1. © 2015 Pearson Education, Inc.

2. The Nerve Stimulus and Action Potential
Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract
Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that neuron
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3. Axon terminals at neuromuscular junctions
Figure 6.4a Motor units.
Axon terminals at neuromuscular junctions
Spinal cord
Motor unit 1
Motor unit 2
Nerve
Axon of motor neuron
Motor neuron cell bodies
Muscle
Muscle fibers
(a)

4. Axon terminals at neuromuscular junctions
Figure 6.4b Motor units.
Axon terminals at neuromuscular junctions
Muscle fibers
Branching axon to motor unit
(b)

5. The Nerve Stimulus and Action Potential
Neuromuscular junction
Association site of axon terminal of the motor neuron and sarcolemma of a muscle
Neurotransmitter
Chemical released by nerve upon arrival of nerve impulse in the axon terminal
Acetylcholine (ACh) is the neurotransmitter that stimulates skeletal muscle
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6. The Nerve Stimulus and Action Potential
Synaptic cleft
Gap between nerve and muscle
Nerve and muscle do not make contact
Filled with interstitial fluid
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7. Transmission of Nerve Impulse to Muscle
When a nerve impulse reaches the axon terminal of the motor neuron,
Calcium channels open, and calcium ions enter the axon terminal
Calcium ion entry causes some synaptic vesicles to release acetylcholine (ACh)
ACh diffuses across the synaptic cleft and attaches to receptors on the sarcolemma of the muscle cell
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8. Transmission of Nerve Impulse to Muscle
If enough ACh is released, the sarcolemma becomes temporarily more permeable to sodium (Na)
Sodium rushes into the cell, and potassium leaves the cell
Depolarization opens more sodium channels that allow sodium ions to enter the cell
Once started, the action potential cannot be stopped, and contraction occurs
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9. Transmission of Nerve Impulse to Muscle
Acetylcholinesterase (AChE) breaks down acetylcholine into acetic acid and choline
AChE ends muscle contraction
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10. Transmission of Nerve Impulse to Muscle
Cell returns to its resting state when:
Potassium ions diffuse out of the cell
Sodium-potassium pump moves sodium and potassium ions back to their original positions
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11. Figure 6.5 Events at the neuromuscular junction.
Slide 2
Myelinated axon of motor neuron
Nerve impulse
Axon terminal of
neuromuscular
junction
Nucleus
Sarcolemma of the muscle fiber
Action potential reaches axon terminal of motor neuron. 1
Synaptic vesicle containing ACh
Axon terminal of motor neuron
Mitochondrion
Ca2+
Ca2+
Synaptic cleft
Sarcolemma
Fusing synaptic vesicle
ACh receptor
ACh
Sarcoplasm of muscle fiber
Folds of sarcolemma

12. Figure 6.5 Events at the neuromuscular junction.
Slide 3
Action potential reaches axon terminal of motor neuron. 1
Synaptic vesicle containing ACh
Calcium (Ca2+) channels open, and Ca2+ enters the axon terminal. 2
Axon terminal of motor neuron
Mitochondrion
Ca2+
Ca2+
Synaptic cleft
Sarcolemma
Fusing synaptic vesicle
ACh receptor
ACh
Sarcoplasm of muscle fiber
Folds of sarcolemma

13. Figure 6.5 Events at the neuromuscular junction.
Slide 4
Action potential reaches axon terminal of motor neuron. 1
Synaptic vesicle containing ACh
Calcium (Ca2+) channels open, and Ca2+ enters the axon terminal. 2
Axon terminal of motor neuron
Mitochondrion
Ca2+
Ca2+
Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis. 3
Synaptic cleft
Sarcolemma
Fusing synaptic vesicle
ACh receptor
ACh
Sarcoplasm of muscle fiber
Folds of sarcolemma

14. Figure 6.5 Events at the neuromuscular junction.
Slide 5
Action potential reaches axon terminal of motor neuron. 1
Synaptic vesicle containing ACh
Calcium (Ca2+) channels open, and Ca2+ enters the axon terminal. 2
Axon terminal of motor neuron
Mitochondrion
Ca2+
Ca2+
Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis. 3
Synaptic cleft
Sarcolemma
Fusing synaptic vesicle
ACh receptor
ACh
Sarcoplasm of muscle fiber
Acetylcholine diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 4
Folds of sarcolemma

15. Figure 6.5 Events at the neuromuscular junction.
Slide 6
ACh binds and channels open that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions leave, producing a local change in the electrical conditions of the membrane (depolarization). This eventually leads to an action potential. 5
Na+ K+
Ion channel in sarcolemma opens; ions pass.

