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

3. 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

4. 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.

5. 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

6. Skeletal Muscle Anatomy

7. Skeletal Muscle Anatomy Continued… 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)

8. Sarcomere

9. Sarcomere Continued… 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)

10. 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

11. 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

12. 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

14. 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

15. QUESTIONS