1. Excitation-Contraction Coupling: At the heart of muscle function Larry M. Frolich, Ph.D. March 17,2011 HOOK Muscle is only biological cell/tissue that can cause rapid, large-scale movement— THE evolutionary innovation that defines animals….and ourselves. Role of excitable membrane and filamentous muscle proteins understood as great and early breakthrough in cell/molecular biology and biochemistry

3. How does muscle work—outline Motor Unit—motor neuron plus skeletal muscle cells (review) Action potential in neurons (reminder) Muscle cell architecture From arrival of an action potential to the contraction of the muscle (excitation-contraction coupling) Molecular basis of muscle movement—sliding filament model Whole muscles and their physiology as explained by the molecular/cellular basis of muscle function (lab activities)

4. The Motor Unit (review) Neurons and Muscle Cells are unique to animals They have “excitable” membranes that transmit action potentials They allow for rapid large-scale movements Motor Unit is one motor neuron plus the muscle cells that it stimulates (or synapses with)--the minimal construct that allows for movement in our body

5. How do neurons carry a message— action potentials REMINDER SLIDE

7. Single muscle cell or muscle “fiber” is composed of myofibrils which contain sarcomeres or contractile “units” Myo (Latin for muscle) Sarco (Greek for flesh) Muscle cell architecture

8. Skeletal muscle fibers are BIG cells—visible to naked eye as fibers in meat, chicken, fish Sarcolemma is muscle cell membrane—”excitable” so has action potentials just like neurons Because cell is large, T-tubules carry action potential— ionic depolarization—into internal parts of cell Ionic depolarization in T-tubules causes sarcoplasmic reticulum to releases calcium Calcium triggers actin-myosin protein filaments to “slide” against each other. Muscle cells

9. The Brain From Top to Bottom From action potential to movement of muscle cell

11. Molecular Basis of Muscle Contraction Actin-Myosin “sliding filament” model Actin and myosin filamentous proteins are packed parallel and end-to-end in sarcomeres When muscle cell is “excited” (experiences action potential), Ca is released causing sarcomeres to contract

12. How does the actin-myosin complex (sarcomere) shorten and contract the muscle? Actin = thin filament “lattice-work” Myosin = thick filament “core” Ca release triggers the formation of molecular cross-bridges from myosin to actin Cross-bridges “row” or “reach” for more adjacent binding site on actin. (would normally draw on board)

13. A Biochemical artwork by David Goodsell –see more herehere

14. Details, details, details… Tropomyosin and troponin create binding site on actin filament Presence of Ca++ exposes binding site “Cocked” cross-bridge on myosin (uses ATP) then attaches to binding site and pulls or “rows” actin filament Cross-bridge linkage is broken and re-cocks to link with next binding site

15. Put the sliding filaments back into a whole muscle… And the result is muscle movement Whole muscle

16. Excitation-Contraction Coupling and Sliding Filament Model explains: Why muscle has peak force at middle lengths: (ideal actin-myosin overlap for cross-bridge formation)—BUCKET DEMO More muscle cells active (“excited”) means more muscle force: (more cross-bridge formation)— EMG’S ISOLATED MUSCLE LAB

17. Excitation-Contraction Coupling and Sliding Filament Model also explains: Concentric/isometric/eccentri c contraction: Cross-bridges continue to form and “reach” even if opposing force is greater. Striations (background of slide)—MICROSCOPE SLIDESMICROSCOPE SLIDES Arm-raising ghost effect after pushing against doorway—DO- AT-HOME DEMODO- AT-HOME DEMO

18. Want more details (from 2008 Nature review) “you'll thank me later” (for protecting from too much detail)

19. Evolutionary tinkering (where did this incredible system come from?: Actin is present in all eukaryotic cell as part of internal cell architecture Myosin is present as “motor protein” that hauls other structures along the actin highways

21. G So, go get your actin and myosin sliding… Gracias por su atención. More info on Frolich website: http://faculty.yc.edu/lfrolich/index.htmhttp://faculty.yc.edu/lfrolich/index.htm

22. Excitation-Contraction Coupling and Sliding Filament Model explains: Why muscle has peak force at middle lengths: (ideal actin- myosin overlap for cross- bridge formation)—BUCKET DEMO More muscle cells active (“excited”) means more muscle force: (more cross- bridge formation)—EMG’S, ISOLATED MUSCLE LABEMG’S ISOLATED MUSCLE LAB Concentric/isometric/eccentri c contraction: Cross-bridges continue to form and “reach” even if opposing force is greater. Striations (background of slide)— MICROSCOPE SLIDES MICROSCOPE SLIDES Arm-raising ghost effect after pushing against doorway—DO-AT-HOME DEMOdoorway—DO-AT-HOME DEMO