Muscle contraction

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Muscle contraction

  1. 1. RK Goit, Lecturer Department of Physiology
  2. 2.  contraction occurs by a sliding filament mechanism  Z discs have been pulled by the actin filaments up to the ends of the myosin filaments  caused by forces generated by interaction of the cross-bridges with the actin filaments  when an action potential travels along the muscle fiber, this causes SR to release Ca++  Ca++ activate forces between myosin & actin filaments  energy comes from ATP molecule
  3. 3. Myosin Filament  composed of multiple myosin molecules (200)  myosin molecule is composed of six polypeptide chains—two heavy chains, & four light chains  two heavy chains wrap spirally around each other to form a double helix, called a myosin tail  one end of each of these chains is folded bilaterally, called a myosin head  four light chains are also part of the myosin head
  4. 4.  tails of the myosin molecules bundled together to form the body of the filament  part of the body of each myosin molecule hangs to the side along with the head- arm  protruding arms & heads together are called cross- bridges  each cross-bridge is flexible at two points called hinges  where the arm leaves the body of the myosin filament  where the head attaches to the arm
  5. 5. ATPase Activity of the Myosin Head  myosin head functions as an ATPase enzyme  this property allows the head to cleave ATP
  6. 6. Actin Filament  composed of: actin, tropomyosin, & troponin Actin  actin filament is a double stranded F-actin protein molecule  F-actin helix is composed of polymerized G-actin molecules  attached to each one of the G-actin molecules is one molecule of ADP  these ADP molecules are the active sites
  7. 7. Tropomyosin  molecules are wrapped spirally around the sides of the F-actin helix  in the resting state, lie on top of the active sites of the actin strands Troponin  are actually complexes of three loosely bound protein subunits  troponin I for actin  troponin T for tropomyosin  troponin C for Ca++
  8. 8.  active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin-tropomyosin complex  in the presence of large amounts of Ca++, the inhibitory effect of the troponin-tropomyosin on the actin filaments is itself inhibited  when Ca++ combine with troponin C, the troponin complex undergoes a conformational change  this “uncovers” the active sites of the actin, thus allowing these to attract the myosin cross-bridge heads & cause contraction to proceed
  9. 9. “Walk-Along” Theory (ratchet theory) of Contraction when a head attaches to an active site, causes the head to tilt toward the arm (power stroke) & to drag the actin filament along with it ↓ head automatically breaks away from the active site ↓ it combines with a new active site ↓ then the head tilts again ↓ pulling the ends of two successive actin filaments toward the center of the myosin filament
  10. 10. ATP as the source of energy for contraction  the heads of the cross-bridges bind with ATP  ATPase activity of the myosin head immediately cleaves the ATP  Ca++ binds with troponin-tropomyosin complex, active sites on the actin filament are uncovered  bond between head of the cross-bridge & the active site of the actin filament
  11. 11.  once the head of the cross-bridge tilts, this allows release of the ADP & phosphate ion  a new molecule of ATP binds  binding of new ATP causes detachment of the head from the actin  the new molecule of ATP is cleaved to begin the next cycle
  12. 12. The process by which depolarization of the muscle fiber initiates contraction is called excitation- contraction coupling.
  13. 13.  myofibrils surrounded by T tubule–sarcoplasmic reticulum system  penetrate all the way from one side of the muscle fiber to the opposite side  they communicate with the extracellular fluid  when an action potential spreads over a muscle fiber membrane, a potential change also spreads along the T tubules to the deep interior of the muscle fiber
  14. 14.  sarcoplasmic reticulum composed of 2 major parts:  large chambers called terminal cisternae  long longitudinal tubules  muscle contraction continues as long as the Ca++ remain in high concentration  a continually active calcium pump located in the walls of the sarcoplasmic reticulum pumps Ca++ away from the myofibrils back into the sarcoplasmic tubules
  15. 15. References  Ganong Review of Medical Physiology, 23/E  Textbook of Medical Physiology, 12/E Guyton & Hall
  16. 16. Thank You
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