1. Skeletal Muscle Activity: Contraction
Chapter 6
Part 2A

2. The Nerve Stimulus & Action Potential
Muscle Contraction Requires:
Electrical current propagation
Stimulation by motor neuron
Motor Unit – one neuron and all the skeletal cells it stimulates
Rise in intracellular Ca2+ levels
Neuromuscular Junction
Location where motor neuron stimulates muscle contractions
Spinal cord
Motor neuron
cell body
Muscle
Nerve
Motor
unit 1
unit 2
fibers
neuron
axon
neuromuscular junctions

3. Anatomy of a Neuromuscular Junction
Synaptic Cleft
Space between motor neuron and
Synaptic Vesicles
Contain a Neurotransmitters
Sarcolemma
Motor End Plate
specialized region with many folds
Receives neurotransmitter
T Tubule
Carries Action Potential SR
Regulates Calcium
Sarcomere
Contracting Unit of Muscle

4. Labeling the NMJ

5. Transmission of Nerve Impulse to Muscle
Neurotransmitter—chemical released by nerve upon arrival of nerve impulse
The neurotransmitter for skeletal muscle is acetylcholine (ACh)
Acetylcholine attaches to receptors on the sarcolemma
Sarcolemma becomes permeable to sodium (Na+)
Sodium rushes into the cell generating an action potential

6. Transmission of Nerve Impulse to Muscle
Once started, muscle contraction cannot be stopped

7. Sliding Filament Theory
Chapter 6
Sliding Filament Theory

8. Sliding Filament Model of Contraction
The generation of force
Does not necessarily cause shortening of the fiber
During Relaxation:
thin and thick filaments overlap only slightly
During contraction:
Myosin heads bind to actin on thin filament
Detach
Bind again to next site on thin filament
Propels the thin filaments toward the M line
H zones shorten and disappear, sarcomeres shorten, muscle cells shorten I
Fully relaxed sarcomere of a muscle fiber
Fully contracted sarcomere of a muscle fiber A Z H

9. Molecular Participants
Myosin
Flexing heads (Cross Bridge)
Provides ‘Power Stroke’
Flexing rods
Allows Actin binding
ATP (Nucleotide)
Transfers energy to myosin and initiates flexing (shape change)
Actin
Contain myosin binding sites
Tropomyosin
Regulates access to actin’s myosin binding sites
Troponin
Exposes actin binding site by moving tropomyosin
Calcium Ions
Activate Troponin

10. Myosin & ATP
ATP transfers its energy to the myosin head, which in turn energizes the power stroke.
ATP disconnects the myosin head from the binding site on actin.

11. Calcium Ions – EC Coupling
Role of Calcium in Starting Muscle Contraction (Excitation-Contraction Coupling):
Action potential moves along sarcolemma to the T tubules
Calcium Ions are Released from SR
Calcium Ions then Bind to Troponin
Tropomyosin Moves Away from the Myosin Binding Sites on Actin
When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

12. 1 2 Action potential is propagated along the sarcolemma and down
the T tubules. 1
Steps in
E-C Coupling:
Sarcolemma
Voltage-sensitive
tubule protein
T tubule
Ca2+
release
channel 2
Calcium
ions are
released.
Terminal
cisterna
of SR
Ca2+

13. Troponin
Tropomyosin
blocking active sites
Myosin
Actin
Ca2+
The aftermath

14. 3 Troponin Tropomyosin blocking active sites Myosin Actin
Active sites exposed and
ready for myosin binding
Ca2+
Calcium binds to
troponin and removes
the blocking action of
tropomyosin.
The aftermath 3

15. 3 4 Troponin Tropomyosin blocking active sites Myosin Actin
Active sites exposed and
ready for myosin binding
Ca2+
cross
bridge
Calcium binds to
troponin and removes
the blocking action of
tropomyosin.
Contraction begins
The aftermath 3 4

16. 4 Steps of Cross Bridge Cycle
Step 1: Cross Bridge Formation
Binding of Myosin to Actin
Tail hinge of the myosin bends and energized myosin head binds to the actin.
Actin
Cross bridge formation.
Ca2+
Myosin
cross bridge
Thick filament
Thin filament
ADP Pi 1

17. 4 Steps of Cross Bridge Cycle
Step 2: Power Stroke of the Cross Bridge
The ADP and Pi are released from the actin.
The activated myosin head tilts backward.
The power stroke occurs as the thin filament is pulled inward toward the center of the sarcomere.
There has been a transfer of energy from the myosin head to the movement of the thin filament.
The power (working) stroke. 2

18. 4 Steps of Cross Bridge Cycle
Step 3: Cross Bridge Detatchment
Disconnecting the Myosin Head
ATP binds to the myosin head, and it disconnects from the actin
Cross bridge detachment.
ATP 3

19. 4 Steps of Cross Bridge Cycle
Step 4: Reactivation of the Myosin Head
ATP hydrolysis transfer energy to the myosin head producing ADP + Pi and returns Myosin to energized position.
Cocking of myosin head.
ATP
hydrolysis
ADP Pi 4