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Chapter 12a
Muscles
About this Chapter
• Skeletal muscle• Mechanics of body movement• Smooth muscle• Cardiac muscle
Three Types of Muscle
Figure 12-1a
Nucleus
Muscle fiber(cell)
Striations
(a) Skeletal muscle
Three Types of Muscle
Figure 12-1b
Muscle fiber
Nucleus
Striations
(b) Cardiac muscle
Intercalated disk
Three Types of Muscle
Figure 12-1c
Muscle fiber
Nucleus
(c) Smooth muscle
Skeletal Muscle
• Usually attached to bones by tendons• Origin: closest to the trunk• Insertion: more distal• Flexor: brings bones together• Extensor: moves bones away• Antagonistic muscle groups: flexor-extensor
pairs
Antagonistic Muscle Groups
Figure 12-2a
(a) Flexion
Biceps musclecontracts (flexor)
Tricepsmusclerelaxes
Antagonistic Muscle Groups
Figure 12-2b
(b) Extension
Triceps musclecontracts (extensor)
Bicepsmusclerelaxes
Organization of Skeletal Muscle
Figure 12-3a (1 of 2)
Skeletal muscle
Muscle fascicle:bundle of fibers
Muscle fiber
Connective tissue
Connectivetissue
Tendon Nerve andblood vessels
Nucleus
(a)
Organization of Skeletal Muscle
Figure 12-3a (2 of 2)
Ultrastructure of Muscle
Figure 12-3b-f
T-tubules
(c)
(d)
(b)
Sarcoplasmicreticulum
Sarcolemma
Mitochondria
Myofibril
Myofibril
Thickfilament
Thinfilament
A band
Z disk
Z disk
(f)
Z disk
I bandM line H zone
Z disk
Sarcomere
Thin filaments
Tropomyosin
Troponin
Actin chain
G-actin molecule
Myosin tail
Myosin heads
Myosin molecule
Thick filaments
Nucleus
Hingeregion
(e)Titin
Nebulin
Titin
M line Myosincrossbridges
M line
ULTRASTRUCTURE OF MUSCLE
ANATOMY SUMMARY
Figure 12-3b
Ultrastructure of Muscle
T-tubules
Sarcoplasmicreticulum
Sarcolemma
Mitochondria
Myofibril
Thickfilament
ThinfilamentNucleus
ULTRASTRUCTURE OF MUSCLE
(b)
Figure 12-3c
Ultrastructure of Muscle
(c)
Myofibril
A bandZ disk
I bandM line H zone
Z diskSarcomere
Figure 12-3d
Ultrastructure of Muscle
(d)Z disk Z disk
Titin
M line Myosincrossbridges
Figure 12-3e
Ultrastructure of Muscle
(e)
Myosin tail
Myosin heads
Myosin molecule
Thick filaments
Hingeregion
M line
Figure 12-3f
Ultrastructure of Muscle
(f)
Thin filaments
Tropomyosin
Troponin
Actin chain G-actin molecule
Titin
Nebulin
Ultrastructure of Muscle
Figure 12-3c-f
(c)
(d)
Myofibril
A bandZ disk
Z disk
(f)
Z disk
I bandM line H zone
Z diskSarcomere
Thin filaments
Tropomyosin
Troponin
Actin chain G-actin molecule
Myosin tail
Myosin heads
Myosin molecule
Thick filaments
Hingeregion
(e)Titin
Nebulin
Titin
M line Myosincrossbridges
M line
T-Tubules and the Sarcoplasmic Reticulum
Figure 12-4
Sarcolemma
Thin filament
Thick filament
Triad Terminalcisterna
T-tubule brings actionpotentials into interiorof muscle fiber.
Sarcoplasmic reticulumstores Ca2+
The Two- and Three-Dimensional Organization of a Sarcomere
Figure 12-5
Sarcomere
(a)
(b)
A band I band H zone I band
Z diskZ disk M line
Thick filament
Thin filament
Z disk Z disk
(c)
I bandthin filaments
only
H zonethick filaments
only
Outer edgeof A band
thick and thinfilaments overlap
M linethick filaments
linked withaccessory proteins
Anatomy Review Animation
PLAY Interactive Physiology® Animation: Muscular System: Anatomy Review: Skeletal Muscle Tissue
Muscle Contraction
• Muscle tension: force created by muscle• Load: weight that opposes contraction• Contraction: creation of tension in muscle• Relaxation: release of tension• Steps leading up to muscle contraction:
1. Events at the neuromuscular junction
2. Excitation-contraction coupling
3. Contraction-relaxation cycle
Summary of Muscle Contraction
Figure 12-7
Events at the Neuromuscular Junction
PLAY Events at the Neuromuscular Junction
PLAY Interactive Physiology® Animation: Muscular System: Events at the Neuromuscular Junction
Changes in a Sarcomere During Contraction
Figure 12-8
Z
Z
Zline
Zline
Mline
Musclerelaxed
Musclecontracted
Sarcomere shortenswith contraction
I band
A bandActin
Z
M
Z
Z
Myosin
Half ofI band
Half ofI band
H zone
H
H
I
A band constant
IH zone and I band both shorten
Z
A band
Half ofI band
Sliding Filament Theory
PLAY Interactive Physiology® Animation: Muscular System: Sliding Filament Theory
The Molecular Basis of Contraction
Figure 12-9a
ADP
Troponin G-Actin
Tropomyosinblocks binding
site on actin
Myosin head
TN
(a) Relaxed state. Myosin head cocked.
Pi
The Molecular Basis of Contraction
Figure 12-9b
Actinmoves
Cytosolic Ca2+
Tropomyosin shifts,exposing bindingsite on actin
TN
Power stroke
(b) Initiation of contraction
Ca2+ levels increasein cytosol.