16. Figure 6.5 Events at the neuromuscular junction.
Slide 7
ACh
Degraded ACh
Na+
Ion channel closed; ions cannot pass.
The enzyme acetylcholinesterase breaks down ACh in the synaptic cleft, ending the process. 6
Acetylcholine- sterase K+

17. Flame spreads rapidly along the twig. 2
Figure 6.6 Comparing the action potential to a flame consuming a dry twig.
Small twig
Match flame
Flame ignites the twig. 1
Flame spreads rapidly along the twig. 2
(a)
Neuromuscular junction
Muscle cell or fiber
Nerve fiber
Striations
Na+ diffuses into the cell. 1
Action potential spreads rapidly along the sarcolemma. 2
(b)

18. Mechanism of Muscle Contraction: The Sliding Filament Theory
Calcium binds to regulatory proteins on thin filaments and exposes myosin-binding sites, allowing the myosin heads on the thick filaments to attach
The attached heads pivot, sliding the thin filaments toward the center of the sarcomere, and contraction occurs
ATP provides the energy for the sliding process, which continues as long as ionic calcium is present
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19. Figure 6.7 Diagrammatic views of a sarcomere.
Myosin
Actin Z I Z I H A
(a) Relaxed sarcomere Z I Z I A
(b) Fully contracted sarcomere

20. Protein complex Myosin myofilament Actin myofilament
Figure 6.8a Schematic representation of contraction mechanism: The sliding filament theory.
Protein complex
In a relaxed muscle cell, the regulatory proteins forming part of the actin myofilaments prevent myosin binding (see a). When an action potential (AP) sweeps along its sarcolemma and a muscle cell is excited, calcium ions (Ca2+) are released from intracellular storage areas (the sacs of the sarcoplasmic reticulum).
Myosin myofilament
Actin myofilament
(a)

21. Upper part of thick filament only (b)
Figure 6.8b Schematic representation of contraction mechanism: The sliding filament theory.
Myosin-binding site
The flood of calcium acts as the final trigger for contraction, because as calcium binds to the regulatory proteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thin filaments. This action exposes myosin-binding sites on the actin, to which the myosin heads can attach (see b), and the myosin heads immediately begin seeking out binding sites.
Ca2+
Upper part of thick filament only
(b)

22. Figure 6.8c Schematic representation of contraction mechanism: The sliding filament theory.
The free myosin heads are “cocked,” much like a set mousetrap. Myosin attachment to actin “springs the trap,” causing the myosin heads to snap (pivot) toward the center of the sarcomere. When this happens, the thin filaments are slightly pulled toward the center of the sarcomere (see c). ATP provides the energy needed to release and recock each myosin head so that it is ready to attach to a binding site farther along the thin filament. When the AP ends and calcium ions are returned to SR storage areas, the regulatory proteins resume their original shape and position, and again block myosin binding to the thin filaments. As a result, the muscle cell relaxes and settles back to its original length.
(c)

23. Contraction of Skeletal Muscle
Muscle fiber contraction is “all or none”
Within a skeletal muscle, not all fibers may be stimulated during the same interval
Different combinations of muscle fiber contractions may give differing responses
Graded responses—different degrees of skeletal muscle shortening
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24. Contraction of Skeletal Muscle
Graded responses can be produced by changing:
The frequency of muscle stimulation
The number of muscle cells being stimulated at one time
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25. Types of Graded Responses
Twitch
Single, brief contraction
Not a normal muscle function
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26. Tension (g) (Stimuli) (a) Twitch
Figure 6.9a A whole muscle’s response to different rates of stimulation.
Tension (g)
(Stimuli)
(a) Twitch

27. Types of Graded Responses
Summing of contractions
One contraction is immediately followed by another
Because stimulations are more frequent, the muscle does not completely return to a resting state
The effects are “summed” (added)
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28. (b) Summing of contractions
Figure 6.9b A whole muscle’s response to different rates of stimulation.
Tension (g)
(Stimuli)
(b) Summing of contractions

29. Types of Graded Responses
Unfused (incomplete) tetanus
Some relaxation occurs between contractions, but nerve stimuli arrive at an even faster rate than during summing of contractions
Unless the muscle contraction is smooth and sustained, it is said to be in unfused tetanus
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30. (c) Unfused (incomplete) tetanus
Figure 6.9c A whole muscle’s response to different rates of stimulation.
Tension (g)
(Stimuli)
(c) Unfused (incomplete) tetanus

