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Muscular System

Muscular System. Chapter 11 Part 2. Gross Structure of Skeletal Muscle. Skeletal muscles is encased by epi mysium : fascia of fibrous connective tissue. Fasciculus , bundle of cylindrical muscle fibers surrounded by peri mysium .

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Muscular System

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  1. Muscular System Chapter 11 Part 2

  2. 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. • Sarcolemma beneath endomysium is thin, elastic membrane.

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

  4. 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.

  5. 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.

  6. Sarcomere H zone A band I band

  7. 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.

  8. 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.

  9. 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.

  10. 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++.

  11. Ultrastructure of Skeletal Muscle • Intracellular tubule system connects inner myofibrils with sarcolemma.

  12. 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.

  13. 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.

  14. 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

  15. 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.

  16. 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.

  17. 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.

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