Cross Bridges

Sliding of the filaments is produced by the action of numerous cross bridges that extend out from the myosin toward the actin. These cross bridges are part of the myosin proteins that extend from the axis of the thick filaments to form "arms" that terminate in globular "heads" (fig. 12.10). A myosin protein has two globular heads that serve as cross bridges. The orientation of the myosin heads on one side of a sarcomere is opposite to that of

Myosin Head Electron Micrograph

■ Figure 12.9 The sliding filament model of muscle contraction. (a) An electron micrograph and (b) a diagram of the sliding filament model of contraction. As the filaments slide, the Z lines are brought closer together and the sarcomeres get shorter. (1) Relaxed muscle; (2) partially contracted muscle; (3) fully contracted muscle.

334 Chapter Twelve

334 Chapter Twelve

Myosin Heads Release The Molecule Adp

■ Figure 12.10 The structure of myosin, showing its binding sites for ATP and actin. Once the myosin head binds to ATP, it is hydrolyzed into ADP and inorganic phosphate (P). This activates the myosin head, "cocking it" to put it into position to bind to attachment sites in the actin molecules.

the other side, so that, when the myosin heads form cross bridges by attaching to actin on each side of the sarcomere, they can pull the actin from each side toward the center.

Isolated muscles are easily stretched (although this is opposed in the body by the stretch reflex, described in a later section), demonstrating that the myosin heads are not attached to actin when the muscle is at rest. Each globular myosin head of a cross bridge contains an ATP-binding site closely associated with an actin-binding site (fig. 12.10, left). The globular heads function as myosin ATPase enzymes, splitting ATP into ADP and Pi.

This reaction must occur before the myosin heads can bind to actin. When ATP is hydrolyzed to ADP and Pi, the myosin head "cocks" (by analogy to the hammer of a gun), putting it into position to bind to actin (fig. 12.10, right).

Once the myosin head binds to actin, forming a cross bridge, the bound Pi is released. This results in a conformational change in the myosin, causing the cross bridge to produce a power stroke (fig. 12.11). This is the force that pulls the thin filaments toward the center of the A band.

After the power stroke, with the myosin head now in its flexed position, the bound ADP is released as a new ATP molecule binds to the myosin head. This release of ADP and binding to a new ATP is required in order for the myosin head to break its bond with actin after the power stroke is completed. If this process were prevented, the myosin heads would remain bound to the actin (see the Clinical box discussion of rigor mortis). The myosin head will then split ATP to ADP and Pj, and—if nothing prevents the binding of the myosin head to the actin—a new cross-bridge cycle will occur (fig. 12.12).

Note that the splitting of ATP is required before a cross bridge can attach to actin and undergo a power stroke, and that the attachment of a new ATP is needed for the cross bridge to release from actin at the end of a power stroke (fig. 12.12).

Because the cross bridges are quite short, a single contraction cycle and power stroke of all the cross bridges in a muscle would shorten the muscle by only about 1% of its resting length. Since muscles can shorten up to 60% of their resting lengths, it is

The detachment of a cross bridge from actin at the end of a power stroke requires that a new ATP molecule bind to the myosin ATPase. The importance of this process is illustrated by the muscular contracture called rigor mortis that occurs due to lack of ATP when the muscle dies. Without ATP, the ADP remains bound to the cross bridges, and the cross bridges remain tightly bound to actin. This results in the formation of "rigor complexes" between myosin and actin that cannot detach. In rigor mortis, the muscles remain stiff until the myosin and actin begin to decompose.

Muscle 335

Muscle 335

Rigor Mortis Medicine

■ Figure 12.11 The power stroke of the cross bridge. After the myosin head binds to actin to form a cross bridge, inorganic phosphate (P) is released. This causes a conformational change in the myosin head, resulting in a power stroke that produces sliding of the thin filament over the thick filament

Atp Binding Myosin Heads

(5) A new ATP binds to myosin head, allowing it to release from actin

(3) Pi is released, causing conformational change in myosin

(5) A new ATP binds to myosin head, allowing it to release from actin

(3) Pi is released, causing conformational change in myosin

Actin Myosin Cross Bridge Cycle Diagram

■ Figure 12.12 The cross-bridge cycle that causes sliding of the filaments and muscle contraction. Hydrolysis of ATP is required for activation of the cross bridge, and the binding of a new ATP is required for the cross bridge to release from the actin at the end of a cycle.

(4) Power stroke causes filaments to slide; ADP is released

(4) Power stroke causes filaments to slide; ADP is released

■ Figure 12.12 The cross-bridge cycle that causes sliding of the filaments and muscle contraction. Hydrolysis of ATP is required for activation of the cross bridge, and the binding of a new ATP is required for the cross bridge to release from the actin at the end of a cycle.

obvious that the contraction cycles must be repeated many times. In order for this to occur, the cross bridges must detach from the actin at the end of a power stroke, reassume their resting orientation, and then reattach to the actin and repeat the cycle.

