Muscle Contractility: An In-Depth Exploration

1. Hair cells, actin bundles, andunconventional myosins

2. Hair cells, actin bundles, andunconventional myosins

3. Hair cells, actin bundles, andunconventional myosins

4. 9.6 Muscle Contractility (1) • A skeletal muscle fiber is a multinucleate cell as a result of fusion of myoblasts in the embryo. • A single muscle fiber is large and highly organized. • Each muscle fiber contains hundreds of cylindrical strands called myofibrils.

5. Structure of skeletal muscle

6. Muscle Contractility (2) • Each myofibril consists of a repeating array of sarcomeres. • Each sarcomere has a banding pattern that gives muscle fiber a striated appearance. • Banding pattern: • Thin filaments (I and A bands) • Thick filaments (H and A bands)

7. The sarcomere

8. Muscle Contractility (3) • The sliding Filament Model of Muscle Contraction • Skeletal muscle works by shortening fibers. • A bands remain constant in length. • H and I bands decrease in width. • Z lines on both ends of sarcomere move inward.

9. The shortening of sarcomere duringmuscle contraction

10. Muscle Contractility (4) • The Composition and Organization of Thick and Thin Filaments • Thin filaments contain actin as well as tropomyosin and troponin. • Tropomyosin occupies the gap between two actin molecules. • Troponin molecules are in contact with both actin and tropomyosin.

11. Molecular organization of the thin filament

12. Muscle Contractility (5) • The third most abundant protein of skeletal muscles is titin. • Titin filaments are elastic and prevent the sarcomere from being pulled apart during muscle stretching.

13. The arrangement of titin molecules within the sarcomere

14. Muscle Contractility (6) • The Molecular Basis of Contraction • During contraction, myosin heads bend thus sliding the thin filaments over the thick filament. • Energy released from ATP hydrolysis induces a conformational change within the head. • Elongated myosin neck acts as a “lever arm”. • Attached actin filament slides a much greater distance than would be possible.

15. Model of the swinging lever arm of myosin II

16. Muscle Contractility (7) • The Energetics of Filament Sliding • Energy is provided by ATPase activity in the myosin head. • Activated myosin attaches to actin initiating the power stroke. • Release of bound ADP is followed by binding of another ATP. • Absence of ATP prevents dissociation of cross-bridges causing rigor mortis.

17. Model of the actino-myosin contractile cycle

18. Muscle Contractility (8) • Excitation-Contraction Coupling • Contact between nerve and muscle is called the neuromuscular junction. • The linking of the nerve impulse to the shortening of the sarcomere is referred to as excitation-contraction coupling. • Action potential in muscles is propagated into the cell interior by transverse (T) tubules.

19. The functional anatomy of a muscle fiber

20. Muscle Contractility (9) • Excitation-contraction coupling (continued) • T tubules terminate near the sarcoplasmic reticulum (SR), which stores Ca2+. • In a relaxed sarcomere, Ca2+ levels are low. • An action potential opens calcium channels in the SR, releasing Ca2+. • Binding of Ca2+ to troponin causes a conformational changes, shifting tropomyosin and exposing the myosin binding site.