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Interaction of thick & thin filaments• __________________
– __________________________________________________________________________
– _____________________________________
sarcomere
sarcomere
Where is ATP needed?
3
4
12
1
1
1
Cleaving ATP ADP allows myosin head to bind to actin filament
thin filament(actin)
thick filament(myosin)
ATP
myosin head
binding site
So that’s where those
10,000,000 ATPs go!Well, not all of it!
ADP
formcrossbridg
ereleas
ecrossbridge
shortensarcomer
e
1
Closer look at muscle cell
multi-nucleated
Mitochondrion
Sarcoplasmicreticulum
Transverse tubules(T-tubules)
Muscle cell organelles• _________________________
– muscle cell cytoplasm– contains many mitochondria
• _________________________– organelle similar to ER
• network of tubes– _________________
• Ca2+ released from SR through channels• Ca2+ restored to SR by Ca2+ pumps
– pump Ca2+ from cytosol– pumps use ATP
Ca2+ ATPase of SR
ATP
The restof theATPs!
But whatdoes theCa2+ do?
Muscle at rest
• Interacting proteins– at rest, ________ molecules hold _____________
fibers so that they cover the myosin-binding sites on actin• troponin has Ca2+ binding sites
The Trigger: motor neurons • Motor neuron triggers muscle contraction
– release ______________ (Ach) neurotransmitter
• Nerve signal travels down ___________– stimulates
sarcoplasmic reticulum (SR) of muscle cell to ________________
– flooding muscle fibers with Ca2+
Nerve trigger of muscle action
• At rest, tropomyosin _______ myosin-binding sites on actin– secured by troponin
• Ca2+ binds to ________– _______________
causes movement of troponin
– releasing tropomyosin
– _____________________________ on actin
Ca2+ triggers muscle action
Coupling Excitation to Contraction1. Ca2+ ____ action potentials to contraction.
2. At rest, Ca2+ is stored in the __________.
3. Spaced along the plasma membrane (_____________) of the muscle fiber are inpocketings of the membrane that form ________ of the “__________". These tubules plunge repeatedly into the interior of the fiber.
4.The tubules of the T system terminate near the ___________________ of the sarcoplasmic reticulum.
5. Each action potential created at the neuromuscular junction sweeps quickly along the sarcolemma and is carried into the T system.
How Ca2+ controls muscle• __________________
– exposed actin binds to myosin
– fibers slide past each other• ratchet system
– shorten muscle cell• muscle contraction
– muscle doesn’t relax until Ca2+ is pumped back into SR • requires ATP
ATP
ATP
Fig. 50-27-4
Thinfilaments
ATP Myosin head (low-energy configuration
Thick filament
Thin filament
Thickfilament
Actin
Myosin head (high-energy configuration
Myosin binding sites
ADP
P i
Cross-bridgeADP
P i
Myosin head (low-energy configuration
Thin filament movestoward center of sarcomere.
ATP
ADP P i+
Fig. 50-28
Myosin-binding site
Tropomyosin
(a) Myosin-binding sites blocked
(b) Myosin-binding sites exposed
Ca2+
Ca2+-binding sites
Troponin complexActin
__________ concentration: muscle contracts
________ concentration: binding sites are covered and contraction stops
Role of Ca and
Regulatory
Proteins
Put it all together…1
ATP
2
3
4
5
7
6
ATP
How it all works…• Action potential causes ______ release from SR
– Ca2+ binds to ______________
• Troponin moves ____________ uncovering ___________________ on actin
• Myosin binds ______– uses ____ to "ratchet" each time– releases, "unratchets" & binds to next actin
• Myosin pulls actin chain along• Sarcomere _________
– Z discs move closer together
• Whole fiber shortens _____________!• Ca2+ pumps restore Ca2+ to SR ___________!
– pumps use _____
ATP
ATP
Fig. 50-26
Z
Relaxedmuscle
M Z
Fully contractedmuscle
Contractingmuscle
Sarcomere0.5 µm
ContractedSarcomere