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Skeletal Muscle: A Specialized Contractile Machine

Skeletal Muscle: A Specialized Contractile Machine. Lauren C. Pope. The Fundamentals. Interesting Facts Importance of Skeletal Muscle Structure of Skeletal Muscle What can trigger muscle contraction? Role of Tropomyosin/Troponin in Skeletal Muscle Contraction. Interesting Facts.

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Skeletal Muscle: A Specialized Contractile Machine

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  1. Skeletal Muscle: A Specialized Contractile Machine Lauren C. Pope

  2. The Fundamentals • Interesting Facts • Importance of Skeletal Muscle • Structure of Skeletal Muscle • What can trigger muscle contraction? • Role of Tropomyosin/Troponin in Skeletal Muscle Contraction

  3. Interesting Facts • Muscle is like a mechanical machine: the power output of muscle can be compared with that of an engine. • A racing car engine and an aircraft engine are only tenfold to fortyfold more powerful than a muscle. • A passenger car engine is only one and a half times more powerful than a muscle.

  4. What is the Significance of Skeletal Muscle? • Skeletal muscles connect the bones in the arms, legs and spine • Used in complex coordinated activities such as walking or positioning of the head • Generate rapid movements by contracting quickly • Hold objects immobile, such as clenching of fists or tensing of muscles • Skeletal muscle contractions occur quickly, repetitively, through long distances and with enough force to move large loads

  5. Skeletal Muscle Anatomy

  6. Skeletal Muscle AnatomyContinued… • A skeletal muscle comprises a bundle of muscle cells or myofibers • A typical cell is cylindrical, large and multinucleated (unlike smooth muscle) • A myofiber is packed with myofibrils, which are bundles of filaments that extend the length of the cell • Myofibrils are subdivided into alternating light and dark bands that are aligned along the length of the muscle cell, giving the myofiber a striated appearance • Dark bands (A bands) and Light bands (I bands)

  7. Sarcomere

  8. The sarcomere is the structural and functional unit of a skeletal muscle The segment from one Z disk to the next is termed a sarcomere During contraction, sarcomeres are shortened to 70% of their resting length. Sarcomere contains two types of filaments: thick (myosin) and thin (actin) Sarcomere Continued…

  9. What Triggers Muscle Contraction?? • Contraction is initiated by an increase in cytosolic calcium concentration • Skeletal muscle cells maintain a low cytosolic calcium level by a unique Calcium ATPase that continually pumps calcium ions from the cytosol into the SR (Sarcoplasmic Reticulum) • This activity establishes a reservoir of calcium in the SR • When a nerve impulse reaches a skeletal muscle cell, it causes a change in the electric potential across the plasma membrane • Skeletal muscle cells can rapidly convert this electric signal (depolarization) into a rise in cystolic calcium, which then initiates contraction

  10. The Role of Tropomyosin/Troponin in Muscle Contraction • Contraction is regulated by 4 accessory proteins on the actin thin filaments: tropomyosin and troponins C, I and T • The cytosolic calcium concentration influences the position of these proteins on the actin • Tropomyosin-ropelike molecule that forms a continuous chain along each actin thin filament and binds to the actin monomers • TN-C (troponin C) controls the position of tropomyosin on the surface of an actin filament

  11. Under the control of calcium and troponin, tropomyosin can occupy 2 positions on a thin filament- an “on” and “off” state • In the absence of calcium (off state), myosin can bind to a thin filament, but the tropomyosin/troponin complex prevents myosin from sliding along the thin filament-no contraction • Binding of calcium ions to TN-C triggers a slight movement of tropomyosin toward the center of the actin filament (on state) which exposes the myosin binding sites on actin • Thus calcium concentrations relieve the inhibition exerted by the tropomyosin/troponin complex and contraction occurs

  12. In Summary… • Skeletal muscle is significant for a variety of factors-generates movement, connects bones and holds objects immobile • Skeletal muscle is composed of myofibers • Sarcomere is the structural and functional unit of a skeletal muscle • Muscle contraction is initiated by an increase in cytosolic calcium concentration • In the presence or absence of calcium, the tropomyosin-troponin complex can either facilitate or inhibit muscle contraction

  13. QUESTIONS

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