BS277 Biology of Muscle.
Fibre Types
Objectives.
After this lecture and associated reading you
should be able to;
• Discuss the relative contributions of genetics and
environment to muscle fibre isotype.
• Discuss the molecular basis of fibre isotype
structure and function.
Twitch characteristics of different (phasic)
muscles reflect fibre type composition.
Note that most muscles contain mixtures of fast (majority) and slow fibres.
As a consequence of twitch
time course, a fast muscle
requires a higher frequency
of stimulation to develop
tetanus tension.
Force generated during
500-ms stimulation at
different frequencies
(isolated mouse
muscles, 25°C).
A, soleus;
B, extensor digitorum
longus (EDL).
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Contraction speed
(Tension)
Fatigue resistance
Colour
Myoglobin content
Capillary supply
Mitochondria
[Glycolytic enzyme]
Substrates used for
ATP generation
Slow
Intermediate
Fast
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension)
Fatigue resistance
Colour
Myoglobin content
Capillary supply
Mitochondria
[Glycolytic enzyme]
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
Colour
Myoglobin content
Capillary supply
Mitochondria
[Glycolytic enzyme]
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Myoglobin content
Capillary supply
Mitochondria
[Glycolytic enzyme]
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Red
White
White
Myoglobin content
Capillary supply
Mitochondria
[Glycolytic enzyme]
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Red
White
White
Myoglobin content
High
Low
Low
Capillary supply
Mitochondria
[Glycolytic enzyme]
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Red
White
White
Myoglobin content
High
Low
Low
Capillary supply
Dense
Intermediate
Scarce
Mitochondria
[Glycolytic enzyme]
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Red
White
White
Myoglobin content
High
Low
Low
Capillary supply
Dense
Intermediate
Scarce
Mitochondria
Many
Intermediate
Few
[Glycolytic enzyme]
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Red
White
White
Myoglobin content
High
Low
Low
Capillary supply
Dense
Intermediate
Scarce
Mitochondria
Many
Intermediate
Few
[Glycolytic enzyme]
Low
High
High
Substrates used for
ATP generation
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Red
White
White
Myoglobin content
High
Low
Low
Capillary supply
Dense
Intermediate
Scarce
Mitochondria
Many
Intermediate
Few
[Glycolytic enzyme]
Low
High
High
Substrates used for
ATP generation
Lipids, CHO,
aas (aerobic)
Primarily
CHO (anaerobic)
CHO
(anaerobic)
TABLE 10-3 Properties of Skeletal Muscle Fibre Types
Property
Slow
Intermediate
Fast
Contraction speed
Slow
Fast
Fast
(Tension
Low
Intermediate
High)
Fatigue resistance
High
Intermediate
Low
Colour
Red
White
White
Myoglobin content
High
Low
Low
Capillary supply
Dense
Intermediate
Scarce
Mitochondria
Many
Intermediate
Few
[Glycolytic enzyme]
Low
High
High
Substrates used for
ATP generation
Lipids, CHO,
aas (aerobic)
Primarily
CHO (anaerobic)
CHO
(anaerobic)
Type IIa, FOG
FR (fast resistant)
Type IIb/x,
FAG, FF
(Fast fatigue)
Alternative names
Type I, S (slow),
SO (slow oxidative)
The molecular basis for fibre isotype
differences.
2xMHC 200kD
4xMLC 20kD
S1 ATPase and actin binding.
S2 aggregation and hinge.
300 mols per thick filament
Thin myofilament is actin
+ regulatory proteins
(Regulatory)
MHC
(essential)
The myosins from different species with different speeds
of muscle contraction break down ATP at different rates.
i.e. the contraction speed of muscle depends on the enzyme
activity of the actomyosin. Fast muscle uses ATP quickly.
From Barany, 1967. In Jones et al 2004
The ATPase activity of the myofibril is a
property of the kind of myosin it contains.
Isotypic variation in myosin
• Myosins within species also differ between fibres
with different shortening velocities.
• Human has 4 heavy chain isotypes.
• Type 1 (slow)
• Type 2 (fast)
– Type 2a
– Type 2x (b)
– Type 2c (Embryo and regenerating fibres)
Histochemical identification of
myosin isotypes.
• Type 1 (slow) inactivated by preincubation at pH 9.4
• Type 2 (fast)
– Type 2a inactivated by pre-incubation at 4.3-4.6
– Type 2x (b) activity reduced by pre-incubation at
4.3-4.6
– Type 2c (Embryo and regenerating fibres)
Preincubation
pH 4.6
pH 9.4
Fibre type
1
2a
2x






Isotypic variation in myosin
expression
• Imunohistochemistry can detect structural
differences between MHC isotypes.
