1. 9
Muscles and Muscle Tissue: Part B

2. How IS contraction of a muscle cell turned on?
I wanna hit a home run…..

3. A nerve cell (brain or spinal cord must signal a muscle cell!
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Nucleus
Sarcolemma of
the muscle fiber
Action potential arrives at
axon terminal of motor neuron. 1
Ca2+
Synaptic vesicle
containing ACh
Ca2+
Voltage-gated Ca2+ channels
open and Ca2+ enters the axon
terminal. 2
Mitochondrion
Synaptic
cleft
Axon terminal
of motor neuron
Ca2+ entry causes some
synaptic vesicles to release
their contents (acetylcholine)
by exocytosis. 3
Fusing synaptic
vesicles
Junctional
folds of
sarcolemma
ACh
Acetylcholine, a
neurotransmitter, diffuses across
the synaptic cleft and binds to
receptors in the sarcolemma. 4
Sarcoplasm of
muscle fiber
ACh binding opens ion
channels that allow simultaneous
passage of Na+ into the muscle
fiber and K+ out of the muscle
fiber. 5
Na+ K+
Postsynaptic membrane
ion channel opens;
ions pass causing a brief
depolarization and repolarization.
ACh effects are terminated
by its enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase. 6
Ach–
Degraded ACh
Postsynaptic membrane
ion channel closed;
ions cannot pass so no further depolarization occurs.
Na+
Acetyl-
cholinesterase K+
Figure 9.8

4. WHY do the ions move the way they do?
Why does Ca+ move INTO the nerve cell?
Why does Na+ move INTO the muscle cell?
Why does K+ move OUT of the muscle cell?
HINT: you learned this in General Biology
OUTSIDE IN IN
Lo Na+
Lo Cl-
Lo Ca+
Hi K+
Hi Na+
Hi Cl-
Hi Ca+
Lo K+
Lo Na+
Lo Cl-
Lo Ca+
Hi K+

5. Ions flow from HI to LOW down their concentration gradient.
Ca++ is higher outside the cell, so tends to move into the cell.
Na+ is higher outside the cell, so tends to move into the cell.
K+ is higher inside the cell, so tends to move out of the cell.
Very few ions move, so the concentrations don’t change.
OUTSIDE IN IN
Lo Na+
Lo Cl-
Lo Ca+
Hi K+
Hi Na+
Hi Cl-
Hi Ca+
Lo K+
Lo Na+
Lo Cl-
Lo Ca+
Hi K+

6. RULES for ION MOVMENT Lo Na+ OCEAN Lo Cl- Hi Na+ Lo Ca+ Hi Cl- Hi K+
IONS FLOW FROM areas of High Concentration to Low Concentration
In other words: IONS MOVE DOWN THEIR CONCENTRATION GRADIENT)
In other words: IONS FLOW FROM HIGH TO LOW
REMEMBER:
The ocean (EC fluid) is salty (Lots of NaCl & CaCl2).
The fish (cell) is just the opposite of the ocean (EC fluid).
Salt tends to flow into the fish (cell).
K+ tends to flow out of the fish (cell).
Lo Na+
Lo Cl-
Lo Ca+
Hi K+
OCEAN
Hi Na+
Hi Cl-
Hi Ca+
Lo K+
FISH

7. ACh opens chemical-gated channels, so small
amount of Na+ enter the muscle. The cell to becomes very briefly POSTIVE (depolarization)
To make an ACTION POTENTIAL requires voltage-gated channels to open, allowing much MORE Na+ in.
Open Na+
Channel
Closed K+
Channel
Synaptic
cleft
Na+
ACh
Na+ K+ K+
ACh + + + + + + + + +
Action potential +
Na+ K+ i o n + + t z a
Generation and propagation of
the action potential (AP) 2 a r i l p o d e o f e W a v
Closed Na+
Channel
Open K+
Channel
Local depolarization:
generation of the end
plate potential on the
sarcolemma 1
Na+ K+ 3
Sarcoplasm of muscle fiber
Repolarization
Figure 9.9

8. Why is it important to know how the NMJ works?
It turns out that many toxins and poisons act on the neuromuscular junction.
These toxins can produce two effects, both resulting in loss of muscle control:
Flaccid paralysis – the muscle cannot contract at all.
Tetanic paralysis – the muscle is in a constant, fully contracted state.
Discuss the following questions with your neighbors, and then choose the appropriate answer.

9. Terminal cisterna of SR
The action potential continues down the muscle cell, falling into a T tubule.
Action potential
is generated
ACh
Sarcolemma
Terminal cisterna of SR
Ca2+
Figure 9.11, step 1

10. At rest, the voltage inside the
muscle cell is NEGATIVE,
Axon terminal
Open Na+
Channel
Closed K+
Channel
Synaptic
cleft
Na+
ACh K+
Na+ K+
ACh + + + + + + + + + n +
Action potential o + +
Na+ K+ a t i z r i l a o p d e o f v e W a 1 1
The cell is negative inside at rest
Sarcoplasm of muscle fiber
Figure 9.9, step 1

11. + ACh is released and crosses the synapse.
ACh binds to the ACh receptor.
The receptor channel opens
and allows Na+ to enter the cell
Na+ is a positive ion and so the
cell briefly becomes POSITIVE inside
Axon terminal
Open Na+
Channel
Closed K+
Channel
Synaptic
cleft
Na+ +
ACh + K+
Na+ K+
ACh + + + + + + n +
Action potential + + + o + +
Na+ K+ t i z a r i 2 a
Generation and propagation of the
action potential (AP) o l
+++++ p d e o f e W a v 1
Sarcoplasm of muscle fiber
Figure 9.9, step 2

