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. Steep Ascending Shallow Ascending Plateau Descending Force length relationship Crossbridge availability – Overlap Structural interference 1.6 um1.25 um 4.1 um 1.25 um 2.5 um Plateau Longest Length

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 Passive tension Total tension Active tension Heat rate

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 Length (% rest) Tension (% max) Passive tension Active tension

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 L opt Above L opt Below L opt

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 loss of 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/A 0 )=2.3e11 M r D (M r, mass; D dose) Titin – 2-4 MD – ~ 5x larger than next largest 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