1. Destruction of Acetylcholine
ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase
This destruction prevents continued muscle fiber contraction in the absence of additional stimuli
Events at the neuromuscular junction
Generation of an action potential

2. Excitation-Contraction Coupling

3. Figure 9.10 Synaptic cleft vesicle 1 ACh SR tubules (cut) SR 2 6 3
ADP Pi
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Removal of Ca2+ by active transport
into the SR after the action
potential ends. SR
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
Ca2+ 1 2 3 4 5 6
Figure 9.10

4. Figure 9.10 Synaptic cleft vesicle ACh
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Figure 9.10

5. Figure 9.10 Synaptic cleft vesicle 1 ACh
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma. 1
Figure 9.10

6. Figure 9.10 Synaptic cleft vesicle 1 ACh SR tubules (cut) SR Ca2+ 2
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol. SR
Ca2+ 1 2
Figure 9.10

7. Figure 9.10 Synaptic cleft vesicle 1 ACh SR tubules (cut) SR 2 3 Ca2+
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed. SR
Ca2+ 1 2 3
Figure 9.10

8. Figure 9.10 Synaptic cleft vesicle 1 ACh SR tubules (cut) SR 2 3 Ca2+
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process. SR
Ca2+ 1 2 3 4
Figure 9.10

9. Figure 9.10 Synaptic cleft vesicle 1 ACh SR tubules (cut) SR 2 3 Ca2+
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Removal of Ca2+ by active transport
into the SR after the action
potential ends. SR
Ca2+ 1 2 3 4 5
Figure 9.10

10. Figure 9.10 Synaptic cleft vesicle 1 ACh SR tubules (cut) SR 2 6 3
ADP Pi
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Removal of Ca2+ by active transport
into the SR after the action
potential ends. SR
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
Ca2+ 1 2 3 4 5 6
Figure 9.10

11. Sequential Events of Contraction
Cross bridge formation – myosin cross bridge attaches to actin filament
Working (power) stroke – myosin head pivots and pulls actin filament toward M line
Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches
“Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state
PLAY
InterActive Physiology ®: Sliding Filament Theory, pages 3-29

12. Figure 9.12 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
Figure 9.12

13. Figure 9.12 Myosin head ADP (high-energy configuration) Pi 1
Myosin head attaches to the actin
myofilament, forming a cross bridge. 1
Figure 9.12

14. Figure 9.12 Myosin head ADP (high-energy configuration) Pi 1
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
Figure 9.12

15. Figure 9.12 Myosin head ADP (high-energy configuration) Pi 1
ATP Pi
Myosin head
(high-energy
configuration)
Myosin head attaches to the actin
myofilament, forming a cross bridge. 1 2
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.
Figure 9.12

16. Figure 9.12 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.
Figure 9.12

17. Figure 9.12 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
Figure 9.12

18. Figure 9.12 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
Figure 9.12

19. Contraction of Skeletal Muscle Fibers
Contraction – refers to the activation of myosin’s cross bridges (force-generating sites)
Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening
Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced

20. Overview – Contraction of a Skeletal Muscle
Principles of muscle mechanics:
Force exerted by a contracting muscle on an object is muscle tension.
Opposing force on the muscle by the weight of the object is called the load.
A contracting muscle does not always shorten and move the load.
Isometric vs. Isotonic
A muscle contracts with varying force and for different periods of time in response to stimuli

21. The Motor Unit
Each muscle is served by at least one motor nerve
Contains axons of up to hundreds of motor neurons.

22. A motor unit = motor neuron and all the muscle fibers it supplies.
The Motor Unit
A motor unit = motor neuron and all the muscle fibers it supplies.
Motor neuron fires - all of the fibers it innervates contract
May have as many as several hundred or as few as four
Fine control vs. gross (large) movements
Muscle fibers are spread out within a unit
Stimulation of a single motor unit causes a weak contraction of the entire muscle.

23. The Muscle Twitch
Muscle twitch = the response of the motor unit to a single action potential
The muscle fibers contract quickly and then relax
Latent: excitation-contraction coupling is occurring
Contraction: cross bridges are active
Relaxation: initiated by reentry of Ca2+ into the SR

24. Developmental Aspects
All muscle tissues develop from embryonic mesoderm cells called myoblasts
Skeletal muscle myoblasts fuse (multiple nuclei)
Cardiac and Smooth develop gap junctions
Smooth muscle has good ability to regenerate throughout life
Muscle mass differs between sexes due to testosterone
Muscle is highly vascularized – resistant to infection