section 3, chapter 9 cross-bridge cycling

Sliding Filament Theory of Contraction

Chapter 9, Section 3

The Sliding Filament Model of Muscle Contraction

Figure 9.11a. Individual sarcomeres shorten as thick and thin filaments slide past one another.

During a muscle contractionThick (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

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.

2. During a contraction, Calcium binds to troponin.

Tropomyosin is repositioned, exposing the myosin binding sites on actin filaments

Cross Bridge Cycling

3. Myosin heads bind to actin filaments.

The phosphate is released.

Cross Bridge Cycling

4. Myosin heads spring forward “Power Stroke” pulling the actin filaments.

ADP is released from Myosin

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 cross-bridges.

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.

Cross Bridge Cycling

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.

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.

RelaxationRigor 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!

End of Chapter 9, Section 3