NEUROMUSCULOSKELETAL SYSTEMS
LECTURE 6
MUSCLE CONTRACTION
Medical Science
Dr R G Luckwill
Unstriated
muscle
Striated
muscle
Skeletal
muscle
Voluntary
muscle
Cardiac
muscle
Smooth
muscle
Involuntary
muscle
Figure 8.1
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Muscle
Tendon
Muscle fiber
(a single
muscle cell)
Connective
tissue
Figure 8.2 (1)
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Muscle
fiber
Dark A band
Light I band
Myofibril
Figure 8.2 (2)
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Z line
Portion
of myofibril
A band
M line
I band
H zone
Sarcomere
Thick filament
A band
Thin filament
Cross
bridges
M line
Myosin
Thick filament
H zone
I band
Z line
Actin
Thin filament
Figure 8.2 (3)
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I band
A band
I band
Cross bridge
Thick filament
Thin filament
Figure 8.4
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Myosin molecule
Actin binding site
Myosin ATPase site
Heads
Tail
Figure 8.5 (1)
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Thick filament
Cross bridge
Myosin molecule
Figure 8.5 (2)
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Actin molecules
Binding site for
attachment with myosin
cross bridge
Actin molecules
Actin helix
+
(See next slide)
Figure 8.6 (1)
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Thin filament
Tropomyosin
Troponin
Thin filament
Figure 8.6 (2)
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Tropomyosin
Actin
Thin filament
Cross-bridge binding sites
Troponin
Myosin cross bridge
Cross-sectional view
Actin
Troponin
Cross-bridge binding site
Tropomyosin
Actin binding site
Myosin cross bridge
Muscle fiber relaxed; no crossbridge binding because the crossbridge binding site on actin is
physically covered by the troponintropomyosin complex
Relaxed
Excited
Muscle fiber excited; released Ca2+
binds with troponin, pulling troponintropomyosin complex aside to
expose cross-bridge binding site;
cross-bridge binding occurs
Longitudinal view
Binding of actin and myosin cross
bridge triggers power stroke that pulls
thin filament inward during contraction
Figure 8.7
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Sarcomere
H zone
Z line
I band
A band
Z line
Relaxed
I band
shorter
H zone
shorter
A band
same
width
Contracted
Thick filament
Thin filament
Sarcomere shorter
Figure 8.8
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Actin molecules in thin myofilament
BINDING Myosin cross
bridge binds to actin
molecule.
Myosin cross bridge
Z line
POWER STROKE Cross
bridge bends, pulling thin
myofilament inward.
DETACHMENT Cross bridge
detaches at end of power
stroke and returns to original
conformation.
BINDING Cross bridge binds
to more distal actin molecule;
cycle repeated.
Figure 8.9 (1)
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Figure 8.9 (2)
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Thin myofilament
Thick myofilament
Figure 8.9 (3)
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Surface membrane of muscle fiber
Segments of
sarcoplasmic
reticulum
Myofibrils
Lateral
sacs
Transverse (T)
tubule
I band
A band
I band
Figure 8.10
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Desmosomes
‘T’ tubule
Lateral sac
(‘L’ tubule)
Desmosome
Ca2+
Figure 8.11 (3)
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T tubule
Terminal button
Surface membrane of muscle cell
Acetylcholine
Acetylcholinegated cation
channel
Lateral
sacs of
sarcoplasmic
reticulum
Tropomyosin
Actin
Troponin
Cross-bridge binding
Myosin cross bridge
Figure 8.12
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Energized
Resting
Binding
Detachment
Bending (power stroke)
Rigor complex
Figure 8.13
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