1. Muscular System Notes

2. Microscopic Muscle Anatomy  Myfibrils  About 1-2 micrometers in diameter  Length of a muscle fiber  Composed of multiple myofilaments  Contains about 10,000 sarcomeres  Myofilaments  Actin (thin filaments)  Myosin (thick filaments)

3. Microscopic Muscle Anatomy  Sarcoplasmic Reticulum  Modified version of the smooth endoplasmic reticulum  Surrounds each myofibril, forms a tubular network (t tubule)  Contains high amounts of Ca 2+ ions

5. Sarcomere  Repeating units of myofilaments  Smallest functional unit of a muscle fiber  Z lines: proteins that interconnect thin filaments in muscle fibers  M line: proteins that connect thick filaments to each other  A band: area of muscle cells that contains thick filaments  I band: area of muscle cells that contains thin filaments

7. Thin and Thick Filaments  Active site on actin is blocked by tropomyosin  Tropomyosin is held in place by troponin  Calcium is required to move tropomyosin out of the way (binds to troponin)  Action potential from brain stimulates muscle cells, sarcoplasmic reticulum dumps calcium ions into the sarcoplasm.  Calcium in sarcoplasm binds to troponin, revealing the active site

8. Sliding Filament Theory  During contraction, I band gets smaller, A band stays the same size  When myosin binds to actin, it creates a cross bridge  Cross bridge pulls (changes shape) thin filament towards the center of the sarcomere  Cross bridge detaches, returns to original shape, then reattaches at a new binding site and repeats

10. Activity  Draw a muscle fiber  Label actin, myosin, troponin, and tropomyosin  Explain the role of calcium in muscle contraction  Take the information on the sliding filament theory and list out 4 steps to muscle contraction