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Outlines some basic structure and functioning of muscles during contraction

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  1. 1. Muscles
  2. 5. Muscle Structure <ul><li>A very high resolution E.M reveals that each myofibril is made up of parallel filaments. </li></ul><ul><li>There are 2 kinds of filament called thick & thin filaments. </li></ul><ul><li>These 2 filaments are linked at intervals called cross bridges , which actually stick out from the thick filaments </li></ul>
  3. 6. Mechanism of muscle contraction <ul><li>The above micrographs show that the sarcomere gets shorter when the muscle contracts </li></ul><ul><li>The light (I) bands become shorter </li></ul><ul><li>The dark bands (A) bands stay the same length </li></ul>
  4. 7. The Sliding Filament Theory <ul><li>So, when the muscle contracts, sarcomeres become smaller </li></ul><ul><li>However the filaments do not change in length. </li></ul><ul><li>Instead they slide past each other (overlap) </li></ul><ul><li>So actin filaments slide between myosin filaments </li></ul><ul><li>and the zone of overlap is larger </li></ul>
  5. 8. What makes the filaments slide past each other? <ul><li>Energy for the movement comes from splitting ATP </li></ul><ul><li>ATPase that does this is located in the myosin heads. </li></ul><ul><li>The energy from the ATP causes the angle of the myosin head to change. </li></ul><ul><li>The myosin heads can attach to actin. </li></ul><ul><li>Movement of the myosin heads and them attaching and detaching from actin causes the filaments to slide relative to one another. </li></ul><ul><li>This movement reduces the sarcomere length. </li></ul>
  6. 9. Repetition of the cycle <ul><li>One ATP molecule is split by each cross bridge in each cycle. </li></ul><ul><li>This takes only a few milliseconds </li></ul><ul><li>During a contraction 1000’s of cross bridges in each sarcomere go through this cycle. </li></ul><ul><li>However the cross bridges are all out of synch, so there are always many cross bridges attached at any one time to maintain force. </li></ul>
  7. 10. The Cross Bridge Cycle
  8. 11. <ul><li>The cycle begins with ATP binding to the myosin head. This causes the myosin head to be released from actin. </li></ul>
  9. 12. 2. The ATP molecule is then hydrolysed while the myosin head is unattached. The ADP & Pi formed remain bound to the myosin head.
  10. 13. <ul><li>3. The energy released by the hydrolysis of ATP is absorbed by the myosin </li></ul><ul><li>This causes the myosin head to change shape (places it in energised state or cocked state – also called the recovery stroke) </li></ul><ul><li>It then binds to the actin filament. </li></ul>
  11. 14. <ul><li>4-5. The ADP and Pi are then released from the myosin head </li></ul><ul><li>Result = Power stroke occurs (the myosin head changes shape) </li></ul><ul><li>This draws the actin filament over the myosin filament. </li></ul>
  12. 15. <ul><li>The cycle begins again when the next ATP binds to the myosin head. Causing the myosin head to be released from actin. </li></ul>
  13. 16. Control of Muscle Contraction <ul><li>How is the cross bridge cycle switched off in a relaxed muscle? </li></ul><ul><li>This is where the regulatory protein on the actin filament, tropomyosin is involved. </li></ul><ul><li>Actin filaments have myosin binding sites. </li></ul><ul><li>These binding sites are blocked by tropomyosin in relaxed muscle. </li></ul><ul><li>When Ca 2+ bind tropomyosin is displaced and the myosin binding sites are uncovered. </li></ul><ul><li>So myosin & actin can now bind together to start the cross bridge cycle </li></ul>
  14. 17. Tropomyosin, Ca 2+ & ATP <ul><li>Ca 2+ causes tropomyosin to be displaced. </li></ul><ul><li>So it no longer blocks the myosin binding site </li></ul><ul><li>So myosin and actin can bind together allowing cross bridge cycling </li></ul>
  15. 18. Neuromuscular junction: Note Ach = Acetylcholine
  16. 19. Sarcoplasmic Reticulum
  17. 20. Sequence of events <ul><li>1. An action potential arrives at the end of a motor neurone, at the neuromuscular junction. </li></ul><ul><li>2. This causes the release of the neurotransmitter acetylcholine. </li></ul><ul><li>3 This initiates an action potential in the muscle cell membrane (Sarcolemma). </li></ul><ul><li>4. This action potential is carried quickly into the large muscle cell by invaginations in the cell membrane called T-tubules. </li></ul>
  18. 21. Sequence of events <ul><li>5. The action potential causes the sarcoplasmic reticulum to release its store of calcium into the myofibrils. </li></ul><ul><li>6. Ca 2+ causes tropomoysin to be displaced uncovering myosin binding sites on actin. </li></ul><ul><li>7. Myosin cross bridges can now attach and the cross bridge cycle can take place. </li></ul><ul><li>Relaxation is the reverse of these steps </li></ul>