1. NOTES: The Muscular System (Ch 8, part 2)
Contraction of a Skeletal Muscle Fiber

2. Let’s Revisit Myofilaments!
Each muscle fiber contains thousands of myofibrils which are made up of myofilaments (proteins that play a key role in contraction)
There are 4 specific types of myofilaments
Myosin
Actin
Troponin
Tropomyosin
Myosin is considered a thick filament, and the remaining 3 are considered thin filaments

3. Myosin: has a moving head that uses energy

5. Skeletal Muscle Fibers:
**The organization of actin and myosin filaments produces the STRIATIONS (bands) seen in skeletal muscle
**The thick (myosin) and thin (actin) filaments are organized into structural units called SARCOMERES (functional units of muscle cells)
border of sarcomere: Z-lines

6. Structure of a Sarcomere

8. Now that we’ve discussed the Anatomy of a Muscle, we need to examine the junction between a nerve cell and a skeletal muscle cell. This junction is what “tells” the muscle to contract.

9. Neuromuscular Junction:
Junction between motor neuron & muscle fiber (cell)
MOTOR NEURONS stimulate muscle fibers to contract
 In response to a nerve impulse, the end of a motor neuron releases a NEUROTRANSMITTER called Acetylcholine (chemical), which stimulates the muscle fiber to contract

12. Excitation and Contraction
A motor neuron forms a synapse with a muscle fiber
Synapse means “chemical junction”
One motor neuron may stimulate many muscle fibers
1) A nerve impulse reaches the end of the motor neuron
2) Synaptic vesicles in the nerve release the neurotransmitter acetylcholine (ACh)
3) ACh diffuses across the synaptic cleft and binds to ACh receptors on the motor endplate of the muscle (part of sarcolemma in contact with neuron)
ANIMATION

14. 4) The stimulation of the ACh receptors causes a new impulse to be propagated along the sarcolemma (plasma membrane) and down T-tubules
5) This impulse causes calcium to be released from the sarcoplasmic reticulum
6) The release of calcium starts the sliding filament theory (the contraction of the muscle!)

16. Contraction occurs at the level of the sarcomere.
Another animation!

17. Sliding Filament Mechanism of Contraction: SKELETAL MUSCLE CONTRACTION
Anatomists noticed that when a muscle fiber (cell) contracts, each individual sarcomere shortens, however, the myofilaments do not change length
How does this occur?

18. SKELETAL MUSCLE CONTRACTION
*Muscle fiber contraction results from a sliding movement of actin and myosin filaments.
SARCOMERE

20. Role of MYOSIN and ACTIN:
 Myosin heads of myosin filaments can form linkages with actin filaments called cross-bridges
 the reaction between actin and myosin filaments generates the force of contraction

21. The myosin heads pull actin towards the center of the sarcomere
                                                                                                                                                                                                                                                                        

22. Exploring the Interaction of Calcium, Troponin, & Tropomyosin:
When a muscle is at rest (no action potential from motor neuron), calcium levels are low within the sarcoplasm (cytoplasm) of the cell.
As a result, the cross bridge binding sites on actin are blocked by the regulatory protein, tropomyosin which is held in place by troponin

23. Stimulation from a nerve impulse causes calcium to flood into the sarcoplasm of the cell from the sarcoplasmic reticulum
Calcium attaches to troponin which causes troponin to change shape
Troponin then drags tropomyosin off of the cross bridge binding sites on actin
Now the myosin head can attach to actin! (= a cross-bridge)

25. Cool
Hyperlink
Animation!

26. STEPS of SLIDING FILAMENT MODEL
Once binding sites on actin are exposed the following events occur:
1. Cross bridge attachment: high energy myosin heads, which are strongly attracted to actin, bind to actin, forming a cross bridge
ADP Pi
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge. 1

27. 2. Power Stroke: myosin head releases its energy; as a myosin head binds, it pulls on actin and slides it toward the center of the sarcomere
ADP Pi
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

28. ATP
ADP Pi
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge.
(low-energy
As new ATP attaches to the myosin head, the link between
myosin and actin weakens, and the cross bridge detaches. 1 2 3
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.
3. Crossbridge detachment: An ATP molecule binds to the myosin head causing it to detach

29. 4. “Cocking” of the myosin head: ATP energy causes the myosin head to return back to its “cocked” position
At this point the cycle is back to where it started!
ATP
ADP
hydrolysis Pi
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
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
As new ATP attaches to the myosin head, the link between
myosin and actin weakens, and the cross bridge detaches.
Thick filament 1 4 2 3
animation

30. Each myosin cross bridge attaches and detaches many times during contraction
Causes the actin to ratchet or move towards the center of the sarcomere
Animation

31. To end a contraction…
the muscle fiber relaxes (and the contraction ends) when
Motor neuron stops sending message from brain
Acetylcholine in the neuromuscular junction is broken down by the enzyme ACETYLCHOLINESTERASE
No more depolarization coming down T-tubules
Calcium ions are actively transported back into the sarcoplasmic reticulum
Without calcium present, the troponin-tropomyosin complex re-covers the myosin-binding sites on actin
Myosin can no longer grab actin
Actin slides away from center of sarcomere

33. CREEPY INFO:
Rigor Mortis- stiffness that occurs in muscles after death
Calcium leaks out of the sarcoplasmic reticulum causing the myofibrils to contract (Ca+2 pumps no longer working)
ATP is no longer being produced so cross bridges are unable to detach leaving the muscles in a state of contraction