Muscular
System Notes
Microscopic Muscle Anatomy
 Myfibrils
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About 1-2 micrometers in diameter
Length of a muscle fiber
Composed of multiple myofilaments
Contains about 10,000 sarcomeres
 Myofilaments
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Actin (thin filaments)
Myosin (thick filaments)
Microscopic Muscle Anatomy
 Sarcoplasmic
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Reticulum
Modified version of the smooth
endoplasmic reticulum
Surrounds each myofibril, forms a tubular
network (t tubule)
Contains high amounts of Ca2+ ions
Sarcomere
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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
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Active site on actin is blocked by tropomyosin
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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
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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