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