2. Interaction of thick & thin filaments __________________ –_____________________________________ _____________________________________ –_____________________________________ sarcomere

3. Where is ATP needed? 3 4 1 2 1 1 1 Cleaving ATP  ADP allows myosin head to bind to actin filament thin filament (actin) thick filament (myosin) ATP myosin head binding site So that’s where those 10,000,000 ATPs go! Well, not all of it! ADP form cross bridge release cross bridge shorten sarcomere 1

4. Closer look at muscle cell multi-nucleated Mitochondrion Sarcoplasmic reticulum Transverse tubules (T-tubules)

5. Muscle cell organelles _________________________ –muscle cell cytoplasm –contains many mitochondria _________________________ –organelle similar to ER network of tubes –_________________ Ca 2+ released from SR through channels Ca 2+ restored to SR by Ca 2+ pumps –pump Ca 2+ from cytosol –pumps use ATP Ca 2+ ATPase of SR ATP The rest of the ATPs! But what does the Ca 2+ do?

6. Muscle at rest Interacting proteins –at rest, ________ molecules hold _____________ fibers so that they cover the myosin-binding sites on actin troponin has Ca 2+ binding sites

7. The Trigger: motor neurons Motor neuron triggers muscle contraction –release ______________ (Ach) neurotransmitter

8. Nerve signal travels down ___________ –stimulates sarcoplasmic reticulum (SR) of muscle cell to ________________ –flooding muscle fibers with Ca 2+ Nerve trigger of muscle action

9. At rest, tropomyosin _______ myosin- binding sites on actin –secured by troponin Ca 2+ binds to ________ –_______________ causes movement of troponin –releasing tropomyosin –___________________ __________ on actin Ca 2+ triggers muscle action

10. Coupling Excitation to Contraction 1. Ca2+ ____ action potentials to contraction. 2. At rest, Ca2+ is stored in the __________. 3. Spaced along the plasma membrane (_____________) of the muscle fiber are inpocketings of the membrane that form ________ of the “__________". These tubules plunge repeatedly into the interior of the fiber. 4.The tubules of the T system terminate near the ___________________ of the sarcoplasmic reticulum. 5. Each action potential created at the neuromuscular junction sweeps quickly along the sarcolemma and is carried into the T system.

11. How Ca 2+ controls muscle __________________ –exposed actin binds to myosin –fibers slide past each other ratchet system –shorten muscle cell muscle contraction –muscle doesn’t relax until Ca 2+ is pumped back into SR requires ATP ATP

12. Fig. 50-27-4 Thin filaments ATP Myosin head (low- energy configuration Thick filament Thin filament Thick filament Actin Myosin head (high- energy configuration Myosin binding sites ADP P i Cross-bridge ADP P i Myosin head (low- energy configuration Thin filament moves toward center of sarcomere. ATP ADP P i +

13. Fig. 50-28 Myosin- binding site Tropomyosin (a) Myosin-binding sites blocked (b) Myosin-binding sites exposed Ca 2+ Ca 2+ -binding sites Troponin complex Actin __________ concentration: muscle contracts ________ concentration : binding sites are covered and contraction stops Role of Ca and Regulatory Proteins

14. Put it all together… 1 ATP 2 3 4 5 7 6

15. How it all works… Action potential causes ______ release from SR –Ca 2+ binds to ______________ Troponin moves ____________ uncovering ___________________ on actin Myosin binds ______ –uses ____ to "ratchet" each time –releases, "unratchets" & binds to next actin Myosin pulls actin chain along Sarcomere _________ –Z discs move closer together Whole fiber shortens  _____________! Ca 2+ pumps restore Ca 2+ to SR  ___________! –pumps use _____ ATP

16. Fig. 50-26 Z Relaxed muscle M Z Fully contracted muscle Contracting muscle Sarcomere 0.5 µm Contracted Sarcomere