During normal contraction, however, only a portion of the cross bridges are attached at any given time. The power strokes are thus not in synchrony, as the strokes of a competitive rowing team would be. Rather, they are like the actions of a team engaged in tug-of-war, where the pulling action of the members is asynchronous. Some cross bridges are engaged in power strokes at all times during the contraction.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • ellie-louise
    How Does Myosin Reattach To Binding Site?
    6 years ago
  • Hamfast
    What causes the release of adp and p from myosin heads?
    5 years ago
  • jouko leskinen
    What is needed to end cross bridge on actin?
    5 years ago
  • Ubalda
    Why must the cross bridge detach?
    5 years ago
  • Almaz
    What causes the myosin heads to bind to actin?
    5 years ago
  • Nebay Teodros
    Where do myosin heads bind to the filament?
    5 years ago
  • mewael
    How is myosin cross bridge binding prevented?
    5 years ago
  • Tobold Brandybuck
    Does atpase split atp before myosin binds to myosin binding sites?
    5 years ago
  • alasdair
    What binds with actin to prevent myosin heads from forming cross bridges?
    5 years ago
  • costantino
    Is energy needed for the myosin head to release from actin?
    5 years ago
  • luke mcintyre
    What is bound to myosin head at rest?
    5 years ago
  • florian baecker
    Does the release of crossbridges require ATP?
    5 years ago
  • JIMMY
    What part of ATP binds to myosin?
    5 years ago
  • CELENDINE
    What is released at the end of the power stroke?
    5 years ago
  • jay docherty
    Is atp needed for cross bridge attachment?
    5 years ago
  • celedor took-took
    What forms cross bridges?
    5 years ago
  • pontus haatainen
    What order for new atp binds, binding makes eay?
    5 years ago
  • SEREDIC
    What is crossbridge in physiology?
    5 years ago
  • AWET
    When does adp leave myosin power stroke?
    5 years ago
  • Eugene
    How does a cross bridge form?
    5 years ago
  • gayle
    What causes the cross bridges between myosin and actin to break?
    5 years ago
  • kevin
    When atp binds to crossbridge and is hydrolyzed?
    5 years ago
  • zula ambessa
    When skeletal muscle dies ,rigor mortis develops as myosin and actin cannot detach?
    5 years ago
  • Bonacata
    How many cross bridges does a myosin form?
    5 years ago
  • Elisabeth M
    When an atp molecule binds to the myosin head, what action occurs?
    5 years ago
  • andres troncoso
    Are the myosin heads attached to actin during rigor mortis?
    5 years ago
  • Elisa
    Where are myosin proteins located?
    5 years ago
  • Anu
    When myosin heads attach to actin this process is called?
    5 years ago
  • sabrina zimmer
    What is released from the myosin head after the power stroke occurs?
    5 years ago
  • Aleena
    When atp attaches to a myosin head?
    5 years ago
  • alfrida
    What is cocking of myosin head?
    5 years ago
  • Tanta
    How short can a sarcomere get?
    5 years ago
  • Gilda
    How does crossbridging cycle occur?
    5 years ago
  • bruna mazzanti
    Do cross bridges split atp before binding to actin?
    5 years ago
  • ernest mcclanahan
    Is atp found on the myosin heads at rest?
    5 years ago
  • Daniel Bisrat
    Is ATP attached to myosin in rest?
    5 years ago
  • halfred
    Is atp split prior to or after actin binding?
    5 years ago
  • kellee
    Are the adp and p considered atp on myosin head?
    5 years ago
  • HUGH STEVENSON
    Where are myosin proteins found?
    5 years ago
  • jussi
    What is required for the myosin heads to form a crossbridge?
    5 years ago
  • Leona
    How is rowing like a cross bridge attachment?
    5 years ago
  • sol
    What is necessary for the release of the cross bridge between myosin and actin?
    5 years ago
  • karin
    When do cross bridges detach?
    5 years ago
  • taneli
    What must happen for myosin to bind to actin?
    5 years ago
  • nora goldworthy
    What must occur before a myosin cross bridge can bind to actin?
    5 years ago
  • Heike
    Where is atp on a model of the sliding filament theory?
    5 years ago
  • Ferdinand
    When does the myosin make contact with the actin to form a crossbridge?
    4 years ago
  • paula
    What does myosin need in order to break the crossbridge with actin?
    4 years ago
  • Samsa
    What molecule is required for the crossbridge to break?
    4 years ago
  • pandora
    What is yosin activation cocking mean?
    4 years ago
  • fastolph
    What is the myosin ATPase normally bound to when at rest?
    4 years ago
  • Dominik
    What molecule is required to disconnect crossbridges?
    4 years ago
  • fethawi
    How does crossbridge cycling occur?
    4 years ago
  • TYTTI
    What is released in the process of crossbridges forming?
    4 years ago
  • blaine
    What molecule is necessary for cross bridge detachment?
    4 years ago
  • Klaudia
    Does cross bridge forming require atp?
    4 years ago
  • sebastian
    What attachment of myosin to crossbridges on actin is required?
    4 years ago
  • Anita
    What is generated when crossbridges form between actin and myosin?
    4 years ago
  • Tesfalem Tewelde
    Do cross bridges bind ATP after they detach actin?
    4 years ago
  • Amparo
    What must happen before a cross bridge can form?
    4 years ago
  • brigitte
    What is necessary for cross bridges to detach from thin filaments?
    4 years ago
  • Nicole Bayer
    What molecule causes the cross bridge to break?
    4 years ago
  • shirley deloach
    What proteins link together forming crossbridges after someone dies?
    3 years ago
  • tanta
    How ATP crossbridges extended?
    2 years ago
  • regina
    What step in the cross bridge cycle occurs after the power stroke?
    1 year ago
  • Mungo
    How heads of cross bridges bind with ATP?
    1 year ago
  • Mewael
    What happens when pi gets detached from myosin head?
    1 year ago
  • kerstin
    What is the actin status needed for cross bridge formation?
    9 months ago
  • hannele
    Do cross bridges contain ATPase?
    9 months ago
  • Bisrat Eyob
    Do cross bridges require ATP?
    7 months ago
  • Samantha Walker
    How ATP break the cross bridges of muscles?
    6 months ago
  • primrose
    What is the cause of cross bridges detaching?
    5 months ago

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