Locomotion [2015]

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  3. 3. Three kinds of muscle: Smooth Unstriated, Unstriped or Involuntary Skeletal, Striated, Striped or
  4. 4. Three kinds of muscle: 1. Smooth Muscle contracts slowly & fatigues slowly 2. Cardiac Muscle is self-stimulating & does not fatigue 3. Skeletal Muscle contracts quickly & fatigues quickly Gap junctions
  5. 5. Cardiac muscle is striated: cells: are smaller than skeletal have one nucleus branch and interdigitate: can withstand tearing
  6. 6. Functions of the Intercalated discs: 1. add to the strength of cardiac muscle 2. provide strong mechanical adhesions between adjacent cells 3. have GAP JUNCTIONS allowing the rapid spread of a depolarisation initiated at one point in the heart
  8. 8. Tendons attach skeletal muscle to bone:
  9. 9. Organisation of Skeletal Muscle 1. Muscle 2. Muscle fibre bundles 3. Muscle fibre 4. Myofibril 5. Myofilaments muscle cell [groups of 10-100 or more muscle fibers] contractile proteins: actin & myosin composed of myofilaments
  10. 10. Each muscle fibre is composed of MYOFIBRILS [6-25 cm long; muscle fibres are multinucleated]
  11. 11. Structure of a muscle fibre Plasma membrane Cytoplasm Endoplasmic reticulum Transverse tubule Release calcium ions Myofibrils fill sarcoplasm
  12. 12. Skeletal muscle is striated i.e. has visible banding Striations = bands Nuclei Connective tissue separates cells Myofibrils fill sarcoplasm Nucleus Striations Sarcolemma Myofibril
  13. 13. Myofibrils are bundles of myofilaments separated by sarcoplasmic reticulum Myofibrils are the contractile organelles of skeletal muscle Myofibrils extend the entire length of a muscle fibre [6-25 cm long]
  14. 14. Myofilaments : MYOSIN thick filaments ACTIN thin filaments Sarcomere Sarcomere: distance between two Z-lines
  15. 15. Sarcomeres are: repeating units of equal length in a myofibril the units of contraction
  16. 16. Capillary Nuclei Sarcoplasmic reticulum T tubule Sarcolemma Bone Mitochondrion Note blood supply to muscle fibre
  17. 17. Sarcomeres are made of overlapping actin & myosin filaments A band dArk [Anisotropic] I band - lIght [Isotropic]
  18. 18. Thick and thin filaments overlap each other in a pattern that creates striations.
  19. 19. Two bands in the Sarcomere Actin + myosin ActinActin I band I bandA band Z line Z lineH zone SARCOMERE Myosin Actin Region of overlap Myosin cross-bridge
  20. 20. Learn to draw sarcomere structure Syllabus says: M line, but some books call it M band M line A band H zone
  21. 21. The structure of Skeletal Muscle A bands - made of actin and myosin I bands - made solely of actin filaments M line
  22. 22. A Sarcomere
  23. 23. How will a TS through the I band look like?
  24. 24. The Sliding Filament Theory of Muscle Contraction: actin slides past myosin
  25. 25. The size of actin & myosin do not change in length as they slide
  26. 26. Size of H zone, I & A bands during contraction: H zone A bandI band RELAXED CONTRACTED
  27. 27. Micrographs showing sarcomere contraction I bands shorten Relaxed muscle Contracted muscle relaxed sarcomere contracted sarcomere A bands stay the same length
  28. 28. The Sliding Filament Theory of Muscle Contraction Myosin cross-bridges pull on thin filaments. Thin filaments slide inward. Z lines come toward each other. Sarcomeres shorten. The muscle fibre shortens. The muscle shortens. Figure 6.7
  29. 29. Proteins required for muscle contraction: 1. Actin 2. Myosin 3. Troponin 4. Tropomyosin
  30. 30. Each actin filament is made up of: two helical strands of globular actin molecules (G-actin) which twist round each another G-actin F-actin The whole assembly of actin molecules is called F-actin (fibrous actin).
  31. 31. Troponin Found periodically along the tropomyosin strand Functions to move the tropomyosin aside, exposing the myosin binding sites. Troponin
  32. 32. Troponin : a globular protein vital to contraction of muscle fibre one to bind: Troponin has three subunits: 1.Actin 2. Tropomyosin 3. Ca2+
  33. 33. Tropomyosin: - forms a fibrous strand around the actin filament
  34. 34. Tropomyosin twists around the actin When the sarcomere is not shortening, the position of the tropomyosin covers the binding sites on the actin subunits and prevents myosin cross bridge binding.
