cross-bridge cycling

1. Chapter 9, Section 3
Sliding Filament Theory of Contraction

2. The Sliding Filament Model of Muscle
Contraction
During a muscle contraction
Thick (myosin) filaments and thin
(actin) filaments slide across one
another
The filaments do not change lengths
Z-bands move closer together causing
the sarcomere to shorten.
I bands appear narrow
Figure 9.11a. Individual sarcomeres shorten as
thick and thin filaments slide past one another.

3. Cross Bridge Cycling
1. When a muscle is relaxed, tropmyosin covers
the binding sites on actin.
A molecule of ADP and Phosphate remains
attached to myosin from the previous
contraction.

4. Cross Bridge Cycling
2. During a contraction, Calcium binds to troponin.
Tropomyosin is repositioned, exposing the myosin
binding sites on actin filaments

5. Cross Bridge Cycling
3. Myosin heads bind to actin filaments.
The phosphate is released.

6. Cross Bridge Cycling
4. Myosin heads spring forward “Power Stroke” pulling the actin
filaments.
ADP is released from Myosin

7. Cross Bridge Cycling
5. Myosin is released from actin.
A new molecule of ATP binds to myosin, causing it to be released from the
actin filament.
• ATP is not yet broken down, but it is essential to release the crossbridges.

8. Cross Bridge Cycling
6. ATP is broken down, providing the energy to
“cock” the myosin filaments (recovery stroke).
7. Steps 1-6 are repeated several times.

9. Figure 9.10. The cross-bridge cycle.
The cycle continues as long as ATP is
present, and nerve impulses release
Acetylcholoine.
Watch the You-Tube video
“Sliding Filament” to view
cross-bridge cycling in action.

10. Relaxation
When a nerve impulse ceases, two events relax muscle fibers.
1. Acetylcholinesterase breaks down Ach in the synapse.
• Prevents continuous stimulation of a muscle fiber.
2. Calcium Pumps (Ca2+ATPase) remove Ca2+ from the sarcoplasm
and returns it to the SR.
• Without calcium, tropomyosin covers the binding sites on actin
filaments.

11. Relaxation
Rigor Mortis is a partial contraction of skeletal muscles that occurs a few
hours after death.
• After death calcium leaks into sarcoplasm, triggering the muscle
contractions.
• But ATP supplies are diminished after death, so ATP is not available to
remove the cross-bridge linkages between actin and myosin.
• muscles do not relax*.
• Contraction is sustained until muscles begin to decompose.
* Notice that ATP is required for muscle relaxation!

12. End of Chapter 9, Section 3