Oo Reconnect to chapter 2 Proteins page 54 Crossbridge Cycling

The force that shortens the sarcomeres comes from cross-bridges pulling on the thin filaments. A myosin cross-bridge can attach to an actin binding site and bend slightly, pulling on the actin filament. Then the head can release, straighten, combine with another binding site further down the actin filament, and pull again (fig. 9.11).

Myosin cross-bridges contain the enzyme ATPase, which catalyzes the breakdown of ATP to ADP and phosphate. This reaction releases energy (see chapter 4, p. 114) that provides the force for muscle contraction. Breakdown of ATP puts the myosin cross-bridge in a "cocked" position (fig. 9.12a). When a muscle is stimulated to contract, a cocked cross-bridge attaches to actin (9.12b) and pulls the actin filament toward the center of the sarcomere, shortening the sarcomere and thus shortening the muscle (9.12c). When another ATP binds, the cross-bridge is first released from the actin binding site (9.12d), then breaks down the ATP to return to the cocked position (9.12a). This cross-bridge cycle may repeat over and over, as long as ATP is present and nerve impulses cause ACh release at that neuromuscular junction.

Relaxation

When nerve impulses cease, two events relax the muscle fiber. First, the acetylcholine that remains in the synapse is rapidly decomposed by an enzyme called acetylcholinesterase. This enzyme is present in the synapse and on the membranes of the motor end plate. The action of acetylcholinesterase prevents a single nerve impulse from continuously stimulating a muscle fiber.

Second, when ACh is broken down, the stimulus to the sarcolemma and the membranes within the muscle fiber ceases. The calcium pump (which requires ATP) quickly moves calcium ions back into the sarcoplasmic reticulum, decreasing the calcium ion concentration of the cytosol. The cross-bridge linkages break (remember, this also requires ATP, although it is not broken down in this step), and tropomyosin rolls back into its groove, preventing any cross-bridge attachment (see fig. 9.11a). Consequently, the muscle fiber relaxes. Table 9.1 summarizes the major events leading to muscle contraction and relaxation.

Actin molecule Myosin cross-bridge Tropomyosin

Actin molecule Myosin cross-bridge Tropomyosin

Blood Vessel Filaments

- Actin filament

- Myosin filament

- Actin filament

- Myosin filament

Binding site

Ca+2

Binding site

Ca+2

Ca+2

Essentials of Human Physiology

Essentials of Human Physiology

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