Ca2+ binds to troponin (TN).
Troponin-Ca2+
complex pulls tropomyosin away from actin’smyosin-binding site.
Myosin binds to actin and completes power stroke.
Actin filament moves.
Pi
1
2 3
4
5
5
4
3
2
1
ADP
The Molecular Basis of Contraction
Figure 12-10
G-actin molecule
Tight binding in the rigor state
ATP binds to myosin.Myosin releases actin.
Myosinbinding sites
Myosinfilament
Myosin hydrolyses ATP. Myosinhead rotates and binds to actin.
ATP
Actin filament movestoward M line.
Myosin releases ADP.Contraction-relaxation
Sliding filament
ADP
Power stroke
Relaxed state with myosin heads cocked
Ca2+
signal
ADPPiPi
1
2
3
4
The Molecular Basis of Contraction
Figure 12-10, step 0
G-actin molecule
Tight binding in the rigor state
Myosinbinding sites
Myosinfilament
The Molecular Basis of Contraction
Figure 12-10, steps 0–1
G-actin molecule
Tight binding in the rigor state
ATP binds to myosin.Myosin releases actin.
Myosinbinding sites
Myosinfilament
ATP
1
The Molecular Basis of Contraction
Figure 12-10, steps 1–2
1 ATP binds to myosin.Myosin releases actin.
Myosin hydrolyses ATP. Myosin head rotates and binds to actin.
ATP
Relaxed state with myosin heads cocked
ADPPi
2
The Molecular Basis of Contraction
Figure 12-10, steps 2–3
2 Myosin hydrolyses ATP. Myosin head rotates and binds to actin.
Actin filament movestoward M line.
Power stroke
Relaxed state withmyosin heads cocked
Ca2+
signal
ADPPi
Pi
3
The Molecular Basis of Contraction
Figure 12-10, steps 3–4
Actin filament movestoward M line.
Myosin releases ADP.
ADP
Power stroke
Pi
43
Excitation-Contraction Coupling
Figure 12-11a
Myosin head
Myosin thick filament
M line
Axon terminal ofsomatic motor neuron
Motor end plate
Sarcoplasmic reticulum
ACh
DHP
DHP = dihydropyridine L-type calcium channel
RyR
RyR = ryanodine receptor-channel
T-tubule
Muscle fiber
(a) Initiation of muscle action potential
Somatic motor neuron releases ACh at neuromuscular junction.
Net entry of Na+ through ACh receptor-channel initiates a muscle action potential
Troponin
Tropomyosin
KEY
Actin
Z disk
Na+
Ca2+
1
2
1
2
Excitation-Contraction Coupling
Figure 12-11a, step 1
Myosin head
Myosin thick filament
M line
Axon terminal ofsomatic motor neuron
Motor end plate
Sarcoplasmic reticulum
ACh
DHP
DHP = dihydropyridine L-type calcium channel
RyR
RyR = ryanodine receptor-channel
T-tubule
Muscle fiber
(a) Initiation of muscle action potential
Somatic motor neuron releases ACh at neuromuscular junction.
Troponin
Tropomyosin
KEY
Actin
Z disk
Ca2+
1 1
Excitation-Contraction Coupling
Figure 12-11a, steps 1–2
Myosin head
Myosin thick filament
M line
Axon terminal ofsomatic motor neuron
Motor end plate
Sarcoplasmic reticulum
ACh
DHP
DHP = dihydropyridine L-type calcium channel
RyR
RyR = ryanodine receptor-channel
T-tubule
Muscle fiber
(a) Initiation of muscle action potential
Somatic motor neuron releases ACh at neuromuscular junction.
Net entry of Na+ through ACh receptor-channel initiates a muscle action potential
Troponin
Tropomyosin
KEY
Actin
Z disk
Na+
Ca2+
1
2
1
2
Excitation-Contraction Coupling
Figure 12-11b
DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel
Ca2+ released
(b) Excitation-contraction couplingDistance actin moves
Myosin thick filament
Action potential in t-tubule altersconformation of DHP receptor.
DHP receptor opens RyR Ca2+
release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm.
Ca2+ binds to troponin, allowing actin-myosin binding.
Myosin heads execute powerstroke.
Actin filament slides toward center of sarcomere.
KEY
3
4
5
6
7
34
5
6
7
Electrical and Mechanical Events in Muscle Contraction
• A twitch is a single contraction-relaxation cycle
Figure 12-12
Muscle fibermembrane
potentialin mV
Developmentof tension
during onemuscle twitch
+30
-70 2msec
Latentperiod
Contractionphase
Motorend plate
Axonterminal
Muscle actionpotential
Time
10–100 msec
Relaxationphase
Time
Neuronmembrane
potentialin mV
+30Muscle fiber
Action potentialfrom CNS
Recordingelectrodes
-70
Time
Ten
sio
n
Phosphocreatine
Figure 12-13
1. Creatine phosphate2. Glycolysis3. Krebs cycle
Locations and Possible Causes of Muscle Fatigue
Figure 12-14
Causes of Muscle Fatigue During Exercise
• Extended submaximal exercise • Depletion of glycogen stores
• Short-duration maximal exertion • Increased levels of inorganic phosphate• May slow Pi release from myosin• Decrease calcium release
• Maximal exercise• Potassium (K+) leaves muscle fiber, leading to
increased concentration that is believed to decrease Ca2+
Skeletal Muscle Metabolism During Fatiguing Submaximal Exercise
Question 12-1
Fast-Twitch Glycolytic and Slow-Twitch Oxidative Muscle Fibers
Figure 12-15
Fast-Twitch Glycolytic and Slow-Twitch Oxidative Muscle Fibers
Table 12-2
Length-Tension Relationships in Contracting Skeletal Muscle
Figure 12-16