Lecture 32- Muscle plasticity: fibre types

Question 1

What changes can we make to muscles?

Answer 1

larger, smaller
stronger, weaker
faster, slower
more resistant to fatigue
repair better after injury

Question 2

there are ___ myHC species indentified in mammalian muscles fibres (could be more)

Answer 2

10

Question 3

limb muscles myHCs include

Answer 3

types 1, IIa, IIx, IId and IIb

Question 4

even very slow muscles (e.g soleus), contain a portion of ___ muscle fibres

Answer 4

fast - and this can be changed

Question 5

Is fibre composition fixed or malleable?

Answer 5

malleable - we can change it under certain circumstances

Question 6

which elements other than myHC can be altered/

Answer 6

Sr density, capillary density, mitochondria, fibre diameter, contractile and regulatory proteins, fibre composition

every element of muscle can be altered

Question 7

My HC isoforms differ in their functional properties. They are influenced by…

Answer 7

hormone levels, exercise, injury and ageing

MyHC inferred from functonal histochemical, electrophoretic and immunohistochemical analysis

Question 8

What are Myosin light chains thought to influence?

Answer 8

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

Question 9

top endurance athletes typically have what amount of type I fibres in their leg muscles?

Answer 9

greater than 80%

Question 10

clenbuterol will do what to fibre composition in rats?

Answer 10

slow fibre to fast fibre (type II)

plus hypertrophy

Question 11

Cross-reinnervation experiments gave clues as to ….

Answer 11

muscle fibre plasticity - muscles properties can change if the nerve input is reversed.

Question 12

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

Answer 12

true

Question 13

low freq stimulation =

high freq stimulation =

Answer 13

low freq stimulation = slow-twitch characteristics

high freq stimulation = fast-twitch characteristics

the total number of impulses determines the fatiguabillity of the fibres

Question 14

What is the S.A.I.D (specific adaptation to imposed demand) principles

Answer 14

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

Question 15

What are the skeletal muscle adaptions in increased use?

Answer 15

chronic electrical stimulation

chronic stretch

compensatory hypertrophy

intermittent electrical stimulations

exercise

Question 16

What are examples of potential inactivity causing disuse muscle atrophy?

Answer 16

himdlimb unweighting

immobilissation

limb (plaster) casting

prolonged bed rest

space flight (weightlessness)

ageing

denervation

Question 17

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?

Answer 17

similar but not as makred as with plaster casting

the length at which the muscle is held is important

Question 18

True or false

immobilization in the shortening position decreases muscle loss

Answer 18

False it accentuates muscle loss

immobilization in a lengthening position leads to better mass and function preservation - stretch is an important stimuli

Question 19

What are the positives of studying muscle plasticity through electrical stimulation?

Answer 19

'’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

Question 20

How does electical stimulation result in muscle plasticity?

Answer 20

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

Question 21

What are the major changes after chronic stimulation?

Answer 21

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

Question 22

With exercise we’re not stimulating the muscle continuously so the time course of changes with electicla stimulation takes ___

Answer 22

weeks

Question 23

true of false

studies have shown changes in fatiguabililty but rarely alteration in myosin isoform expression cnverting type II fibres to type I

Answer 23

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

Question 24

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

Answer 24

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

Question 25

The sequence of adaptive events is:

Answer 25

Ca2+ handling (SR is overwhelmed) —> enzyme activities –> myosin heavy chains

Question 26

True or false

Muscle metabolic enzymes, capillaries, SR and T-system are much harder to change than contractile proteins

Answer 26

False

much easier to change

Question 27

With the change from fast twitch to slow twitch the atrophy should be viewed in what context?

Answer 27

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