1. Initially Sarcolemma is in the Resting Membrane state
Physiology of Skeletal Muscle Fibers … Binding of Ach to Receptors on Sarcolemma then triggers changes in Muscle Fibers and leads to C. Excitation-Contraction Coupling: events from transmission of action potential along sarcolemma to the beginning of contraction
Initially Sarcolemma is in the Resting Membrane state
2. #2 Action Potential
Receptors allow Na+ ions to enter and K+ ions to leave the muscle fiber; but more Na+ enters making the inside of the cell positive
More Positive outside
Negative inside
Outside Cell
Inside Cell
Figure 9.8a

2. Excitation-Contraction Coupling …
2. #2 Action Potential …
Charge inside cell changes from negative to positive
If enough Na+ enters, an action potential travels down Sarcolemma
It moves out in all directions via Depolarization and Repolarization
Outside Cell + + + + + + +
Inside Cell +
Na++ K+
Figure 9.8b

3. Excitation-Contraction Coupling … 2. Action Potential …
©2007 Thomson Higher Education
After Action Potential has ended, the Resting state is restored by the Na+-K+ pump
Na+
Figure 9.8d

4. Excitation-Contraction Coupling … Action Potential …
Action potential travels down the T-tubules
The Sarcoplasmic Reticulum releases Ca+2 into the cell
High Ca+ in the Sarcoplasm ultimately leads to muscle contraction
Figure 9.8c

5. Run the Excitation-Contraction Coupling

6. D. Sliding Filament Model & Cross-Bridge Cycling
OVERVIEW
Calcium ions cause the sarcomeres to shorten
Requires ATP

7. © 2013 Pearson Education, Inc.
Figure 9.6 Sliding filament model of contraction.
Slide 1
Slide 1
Overview … 1
Fully relaxed sarcomere of a muscle fiber Z H Z I A I 2
Fully contracted sarcomere of a muscle fiber Z Z I A I
© 2013 Pearson Education, Inc.

8. Sliding Filament Model …
1. At ‘rest’, myosin heads are already ‘cocked’
High energy state from the previous ATP hydrolysis
2. Ca+2 attaches to Troponin  moves off head binding site
Figure 6.8a

9. Myosin head binding and cycling…
ADP
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge. 1 Pi
Myosin head binding and cycling…
3. Cross bridge formation – myosin cross bridge attaches to actin filament
Figure 9.12

10. ADP
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge.
Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released. 1 2 Pi
4. Working (power) stroke – myosin head pivots and pulls actin filament toward M line. Occurs as Phosphate is released and then the ADP
Figure 9.12

11. ATP
ADP
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge.
Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
(low-energy 1 2 3 Pi
As new ATP attaches to the myosin head,
the link between myosin and actin weakens,
and the cross bridge detaches.
5. Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches
Figure 9.12

12. 6. Myosin Head is “cocked” when Energy is released by ATP hydrolysis into P and ADP
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
hydrolysis
As ATP is split into ADP and Pi, the myosin
head is energized (cocked into the high-energy
conformation).
Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
(low-energy
Thick filament 1 4 2 3 Pi
As new ATP attaches to the myosin head,
the link between myosin and actin weakens,
and the cross bridge detaches.
Figure 9.12

13. The Sliding Filament Theory …
7. Actin is pulled all the way inward as Head attachment, movement of Actin, and Detachment of Head Continues.
Figure 6.8a

14. Depends on Nervous stimulation
ATP
ADP
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
hydrolysis
As ATP is split into ADP and Pi, the myosin
head is energized (cocked into the high-energy
conformation).
Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
(low-energy
Thick filament 1 4 2 3 Pi
As new ATP attaches to the myosin head,
the link between myosin and actin weakens,
and the cross bridge detaches.
8. If Ca2+ and ATP continue to be available, the muscle will continue to contract.
Depends on Nervous stimulation
9. Rigor mortis = cross-bridge formation w/out ATP to detach (3-4 hrs after death before protein breakdown)
Figure 9.12

15. Run the Cross-Bridge Cycle video

16. REVIEW OVERVIEW OF STEPS TO MUSCLE CONTRACTION
Brain: message  spinal cord  nerve  Axon Terminal
1. Events at Neuromuscular Junction: Axon of Motor Neuron sends message to Sarcolemma
2. EXCITATION-CONTRACTION COUPLING:
Action Potential moves along sarcolemma and message transferred to Inside the Muscle cell
Sarcoplasmic Reticulum releases Calcium
Calcium binds to troponin …
Contraction begins
3. CROSS BRIDGE CYCLE (using Sliding Filament Model): Myofilaments shorten  Muscle fiber shortens  Muscle contracts

17. E. Role of Ionic Calcium (Ca2+) in Contraction
1. At low intracellular Ca2+ concentration:
Tropomyosin blocks binding sites on actin
Myosin cross bridges cannot attach to binding sites on actin
The relaxed state of the muscle is enforced
Figure 9.11a

18. Ionic Calcium (Ca2+) in Contraction Mechanism
2. At higher Ca2+ concentrations:
Extra Ca2+ binds to troponin
Calcium-activated troponin undergoes conformation change
Change moves tropomyosin away from actin’s binding sites
Figure 9.11b

19. 2. At higher Ca+2 Concentrations …
Myosin head can now bind and cycle
Permits contraction (sliding of thin filaments by myosin cross bridges) to begin
Figure 9.11d

20. END