential inactivity causing disuse muscle atrophy?
himdlimb unweighting
immobilissation
limb (plaster) casting
prolonged bed rest
space flight (weightlessness)
ageing
denervation
In limb casting there is rapid changes in the size of the muscle, 40% of fire less in the 6 weeks.
are the changes the same with bedrest?
similar but not as makred as with plaster casting
the length at which the muscle is held is important
True or false
immobilization in the shortening position decreases muscle loss
False it accentuates muscle loss
immobilization in a lengthening position leads to better mass and function preservation - stretch is an important stimuli
What are the positives of studying muscle plasticity through electrical stimulation?
'’clean’’ way of investigating muscle adaptation
repeatable, quantifiable amount of ‘‘exercise’’
provides insights into other forms of muscle adaptation such as physical activity
has clinical application e.g cardiac assist
most extensively studied
How does electical stimulation result in muscle plasticity?
activates the ‘‘fast’’ muscle with ‘‘slow’’ muscle like freq.
fast muscle changes its metabolic and then its contractile properties to compltely transform into a slow muscle
these changes result from a transfomation of fast fibres into slow fibres
NOT from a loss of fast fibres with replacement of slow fibres through regeneration
What are the major changes after chronic stimulation?
increased capillary density
proliferation of mitochondria - increased oxidative status
decrase in SR and Ca2+ ATP ase
expression of different myosin and troponin isoforms
these changes all occur with different time courses
With exercise we’re not stimulating the muscle continuously so the time course of changes with electicla stimulation takes ___
weeks
true of false
studies have shown changes in fatiguabililty but rarely alteration in myosin isoform expression cnverting type II fibres to type I
true
stimulation in some patients with SC injury have shown some evidence of a transformation of fast to slow myosins and increases in oxidative enzymes
Within the first few hours of transformation, the SR begins to ____
within 2-12 days, incraeses in
- ___________ ____________
- ___________ ____________
these changes reflect an increased metabolic activity of the muscle
Within the first few hours of transformation, the SR begins to swell
within 2-12 days, incraeses in
- mitochondrial number, oxidative enzyme activity
- capillary blood flow, o2 consumption
these changes reflect an increased metabolic activity of the muscle
The sequence of adaptive events is:
Ca2+ handling (SR is overwhelmed) —> enzyme activities –> myosin heavy chains
True or false
Muscle metabolic enzymes, capillaries, SR and T-system are much harder to change than contractile proteins
False
much easier to change
With the change from fast twitch to slow twitch the atrophy should be viewed in what context?
a deliberate response of the muscle fibre to chronic stimulation - perhaps to decrease diffusion distances from the muscle fibre to the interstitial spaces which contain the capillaries
