1. Anatomy and Physiology
Sliding Filament Model

2. Excitation-Contraction
Coupling
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• Muscle impulses cause SR to Tropomyosin
Troponin Thin filament
release calcium ions into cytosol ADP P
+ ADP P
+
Actin monomers
• Calcium binds to troponin to
Thick filament
1 Relaxed muscle
Muscle contraction Muscle relaxation
change its shape Ca+2 Ca+2
Release of Ca from sarcoplasmic
+2
Active transport of Ca+2 into sarcoplasmic
reticulum exposes binding sites on actin: reticulum, which requires ATP, makes
• The position of tropomyosin is Ca+2 binds to troponin
myosin binding sites unavailable.
ATP
Tropomyosin pulled aside
altered Binding sites on
actin exposed
• Binding sites on actin are now Ca+2
ADP +
P
Ca+2
ADP P
+
Ca+2
exposed 2 Exposed binding sites on actin molecules
allow the muscle contraction cycle to occur
• Actin and myosin molecules bind
via myosin cross-bridges ADP P
+ ADP +
P Contraction cycle ADP +
P ADP +
P
6 ATP splits, which provides 3 Cross-bridges bind actin to
power to “cock” the myosin myosin
cross-bridges
ADP ADP
ATP ATP ATP P P
ATP ADP +P
5 New ATP binds to myosin, releasing linkages 4 Cross-bridges pull thin filament
(power stroke), ADP and P released
from myosin
2

3. The Sliding Filament Model
of Muscle Contraction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• When sarcromeres Sarcomere
shorten, thick and thin Z line
A band
Z line
filaments slide past one
another
• H zones and I bands
narrow
1 Relaxed
Thin Thick
filaments filaments
• Z lines move closer
together
2 Contracting
3
3 Fully contracted
(a)

4. Cross Bridge Cycling
• Myosin cross-bridge attaches
to actin binding site Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Myosin cross-bridge pulls
Tropomyosin
Troponin Thin filament
Actin monomers
thin filament ADP P
+ ADP P
+
Thick filament
1 Relaxed muscle
• ADP and phosphate Muscle contraction
Release of Ca from sarcoplasmic
+2
Ca+2 Ca+2
Muscle relaxation
Active transport of Ca+2 into sarcoplasmic
released from myosin reticulum exposes binding sites on actin:
Ca+2 binds to troponin
reticulum, which requires ATP, makes
myosin binding sites unavailable.
ATP
Tropomyosin pulled aside
• New ATP binds to myosin Binding sites on
actin exposed
Ca+2 Ca+2 Ca+2
ADP +
P ADP P
+
2 Exposed binding sites on actin molecules
• Linkage between actin and allow the muscle contraction cycle to occur
myosin cross-bridge break
ADP P
+ ADP +
P Contraction cycle ADP +
P ADP +
P
6 ATP splits, which provides 3 Cross-bridges bind actin to
power to “cock” the myosin myosin
• ATP splits cross-bridges
• Myosin cross-bridge goes back ATP ATP ATP P
ADP
P
ADP
ATP ADP +P
to original position 5 New ATP binds to myosin, releasing linkages 4 Cross-bridges pull thin filament
(power stroke), ADP and P released
from myosin
4

5. Animation: The Cross-Bridge Cycle
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6. Animation: Breakdown of ATP
and Cross-Bridge Movement
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operating systems, some animations
will not appear until the presentation is
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6

7. Relaxation
• Acetylcholinesterase – rapidly decomposes Ach remaining in
the synapse
• Muscle impulse stops
• Stimulus to sarcolemma and muscle fiber membrane ceases
• Calcium moves back into sarcoplasmic reticulum (SR)
• Myosin and actin binding prevented
• Muscle fiber relaxes
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8. References
• Shier, D., Butler, J., and Lewis, R. 2010.
Hole’s Human Anatomy & Physiology, New
York.
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