– About 3% of fibres express both type 1 and type 2
MHC.
– Up to 40% can express both 2a and 2x.
• 2 light chains for each heavy chain; essential and
regulatory (only regulate in smooth muscle).
– Light chains also have fast and slow isotypes.
Isotypic variation in other
myofibrillar proteins
• Actin has skeletal and cardiac forms but no differential
association with fast / slow fibre isotypes.
• Tropomyosin is an αβ dimer.
– Can be homo or hetero dimeric (αα; ββ; αβ)
– α chain has fast and slow forms.
– Fast fibres tend to have ββ and the fast form of α
• Troponin
– TnC, TnI(inhibitory), TnT(tropomyosin binding)
– All have fast and slow forms.
Schaffiano, S. and Reggiani, C. (1996) Physiol. Rev 76, 371-421.
Methods of fibre typing
• Biopsies in humans; cryostat sections cut.
• Enzyme histochemistry
– myosin ATPase
• Fast; inactivated at pH 4.3-4.6.
• Slow; inactivated by pH 9.4.
– ‘glycolytic’ enzymes
•
•
•
•
LDH
-glycerophosphate dehydrogenase
PFK
glycogen (myo)phosphorylase
– ‘aerobic’ enzymes
• Kreb’s cycle
–
Succinate dehydrogenase (SDH)
• Lipid / ketone metabolising enzymes
– beta hydroxybutyrate dehydrogenase
– Non-specific esterase
• Immunohistochemistry
– Myosin heavy/light chains
Identification of human skeletal muscle
fibre isotype by enzyme histochemistry.
(From Jones et al 2004)
Micrograph shows sequential transverse sections of
quadriceps stained for myosin ATPase (pH 9.4;
inactivates type 1 myosin)), NADH transferase
(complex 1; oxidative) and (myo)phosphorylase.
Identification of human skeletal muscle fibre isotype
by enzyme histochemistry.
(From Jones et al 2004)
Enzyme profiles from serial sections stained using different
substrates, identify clusters of properties that differentiate between
fibre types.
SDH
= succinate dehydrogenase (‘aerobic’)
-GPDH = glycerophosphate dehydrogenase (glycolytic)
Proportions of Type 1 (slow)
fibres in different populations
(From Spurway, 2006)
Motor units.
Contractile characteristics of the main types of motor unit.
Upper traces are single twitches; lower traces show the fatigue
curves during repetitive tetanic stimulation.
Note the differences in force, speed and fatigability.
Tension
(One stimulus)
(Repeated stimuli)
( number of fibres)
Contractile properties
of motor units.
The size, speed and
fatigue resistance of
individual motor units,
correlates with fibre type
as defined by
histochemical properties.
Neural determination of fibre
isotype.
•
•
•
•
•
•
•
Similarity of fibres within a unit suggest a neural influence.
Cross-innervation changes phenotype (Buller et al, 1960).
Denervated/tenotomised postural muscle becomes faster
Electrical stimulation at 10Hz  type 1.
Brief, infrequent 40 Hz  type 2X
More frequent 40 Hz  type 2A
Frequent stimulation elevates Ca++ that regulates transcription
factor (NFAT) activity via calcineurin and NFAT
dephosphorylation. (Chin, 1998)
• Inheritance of fibre type via motor neurones?
Other factors.
• If the nervous system does not develop, fibre types still
differentiate.
• Steroids effect fibre size, but [thyroid hormone]
influences fibre type (higher T3/4, faster isoforms).
– Interaction between muscle activity and hormones. More active
muscles more susceptible to hormone influence
• Stretch ( tension) fast-slow transformation.
Overview of differences
between athletes.
• % of type 1:type 2 determined genetically (motor
neurones? Myoblasts?).
• Proportions of type 2A and type 2X influenced by training
– (high frequency of recruitment induces 2X  2A, but not type 2
 type 1).
• Frequency of recruitment influences hypertrophy (type 2
> type 1).
• Frequency of recruitment influences levels of metabolic
enzymes.
• Training intensity and volume influence hormones;
synergistic effect with direct influence on muscles?
Luquet, S. et al. (2003). Peroxisome proliferator-activated receptor δ controls muscle development
and oxidative capability. FASEB J. 17, 2299-2301
PPAR is a ligand activated transcription factor.