  35. 35. Role of Calcium in Muscle Contraction: Action Potential Occurs Calcium Ions are Released from the Terminal Cisternae Calcium Ions then Bind to Troponin Tropomyosin Moves Away from the Myosin Binding Sites on Actin [see next slide]
  36. 36. Depolarisation travels along T-tubules
  37. 37. Closer look at T-Tubules
  38. 38. The Myosin molecule consists of two long polypeptide chains coiled together : each chain ends in a globular head [cross bridge] Tail(a) A myosin molecule Many myosins (about 200) make up each thick filament
  39. 39. Myosin head changes position The two heads (called cross bridges) move back and forth, providing the power stroke for muscle contraction. The tail of myosin has a hinge which allows vertical movement so that the cross bridge can bind to actin. Power stroke
  40. 40. Cross bridge cycle in muscle contraction During the contraction of a sarcomere about half of the cross bridges are attached to actin and about half are bound at any given time.
  41. 41. The Myosin Heads have two sites: binds & hydrolyses ATP ATPase site Tropomysoin Actin-binding site myosin binds to actin, forming a cross-bridge
  42. 42. Proteins often change their shape or conformation as they function.
  43. 43. As myosin functions within muscle cells, it undergoes the following four steps: Myosin is in a high energy state. The tail hinge bends allowing the myosin to make contact with actin. Myosin is in a high energy state. The ADP and Pi are released from the head (cross bridge) and the head tilts backward, causing a power stroke. The cross bridge goes from a high energy state to a low energy state. Myosin is in a low energy state. ATP binds to the head (cross bridge) but does not transfer its energy to the head yet. Myosin is in a low energy state. ATP is hydrolysed into ADP and Pi, releasing its energy, which is transferred to the myosin head (cross bridge). 1 23 4
  44. 44. Cocked: Store energy
  45. 45. Details of Sliding Filament Theory start
  46. 46. 1 2 3 4 REMEMBER: Myosin binds ATP: cross-bridge is disconnected ATP is hydrolysed: myosin head is repositioned able to form another cross- bridge
  47. 47. Summary of the role that ATP plays in the contraction of muscle: 1. ATP transfers its energy to the myosin cross bridge, which in turn energizes the power stroke. 2. ATP disconnects the myosin cross bridge from the binding site on actin. 3. ATP fuels the pump that actively transports calcium ions back into the sarcoplasmic reticulum.
  48. 48. What happens if Ca2+ levels are: High: Ca2+ binds to troponin tropomyosin is displaced, allowing the formation of actin-myosin cross- bridges Low: tropomyosin inhibits cross-bridge formation
  49. 49. How is the cross bridge broken? The myosin head binds a molecule of ATP, which causes it to release the actin
  50. 50. What happens in the absence of ATP? This explains why muscles stiffen soon after animals die, a condition known as RIGOR MORTIS the actin-myosin bonds cannot be broken the muscles stiffen
  51. 51. Do the muscles remain stiff forever in a dead animal? NO Eventually the proteins begin to lose their integrity, and the muscles soften. Dead !!
  52. 52. The Neuromuscular Junction
  53. 53. A Motor Unit is made up of: all the fibres activated by a single motor neurone all the fibres contract simultaneously
  54. 54. Number of Motor Units involved varies
  55. 55. Question: [MAY, 2000] This question concerns muscle. a) Distinguish between sarcomeres and myofibrils. (2) Sarcomeres are the units of contraction. They lie between two Z-lines. Myofibrils are bundles of myofilaments made of actin and myosin.
  56. 56. Question: [MAY, 2000] b)Briefly explain the role of actin and myosin in contraction of striated muscle. (4) When calcium ions bind to troponin, myosin-binding sites on actin filaments are exposed and myosin heads bind to actin, releasing ADP. The myosin head changes position and filaments slide past each other. ATP binds to myosin, causing it to release actin. Hydrolysis of ATP makes the myosin head return to original position.
  57. 57. Question: [MAY, 2000] c) What role is played by the Z line (or Z disc) during the contraction of striated muscle fibres? Z-lines hold actin filaments together. Distance between Zlines shortens on contraction. (2) d)The presence of calcium ions is necessary for the hydrolysis of ATP. How would removal of calcium ions from the muscle fibre sarcoplasm affect contraction? (2) Contraction stops. ATP is needed to break the cross-bridges between myosin and actin.
  58. 58. Summary NAME FUNCTION Actin filaments Slide past myosin, causing contraction Ca2+ Needed for myosin to bind to actin Myosin filaments Pull actin filaments by means of cross-bridges; are enzymatic and split ATP ATP Supplies energy for muscle contraction
  59. 59. Energy Supply for Contraction: Glucose: is usually the source of energy for muscle contraction Phosphocreatine: is a PHOSPHAGEN a high energy phospha