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Vertebrate muscles use __ contractile mechanism
sliding filament
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Muscle cells are called __ & muscles form from fusion of embryonic __
muscle fibers; myoblasts
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Skeletal muscles are __ (in comparison to cardiac muscles)
large & multinucleate
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One muscle consists of many muscle fibers bundled together by __. Each muscle fiber has several __ - bundles of actin & myosin filament
connective tissue; myofibrils
6 actin filaments surround one myosin filament
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thin filaments
actin
Actin filament is actin monomers in a long, twisted molecule
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Each myofibril consists of __ - repeating units of overlapping actin & myosin filaments
sarcomeres
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Each sarcomere is bounded by __ which anchor actin
Z lines
(page 9 and 13 on slide)
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band of sarcomere that contains myosin
A band
doesn't move during contraction
(page 14 on slide)
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parts of sarcomere that has no overlap of actin & myosin
H zone & I band
get smaller as muscles contract
(page 14 on slide)
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Bundles of myosin filaments are held in the center of the sarcomeres by __
titin
the largest protein in the body, runs the full length of the sarcomere
(page 13 on slide)
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When muscle contracts, sarcomeres __ and band pattern changes
shorten
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__ twists around actin; __ attached at intervals
Tropomyosin; troponin
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When myosin heads bind to specific sites on actin molecules to form cross bridges, what happens?
Myosin changes conformation, causes actin filament to slide 5-10 nm
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Muscle contraction is initiated by action potentials from a __ at the __
motor neuron; neuromuscular junction
(page 19 on slide)
Action potentials in muscle fiber also travel deep within the cell.
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All muscle fibers are activated by one motor neuron. This is called a __
motor unit
One muscle may have many motor units.
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To increase the strength of muscle contraction: __ or __
- increase rate of firing of motor neuron;
- recruit more motor neurons to fire (more motor units activated
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__, which spread action potentials into the fiber, descend into the sarcoplasm (muscle fiber cytoplasm) and run close to the __ - a closed compartment that surrounds every myofibril
T tubules (transverse tubules); sarcoplasmic reticulum
(page 24 on slide)
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2 proteins that span space between T tubules and sarcoplasmic reticulum; physically connected
- dyhydropyridine (DHP) receptor: on the T tubule membrane, voltage-sensitive
- ryanodine receptor: in the sarcoplasmic reticulum membrane; is a Ca2+ channel
(page 26 on slide)
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steps in skeletal muscle contraction
- 1. Motor neurons release AcH to nicotinic receptors on end plate.
- 2. Na+ enters. EPP (end plate potential) occurs as suprathreshold is reached.
- 3. Sarcolemma depolarizes. Action potential reaches DHP receptor on T tubule membrane & changes conformation.
- 4. Ryanodine receptor then allows Ca2+ to leave the sarcoplasmic reticulum.
- 5. Ca2+ ions diffuse into the sarcoplasm (At rest, Ca2+ concentration is high in sarcoplasmic reticulum & lower in sarcoplasm)6. Ca2+ binds to troponin, which changes conformation.
- 7. Troponin is bound to tropomyosin - twisting of tropomyosin exposes binding sites on actin.
- 8. Myosin head (with ADP attached) then binds to actin, forming a cross-bridge.
- 9. Myosin changes conformation, causing a power stroke: actin slides 5-10 nm.
- 10. ADP falls off. ATP attaches & breaks the cross-bridge until ATP hydrolyzes.
- 11. Then the cycle continues as long as Ca2+ is in the sarcoplasm.
When Ca2+ pumps remove Ca2+ from sarcoplasm, contraction stops.
Crossbridge is stuck when no ATP is present.
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Troponin has 3 subunits: one binds __, one binds __, & one binds __
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At rest, __ blocks the binding sites on actin.
tropomyosin
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