The document describes the sliding filament model of muscle contraction. It explains that (1) muscle impulses cause calcium ions to be released from the sarcoplasmic reticulum, which then bind to troponin and alter the position of tropomyosin, exposing binding sites on actin. (2) Actin and myosin can then bind via cross-bridges, causing contraction. (3) Muscle relaxation occurs when calcium is actively transported back into the sarcoplasmic reticulum, making the binding sites on actin unavailable again.

1. Anatomy and Physiology
Sliding Filament Model

2. Excitation-Contraction
Coupling
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• 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
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
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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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