1. Sliding Filament

2. Thick & thin filaments
Myosin tails aligned together & heads pointed away from center of sarcomere

3. Interaction of thick & thin filaments
Cross bridges
connections formed between myosin heads (thick filaments) & actin (thin filaments)
cause the muscle to shorten (contract)
sarcomere
sarcomere

4. thick filament (myosin) So that’s where those 10,000,000 ATPs go!
Where is ATP needed?
binding site
thin filament (actin)
myosin head
ADP
thick filament (myosin) 1 2
ATP
So that’s where those 10,000,000 ATPs go!
Well, not all of it!
form cross bridge
release cross bridge 1 1 1 3
shorten sarcomere
Cleaving ATP  ADP allows myosin head to bind to actin filament 1 4

5. Closer look at muscle cell
Sarcoplasmic reticulum
Transverse tubules (T-tubules)
Mitochondrion
multi-nucleated

6. Muscle cell organelles
Ca2+ ATPase of SR
Sarcoplasm
muscle cell cytoplasm
contains many mitochondria
Sarcoplasmic reticulum (SR)
organelle similar to ER
network of tubes
stores Ca2+
Ca2+ released from SR through channels
Ca2+ restored to SR by Ca2+ pumps
pump Ca2+ from cytosol
pumps use ATP
There’s the rest of the ATPs!
But what does the Ca2+ do?
ATP

7. Muscle at rest Interacting proteins
at rest, troponin molecules hold tropomyosin fibers so that they cover the myosin-binding sites on actin
troponin has Ca2+ binding sites

8. The Trigger: motor neurons
Motor neuron triggers muscle contraction
release acetylcholine (Ach) neurotransmitter

9. Nerve trigger of muscle action
Nerve signal travels down T-tubule
stimulates sarcoplasmic reticulum (SR) of muscle cell to release stored Ca2+
flooding muscle fibers with Ca2+

10. Ca2+ triggers muscle action
At rest, tropomyosin blocks myosin-binding sites on actin
secured by troponin
Ca2+ binds to troponin
shape change causes movement of troponin
releasing tropomyosin
exposes myosin-binding sites on actin

11. Coupling Excitation to Contraction
Calcium ions (Ca2+) link action potentials to contraction.
At rest, Ca2+ is stored in the sarcoplasmic reticulum.
Spaced along the plasma membrane (sarcolemma) of the muscle fiber are inpocketings of the membrane that form tubules of the "T system". These tubules plunge repeatedly into the interior of the fiber.
The tubules of the T system terminate near the calcium-filled sacs of the sarcoplasmic reticulum.
Each action potential created at the neuromuscular junction sweeps quickly along the sarcolemma and is carried into the T system.

12. How Ca2+ controls muscle
Sliding filament model
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

13. Myosin head (low- energy configuration
Thick filament
Thin filaments
Thin filament
Myosin head (low- energy configuration
ATP
ATP
Thick filament
Myosin binding sites
Thin filament moves toward center of sarcomere.
Actin
Myosin head (low- energy configuration
ADP
Myosin head (high- energy configuration
P i
ADP
ADP +
P i
P i
Cross-bridge

14. Ca low concentration: binding sites are covered and contraction stops
Role of Ca and
Regulatory
Proteins
Fig
Tropomyosin
Ca2+-binding sites
Actin
Troponin complex
(a) Myosin-binding sites blocked
Ca low concentration: binding sites are covered and contraction stops
Ca hi concentration: muscle contracts
Ca2+
Myosin- binding site
(b) Myosin-binding sites exposed

15. Put it all together… 1 2 3
ATP 7 4 6
ATP 5

16. How it all works… Action potential causes Ca2+ release from SR
Ca2+ binds to troponin
Troponin moves tropomyosin uncovering myosin binding site on actin
Myosin binds actin
uses ATP to "ratchet" each time
releases, "unratchets" & binds to next actin
Myosin pulls actin chain along
Sarcomere shortens
Z discs move closer together
Whole fiber shortens  contraction!
Ca2+ pumps restore Ca2+ to SR  relaxation!
pumps use ATP
ATP
ATP

17. Fully contracted muscle
Fig
Sarcomere
0.5 µm Z M Z
Relaxed muscle
Contracting muscle
Fully contracted muscle
Contracted Sarcomere