1. Sliding Filament Mechanism
Ch notes

2. Sliding Filament Mechanism
Describes the cause and mechanism of muscle contraction
Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere
The thin filaments are pulled toward the M line and meet at the center of the sarcomere
The Z discs come closer together and the sarcomere shortens.
This shortening shortens the entire muscle

3. Sliding Filament Mechanism

4. The Contraction Cycle Contraction
SR releases Ca2+ ions into the cytoplasm
Ca2+ binds to troponin to move tropomyosin out of the way of the myosin-binding sites on actin
Contraction cycle – repeating sequence of events that causes the filaments to slide
4 steps

5. 4 Steps to Contraction Cycle
1. ATP hydrolysis
Breaking ATP into ADP + P adds energy to the myosin head so it can turn
2. Attachment of myosin to actin to form cross-bridges
Myosin head attaches to binding site on actin
Releases P
Crossbridge forms

6. 4 Steps to Contraction Cycle
3. Power Stroke
Myosin crossbridges rotate
4. Detachment of myosin from actin
Myosin remains attached until another ATP binds causes it to detach
* rigor mortis
Contraction cycle continues
- with ATP
- with Ca2+ levels high enough

7. Contraction Cycle

8. Contraction The myosin heads “walk” down the actin
They can attach and detach about 5 times per second

9. Neuromuscular Junction (NMJ)
Somatic motor neurons – neurons that stimulate skeletal muscle fibers
Place where action potentials arise
Synapse – region where 2 neurons or 1 neuron and a target cell (muscle fiber) meet
Synaptic cleft – gap that separates two cells
Neurotransmitter – chemical released for the 2 cells to communicate

10. NMJ

11. NMJ

12. NMJ

13. NMJ Parts
At the end of each axon are synaptic vesicles filled with acetylcholine (ACh)
ACh is the neurotransmitter
Motor end plate – sarcolemma opposite the synapse
Contains acetylcholine receptors
Action potential starts here with stimulation from neurons

14. Action Potential 1. Release of ACh 2. Activation of ACh receptors
Vesicles do exocytosis
Diffuse to motor end plate
2. Activation of ACh receptors
2 Ach bind to motor end plate and opens the receptor
Na+ flows across membrane through channel

15. Action Potential 3. Production of muscle action potential
Na+ makes fiber more + charged
This causes release of Ca+
Contraction cycle!
4. Termination of ACh activity
Acetylcholinesterase (AChE) – breaks down ACh so further muscle contractions don’t occur
Unless more ACh is released

16. Overview