1. Copyright © 2006 Lippincott Williams & Wilkins. Muscular System Chapter 11 Part 2

2. Copyright © 2006 Lippincott Williams & Wilkins.

3. Gross Structure of Skeletal Muscle Skeletal muscles is encased by epimysium: fascia of fibrous connective tissue. Fasciculus, bundle of cylindrical muscle fibers surrounded by perimysium. Muscle fibers is cylindrical with length of few mm to many cm, contains nuclei. Endomysium surrounds muscle fiber. SarcolemmaSarcolemma beneath endomysium is thin, elastic membrane.

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5. Sarcoplasm (cytosol) is fluid portion in muscle fiber contains contractile proteins, nuclei, mitochondria, sarcoplasmic reticulum.

6. Copyright © 2006 Lippincott Williams & Wilkins. Gross Structure of Skeletal Muscle Contractile proteins of cell are myofibrils. Chemical composition: 75% water, 20% protein, 5% other. Blood supply. Milking action of rhythmic exercise; compressive force of resistive exercise retards blood flow.

7. Copyright © 2006 Lippincott Williams & Wilkins. Ultrastructure of Skeletal Muscle Sarcomere is basic functional unit. Distinguished area between Z lines. Thicker filaments confined to A band, a lighter middle region called the H zone. Thinner filaments arise in middle region of I band, at the Z line. During contraction, neither thick nor thin filaments change in length. They slide.

8. Copyright © 2006 Lippincott Williams & Wilkins. Sarcomere A band H zone I band

9. Copyright © 2006 Lippincott Williams & Wilkins. During contraction, neither myosin nor actin filaments change in length. They slide past each other. A band remains same, I band decreases. H zone decreases on contraction.

10. Copyright © 2006 Lippincott Williams & Wilkins. Ultrastructure of Skeletal Muscle Actin-Myosin Orientation Myosin filament (thick). Myosin molecules have long rod-shaped tails with 2 globular heads. The heads form cross bridges.

11. Copyright © 2006 Lippincott Williams & Wilkins. Ultrastructure of Skeletal Muscle Actin filament (thin). Actin molecules are pear-shaped double helix. Tropomyosin is a rigid, rod-shaped protein lies in groove on either side of actin. Troponin is complex of 3 globular proteins.

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14. Ultrastructure of Skeletal Muscle Tropomyosin is rigid, rod-shaped protein which lies in groove on either side of actin. Troponin is complex of three polypeptides embedded at regular intervals along tropomyosin. Binds Ca ++.

15. Copyright © 2006 Lippincott Williams & Wilkins. Ultrastructure of Skeletal Muscle Intracellular tubule system connects inner myofibrils with sarcolemma.

16. Copyright © 2006 Lippincott Williams & Wilkins. Sarcoplasm is the cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria. Sarcolemma has holes in it. Holes lead into tubes called Transverse tubules or T tubules. T tubules pass down into muscle cells and go around the myofibrils. Function of T tubules is to conduct impulses from the surface of the cell (sarcolemma) down into the cell to the sarcoplasmic reticulum. Sarcoplasmic reticulum is a bit like endoplasmic reticulum of other cells, it is hollow. Function of sarcoplasmic reticulum is to store calcium ions.

17. Copyright © 2006 Lippincott Williams & Wilkins. Sliding-Filament Theory The sliding-filament theory proposes that muscle fibers shorten or lengthen because thick and thin myofilaments slide past each other without the filaments themselves changing length. See http://thepoint.lww.com Animation: Sliding Filament Theory.http://thepoint.lww.com

18. Copyright © 2006 Lippincott Williams & Wilkins. Sliding-Filament Theory (cont’d) The myosin crossbridges, which cyclically attach, rotate, and detach from the actin filaments with energy from ATP hydrolysis, provide the molecular motor to drive fiber shortening

19. Copyright © 2006 Lippincott Williams & Wilkins. Excitation-Contraction Coupling Provides the physiologic mechanism whereby an electrical discharge at the muscle initiates the chemical events that cause activation. When stimulated to contract, Ca ++ released from SR. Rapid binding of Ca ++ to troponin in actin filaments releases troponin’s inhibition of actin-myosin inhibition.

20. Copyright © 2006 Lippincott Williams & Wilkins. Actin combines with myosin-ATP. Actin activates ATPase, which splits ATP. Energy release produces crossbridge movement (power stroke). New ATP attaches to myosin crossbridge to dissociate from actin.

21. Copyright © 2006 Lippincott Williams & Wilkins. Relaxation When muscle stimulation ceases, intracellular Ca ++ decreases as Ca ++ is pumped back in SR by active transport. Ca ++ removal restores inhibitory action of troponin-tropomyosin.