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Length tension relationship

Length tension relationship. Sliding filament theory Tension is produced by interaction of thick & thin filaments Interference at short lengths (ascending limb) Reduced interaction at long lengths (descending) Supporting evidence Single fibers Special conditions for descending limb.

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Length tension relationship

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  1. Length tension relationship • Sliding filament theory • Tension is produced by interaction of thick & thin filaments • Interference at short lengths (ascending limb) • Reduced interaction at long lengths (descending) • Supporting evidence • Single fibers • Special conditions for descending limb

  2. Force length relationship • Crossbridge availability • Overlap • Structural interference 4.1 um Plateau Longest Length Shallow Ascending 1.25 um 1.25 um 1.6 um Descending 2.5 um Steep Ascending Plateau 1.25 um 1.25 um

  3. Historical context • Blix 1893 • Total force follows an “S-shaped” relation to length • Heat production continuously increases • Evans & Hill 1914 • Active vs total tension • Heat production parallels active tension Total tension Active tension Heat rate Passive tension

  4. Historical context • Ramsey & Street, 1940 • Single frog fibers • Passive tension (myofibrils vs sarcolemma) • Distinct force maximum, both total and active • Loss of sarcomere alignment with long stretch Active tension Tension (% max) Passive tension Length (% rest)

  5. Possible mechanisms • Coiling of ‘kinked’ fibers • Mechanical spring • Striation & changes during stretch • Shortening of one structure • eg, dehydration • Only I-band changes length • Bi-molecular interaction • X-ray (1935) • Structural derangements “delta” state (R&S 1940)

  6. The Big Key • Hugh Huxley 1957 • Visibly interdigitating filament arrays • Visible molecular interactions (crossbridges)

  7. AF Huxley & Peachey 1961 • Single frog fibers • Monitor striation • “Isometric” fiber does not have isometric striations

  8. Gordon, Huxley & Julian (1966) • Single fiber segments • “Spot follower” • Control sub-segment of larger fiber • Assume intervening material is functionally static • Still not measuring actual striations

  9. GHJ raw measurements Near Lopt Above Lopt Below Lopt

  10. GHJ Long lengths • Continuous tension rise • Striation irregularities (instability) • Internal rearrangement w/o membrane motion • Extrapolation • Undesirable but consistent

  11. GHJ Synthesis

  12. Mammalian fibers • Actin filament 1.1 um • Myosin filament 1.63 um Edman 2005

  13. Fiber segment summary • Peak force corresponds with max overlap of thin filaments and crossbridges (± bare zone) • Force decreases linearly with decreasing overlap (descending limb) • Force decreases slowly as thin filaments overlap (shallow ascending limb) • Force decreases rapidly as thick filament overlaps Z-disk (steep ascending limb)

  14. Single myofibrils • Rassier, Herzog & Pollack (2003) • Isolate myofibril segments ~ 20 sarcomeres • Activate by direct calcium bath Fibril image Intensity profile

  15. Sarcomeres are not all equal • Heterogeneity increases with movement • Just like R&S • GHJ • ~200 sarcomeres • ~2000 myofibrils

  16. Single Sarcomere • Rassier & Pavlov 2008 • Even this is not constant • A-band wobbles between Z-disks

  17. Other length trajectories • GHJ: start long passive, unloaded shortening to test length • Abbot & Aubert (1952) • Allow force development before length change • Residual force enhancement • Persistent lossof force

  18. Residual force enhancement • Joumaa, Leonard & Herzog (2008) • Single myofibrils • Generate greater than ‘maximum’ tension on descending limb

  19. Residual force enhancement • Nonuniformity • Fiber, fibril, sarcomere • “Weak” sarcomere/half-sarcomere stretches, gaining from force-velocity property • Other sources of force • Titin • Myofilament shortening Nagornyak & al., 2004

  20. Submaximal activation • Rack & Westbury, 1969 • “Normal” activation frequency low, subfused • Distributed stim allows lower f but steady force At lower activation, length-tension shifts to longer lengths

  21. Passive tension • Banus & Zetlin (1938) • Muscles with fibers “scooped out” have same passive tension  epi-/peri-mysium gives passive tension • Ramsey & Street (1940) • Pinched sections of fiber w/o sarcomeres carry same tension as intact sections  sarcolemma gives passive tension • DK Hill (1950) • Passive tension is viscoelasticresidual crossbridges • Magid & Law (1985) • Skinned fiber passive elasticity is the same as whole muscle and not visco-elastic  myofibrils give passive tension

  22. Titin hypothesis • Horowits & al 1986 • Skinned, irradiated fibers • ln(A/A0)=2.3e11 Mr D (Mr, mass; D dose) • Titin • 2-4 MD • ~ 5x larger than nextlargest protein Normal fiber Irradiated fiber

  23. Horowits & al • Tension declines with dose • ~3.4 MD passive • ~3.2 MD active • Experimental measures match theory quantitatively

  24. Titin Model • Modular spring • Discrete, independent elastic domains • Segmental association with thick filament • Spring + yield • Linear elastic • Perfectly plastic • ECM dominates at long lengths

  25. Summary • Sliding filament theory • Steep ascending limb • Shallow ascending limb • Plateau • Descending limb • Passive tension • ECM: chinese finger trap • Titin: modular spring

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