31. Types of Graded Responses
Fused (complete) tetanus
No evidence of relaxation before the following contractions
Frequency of stimulations does not allow for relaxation between contractions
The result is a smooth and sustained muscle contraction
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32. (d) Fused (complete) tetanus
Figure 6.9d A whole muscle’s response to different rates of stimulation.
Tension (g)
(Stimuli)
(d) Fused (complete) tetanus

33. Muscle Response to Strong Stimuli
Muscle force depends upon the number of fibers stimulated
Contraction of more fibers results in greater muscle tension
Muscles can continue to contract unless they run out of energy
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34. Concept Link
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35. Energy for Muscle Contraction
ATP
Immediate source of energy for muscle contraction
Stored in muscle fibers in small amounts that are quickly used up
After this initial time, other pathways must be utilized to produce ATP
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36. Energy for Muscle Contraction
Three ways to generate ATP
Direct phosphorylation of ADP by creatine phosphate
Aerobic respiration
Anaerobic glycolysis and lactic acid formation
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37. Energy for Muscle Contraction
Direct phosphorylation of ADP by creatine phosphate (CP)—fastest
Muscle cells store CP, a high-energy molecule
After ATP is depleted, ADP remains
CP transfers a phosphate group to ADP to regenerate ATP
CP supplies are exhausted in less than 15 seconds
About 1 ATP is created per CP molecule
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38. Figure 6.10a Methods of regenerating ATP during muscle activity.
(a) Direct phosphorylation
Coupled reaction of creatine phosphate (CP) and ADP
Energy source: CP CP
ADP
Creatine
ATP
Oxygen use:
None
1 ATP per CP, creatine
Products:
Duration of energy provision:
15 seconds

39. Energy for Muscle Contraction
Aerobic respiration
Glucose is broken down to carbon dioxide and water, releasing energy (about 32 ATP)
A series of metabolic pathways occurs in the mitochondria
This is a slower reaction that requires continuous oxygen
Carbon dioxide and water are produced
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40. Figure 6.10c Methods of regenerating ATP during muscle activity.
(c) Aerobic pathway
Aerobic cellular respiration
Energy source:
glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism
Glucose (from glycogen breakdown or delivered from blood) O2
Pyruvic acid
Fatty acids O2
Aerobic respiration in mitochondria
Aerobic respiration in mitochondria
Amino acids 32
ATP
CO2
H2O
net gain per glucose
Oxygen use:
Required
32 ATP per glucose, CO2, H2O
Products:
Duration of energy provision:
Hours

41. Energy for Muscle Contraction
Anaerobic glycolysis and lactic acid formation
Reaction that breaks down glucose without oxygen
Glucose is broken down to pyruvic acid to produce about 2 ATP
Pyruvic acid is converted to lactic acid
This reaction is not as efficient, but is fast
Huge amounts of glucose are needed
Lactic acid produces muscle fatigue
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42. Figure 6.10b Methods of regenerating ATP during muscle activity.
(b) Anaerobic pathway
Glycolysis and lactic acid formation
Energy source:
glucose
Glucose (from glycogen breakdown or delivered from blood)
Glycolysis in cytosol O2 2
ATP
Pyruvic acid
net gain O2
Released to blood
Lactic acid
Oxygen use:
None
Products:
2 ATP per glucose, lactic acid
Duration of energy provision:
40 seconds, or slightly more

43. Muscle Fatigue and Oxygen Deficit
If muscle activity is strenuous and prolonged, muscle fatigue occurs because:
Ionic imbalances occur
Lactic acid accumulates in the muscle
Energy (ATP) supply decreases
After exercise, the oxygen deficit is repaid by rapid, deep breathing
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44. Types of Muscle Contractions
Isotonic contractions
Myofilaments are able to slide past each other during contractions
The muscle shortens, and movement occurs
Example: bending the knee; rotating the arm
Isometric contractions
Tension in the muscles increases
The muscle is unable to shorten or produce movement
Example: pushing against a wall with bent elbows
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45. Muscle Tone Muscle tone keeps muscles healthy and ready to react
Result of a staggered series of nerve impulses delivered to different cells within the muscle
If the nerve supply is destroyed, the muscle loses tone, becomes paralyzed, and atrophies
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46. Effect of Exercise on Muscles
Exercise increases muscle size, strength, and endurance
Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue
Makes body metabolism more efficient
Improves digestion, coordination
Resistance (isometric) exercise (weight lifting) increases muscle size and strength
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47. Figure 6.11 The effects of aerobic training versus strength training.

48. Table 6.2 The Five Golden Rules of Skeletal Muscle Activity.