12. The positive charge causes DIFFERENT Na+ channels to open, and more Na+enters, SO, the +++ charge is carried down the cell membrane
Axon terminal
Open Na+
Channel
Closed K+
Channel
Synaptic
cleft
Na+ +
ACh + K+
Na+ K+
ACh + + + + + + +
Action potential + + + n o + +
Na+ K+ t i a r i z 2 a
Generation and propagation of the
action potential (AP) o l p d e o f v e W a 1 1
Local depolarization: generation of the
end plate potential on the sarcolemma
Sarcoplasm of muscle fiber
Figure 9.9, step 2

13. + THE TRAVELING POSITIVE CHARGE IS CALLED
AN ACTION POTENTIAL. THE AP IS A SIGNAL THAT
TURNS ON CONTRACTION (LIKE A DOORBELL)
Axon terminal
Open Na+
Channel
Closed K+
Channel
Synaptic
cleft
Na+ +
ACh + K+
Na+ K+
ACh + + + + + + +
Action potential + + + n o + +
Na+ K+ t i a r i z 2 a
Generation and propagation of the
action potential (AP) o l p d e o f v e W a 1 1
Local depolarization: generation of the
end plate potential on the sarcolemma
Sarcoplasm of muscle fiber
Figure 9.9, step 2

14. Ca++ is released from the SR into the cytoplasm
The Action potential is a signal that moves along the sarcolemma and down the T tubules, and then
Ca++ is released from the SR into the cytoplasm
Steps in
E-C Coupling:
Sarcolemma
Voltage-sensitive
tubule protein
T tubule
Ca2+
release
channel
Terminal
cisterna
of SR
Ca2+
Figure 9.11, step 3

15. Can you explain why calcium is able to diffuse OUT of the sarcoplasmic reticulum and into the cytoplasm of the cell? ? .
T tubule
Ca2+
release
channel .
Ca2+
Figure 9.11, step 4

16. Into the cytoplasm starts the thin and thick filament interaction.
Steps in E-C Coupling:
Sarcolemma
Voltage-sensitive
tubule protein
T tubule 1
Action potential is propagated along
the sarcolemma and down the T tubules.
Calcium flowing
Into the cytoplasm starts the thin and thick filament interaction.
Ca2+
release
channel
Calcium ions are released. 2
Terminal
cisterna
of SR
Ca2+
Actin
Troponin
Tropomyosin
blocking active sites
Ca2+
Myosin 3
Calcium binds to troponin and
removes the blocking action of
tropomyosin.
Active sites exposed and
ready for myosin binding 4
Contraction begins
Myosin
cross
bridge
The aftermath
Figure 9.11, step 2

17. Actin Troponin Tropomyosin blocking active sites Ca2+ Myosin
The aftermath
Figure 9.11, step 5

18. 3 Actin Troponin Tropomyosin blocking active sites Ca2+ Myosin
Calcium binds to
troponin and removes
the blocking action of
tropomyosin. 3
Active sites exposed and
ready for myosin binding
The aftermath
Figure 9.11, step 6

19. 3 4 Actin Troponin Tropomyosin blocking active sites Ca2+ Myosin
Calcium binds to
troponin and removes
the blocking action of
tropomyosin. 3
Active sites exposed and
ready for myosin binding
Contraction begins 4
Myosin
cross
bridge
The aftermath
Figure 9.11, step 7

20. Figure 9.11, step 8 Steps in E-C Coupling: The aftermath Sarcolemma
Voltage-sensitive
tubule protein
T tubule 1
Action potential is propagated along
the sarcolemma and down the T tubules.
Ca2+
release
channel
Calcium ions are released. 2
Terminal
cisterna
of SR
Ca2+
Actin
Troponin
Tropomyosin
blocking active sites
Ca2+
Myosin
Calcium binds to troponin and
removes the blocking action of
tropomyosin. 3
Active sites exposed and
ready for myosin binding
Contraction begins 4
Myosin
cross
bridge
The aftermath
Figure 9.11, step 8

21. ATP is needed for this to occur.
The thick filaments pull on the thin filaments causing the cell to contract.
ATP is needed for this to occur.
Thin filament
Actin
Ca2+
Myosin
cross bridge
ADP Pi
Thick
filament
Myosin 1
Cross bridge formation.
ADP
ADP Pi
ATP
hydrolysis Pi 4
Cocking of myosin head. 2
The power (working)
stroke.
ATP
ATP 3
Cross bridge
detachment.
Figure 9.12

22. Cross bridge formation.
Actin
Ca2+
Thin filament
ADP
Myosin
cross bridge Pi
Thick filament
Myosin 1
Cross bridge formation.
Figure 9.12, step 1

23. The power (working) stroke.
ADP Pi 2
The power (working) stroke.
Figure 9.12, step 3

24. Cross bridge detachment.
ATP 3
Cross bridge detachment.
Figure 9.12, step 4

25. ADP
ATP
hydrolysis Pi 4
Cocking of myosin head.
Figure 9.12, step 5

26. Figure 9.12 Thin filament Actin Ca2+ Myosin cross bridge Thick
ADP Pi
Thick
filament
Myosin 1
Cross bridge formation.
ADP
ADP Pi
ATP
hydrolysis Pi 4
Cocking of myosin head. 2
The power (working)
stroke.
ATP
ATP 3
Cross bridge
detachment.
Figure 9.12

27. Lets look at ATP’s role in the cross-bridge cycle