Muscular System Notes

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)

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 Ca2+ ions

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

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

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

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