Muscle fibre heterogeneity and performance Flashcards
composition of endurance athlete
- low muscle mass
- slow twitch muscle composition
- low power output maintained
- high fatigue resistance
- adapted CV and respiratory systems
composition of sprinters
- large muscle mass
- fast twitch muscle composition
- high power output that is instantaneous
- low fatigue resistance
muscle classifications (new)
- colour
- twitch characterisitics
- twitch & fatiguability
- contractile & metabolic
- myosin ATPase activity
- molecular motor
early muscle classifications
- colour
- twitch characterisitics
- twitch & fatiguability
- contractile & metabolic
different muscle colours
red = slow, high myoglobin content white = fast, low myoglobin content
what gives muscle its colour
myoglobin content
colour of fast muscle
white
colour of slow muscle
red
which muscle has high myoglobin content
slow, red
what determines the speed of a twitch
- activation caused by release of calcium from SR
- cross-bridge kinetics of myosin ATPase and isoform composition
can you activate one muscle fibre
no, you can activate one motor unit
characteristics of slow fibre
has not time to relax = fused twitches at lower frequenies
characteristics of fast fibre
has time to relax giving unfused at higher frequencies
how do fast and slow fibre twitch force compare
high frequency of stimulation is needed of fast fibres to give some force as slow fibres
different types of twitch fatiguability
based on selective activation of motor units
- slow-fatigue resistant SFR
- fast-fatigue resistant FFR
- fast-fatiguable FF (force is reduced sooner)
contractile and metabolic classification
based on: - speed of contraction - substrate and enzyme characteristics called: - slow twitch - fast twitch oxidative - fast twitch glycolytic
different types of metabolic enzymes for contraction
oxidative (HAD, SDH, CS)
glycolytic (LDG, PFK, PHOS)
enzymes of slow twitch
high oxidative
low glycolytic
enzymes of fast twitch oxidative
medium oxidative
medium glycolytic
enzymes of fast twitch glycolytic
low oxidative
high glycolytic
myosin ATPase activity characterisation
low [ATPase] in slow fibres
high [ATPase] in fast fibres
acid inhibits fast fibres
base inhibits slow fibred
using ATPas histochemistry with pre-incubation at different pH to identify fibre types
- slow are dark at low ph, light at high pH
- fast A and B are dark at high and light at pH 4.4
- fast A is dark at 4.6
- fast B is light at 4.6
molecular motor classification
heavy chain myosin isoforms
MHC-I
MHC-IIA
MHC-IIX
Three overall groups of muscle fibres
slow
fast
fastest
SLOW
- colour
- twitch characterisitics
- twitch & fatiguability
- contractile & metabolic
- myosin ATPase activity
- molecular motor
- colour = red
- twitch characterisitics = ST
- twitch & fatiguability = ST
- contractile & metabolic = SO
- myosin ATPase activity = Type I
- molecular motor = MHC-I
FAST
- colour
- twitch characterisitics
- twitch & fatiguability
- contractile & metabolic
- myosin ATPase activity
- molecular motor
- colour = white
- twitch characterisitics = FTa
- twitch & fatiguability = FFR
- contractile & metabolic = FO
- myosin ATPase activity = Type IIa
- molecular motor = MHC-IIa
FASTEST
- colour
- twitch characterisitics
- twitch & fatiguability
- contractile & metabolic
- myosin ATPase activity
- molecular motor
- colour = white
- twitch characterisitics = FTB
- twitch & fatiguability = FF
- contractile & metabolic = FG
- myosin ATPase activity = Type IIb
- molecular motor = MHC-IIx
- colour = white
- twitch characterisitics = FTa
- twitch & fatiguability = FFR
- contractile & metabolic = FO
- myosin ATPase activity = Type IIa
- molecular motor = MHC-IIa
fast muscle fibre
- colour = white
- twitch characterisitics = FTB
- twitch & fatiguability = FF
- contractile & metabolic = FG
- myosin ATPase activity = Type IIb
- molecular motor = MHC-IIx
fastest muscle fibre
- colour = red
- twitch characterisitics = ST
- twitch & fatiguability = ST
- contractile & metabolic = SO
- myosin ATPase activity = Type I
- molecular motor = MHC-I
slow muscle fibre
why are there difference in metabolic and contractile functions of fibre types
muscles with different functional demands have different compositions
- some muscles need fatigues resistant
- some muscles need rapid power
what determines expression of fibre types
genetics
sprinter fibre type
primarily TII
endurance athlete fibre type
primarily TI
problem with removing muscle fibres for analysis
muscle fibres can be removed and single fibres teased out but by removing them from tendons, there is no longer a membrane potential for them to be activated for contraction
what maintains membrane potential of muscle
Na+ K+
alternative method for analysing muscle fibres
chemically skinned and then activated by immuring in a solution of calcium
- force is normalised at the cross section of the fibre for relative comparison
method of analysing function of myosin
in vitro motility assay
- speed of actin filament sliding on a rail of myosin is calculated
why is the speed of a rat fibre faster than human
- rat sarcomere is same length as a human sarcomere
- humans gain power from having large muscles with lots of sarcomeres in parallel
- rats rely on high velocity sarcomeres in series because they have small muscles
- different muscle compostions will have different optimal velocities to produce the sam power
when all other things are equal, what does the arrangement of sarcomere determine?
- force (parallel gives large CSA)
- velocity (in series gives length)
- power (total sarcomere gives volume)
why does muscle power reduce with ageing
- loss of MCH-II isoforms
older muscle has less fast fibres following selective atrophy and hypertrophy
what modulates force from a whole muscle
- motor unit recruitment
- rate of firing of individual motor unit (frequency)
Henneman’s size principle
order of recruitment
- muscle fibres in a single motor unit are all the same size
- motor units are recruited on the basis of the size of their nerve cell body (smallest to largest)
- TI recruited first then TIIa, TIIx
are motor units recruited on basis of their size
yes, but of their nerve cell body size (smallest to largest)
not the size or number of fibres in the motor unit
what gives a muscle good endurance
oxygen delivery
oxygen utilisation
what determines a muscle’s oxygen utilisation
mitochondrion
what gives muscles oxygen delivery
capillary supply
what fibres favour endurance and why
Type I
- high mitochondrial density
- high capillary density
- high myoglobin content
- high TAG content
- high oxidative enzyme activity
how does exercise effect muscle fibre type
we cannot change fibre type but we can improve endurance through increased mitochondria and improved capillary performance
we can train to improve the metabolism of fibres
three essential muscle fibre protein isoforms
Type I
Type IIa
Type IIx
(Type IIb in rodents)
why doe fibres differentiations in mechanical properties
myosin heavy chain isoform composition
range from slow to fast
I -> IIa -> IIx
how are TI designed for endurance (summary)
metabolic properties of TI designed for more oxidative metabolism because of their higher mitochondrial content
what what does [ATPase] tell about fibre type
ATPase activity postively correlated with muscle contraction and so interpreted for contraction speed
- during the cross-bridge cycle, the myosin molecule itself binds and hydrolyzes ATP during force generation
why are muscle fibres preincubated for ATPase analysis
- acid inhibts ATPase actiity in fast fibres but not slow fibres
- basic condtions inhibit ATPase activity in slow fibres bt now fast fibres
= dark colour shows which fibre types are present
how does acid affect ATPase acitivty
inhibts it in fast fibres only
therefore dark colour in acid = slow fibres present
how do basic conditions affect ATPase acitivity
inhibits it in slow fibres only
therefore dark colour in basic conditions = fast fibres present
