Chapter 12a
Muscles
About this Chapter
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Skeletal muscle
Mechanics of body movement
Smooth muscle
Cardiac muscle
Three Types of Muscle
Nucleus
Muscle fiber
(cell)
Striations
(a) Skeletal muscle
Figure 12-1a
Three Types of Muscle
Striations
Muscle fiber
Intercalated disk
(b) Cardiac muscle
Nucleus
Figure 12-1b
Three Types of Muscle
Muscle fiber
Nucleus
(c) Smooth muscle
Figure 12-1c
Skeletal Muscle
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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
Triceps
muscle
relaxes
Biceps muscle
contracts (flexor)
(a) Flexion
Figure 12-2a
Antagonistic Muscle Groups
Triceps muscle
contracts (extensor)
Biceps
muscle
relaxes
(b) Extension
Figure 12-2b
Organization of Skeletal Muscle
Skeletal muscle
Tendon
Nerve and
blood vessels
Connective tissue
Muscle fascicle:
bundle of fibers
Connective
tissue
Nucleus
Muscle fiber
(a)
Figure 12-3a (1 of 2)
Organization of Skeletal Muscle
Figure 12-3a (2 of 2)
Ultrastructure of Muscle
ANATOMY SUMMARY
ULTRASTRUCTURE OF MUSCLE
Mitochondria
Sarcoplasmic
reticulum
Thick
filament
Nucleus
Thin
filament
T-tubules
Myofibril
Sarcolemma
(b)
A band
Sarcomere
Z disk
Z disk
Myofibril
(c)
M line
I band
H zone
Titin
(d)
Z disk
Z disk
M line
Myosin
crossbridges
M line
Thick filaments
Thin filaments
Titin
(e)
Troponin Nebulin
Myosin heads
Myosin tail
Hinge
region
Tropomyosin
Myosin molecule
(f)
G-actin molecule
Actin chain
Figure 12-3b-f
Ultrastructure of Muscle
ULTRASTRUCTURE OF MUSCLE
Mitochondria
Sarcoplasmic
reticulum
Thick
Thin
filament filament
Nucleus
T-tubules
Myofibril
Sarcolemma
(b)
Figure 12-3b
Ultrastructure of Muscle
Sarcomere
A band
Z disk
Z disk
Myofibril
(c)
M line
I band
H zone
Figure 12-3c
Ultrastructure of Muscle
Titin
(d)
Z disk
M line
Myosin
crossbridges
Z disk
Figure 12-3d
Ultrastructure of Muscle
M line
Thick filaments
(e)
Myosin
heads
Myosin tail Hinge
region
Myosin molecule
Figure 12-3e
Ultrastructure of Muscle
Thin filaments
Titin
Troponin Nebulin
Tropomyosin
G-actin molecule
Actin chain
(f)
Figure 12-3f
Ultrastructure of Muscle
Sarcomere
A band
Z disk
Z disk
Myofibril
(c)
M line
I band
H zone
Titin
(d)
Z disk
M line
Thick filaments
M line
Myosin
crossbridges
Z disk
Thin filaments
Titin
(e)
Myosin
heads
Myosin tail Hinge
region
Myosin molecule
Troponin Nebulin
Tropomyosin
G-actin molecule
Actin
chain
(f)
Figure 12-3c-f
T-Tubules and the Sarcoplasmic Reticulum
T-tubule brings action
potentials into interior
of muscle fiber.
Triad
Thin filament
Sarcolemma
Sarcoplasmic reticulum
stores Ca2+
Thick filament
Terminal
cisterna
Figure 12-4
The Two- and Three-Dimensional Organization of a
Sarcomere
I band
Sarcomere
A band
H zone
I band
Thin filament
Thick filament
(a)
Z disk
(b)
Z disk
Z disk
I band
thin filaments
only
(c)
M line
Z disk
H zone
thick filaments
only
M line
thick filaments
linked with
accessory proteins
Outer edge
of A band
thick and thin
filaments overlap
Figure 12-5
Anatomy Review Animation
PLAY
Interactive Physiology® Animation: Muscular System:
Anatomy Review: Skeletal Muscle Tissue
Muscle Contraction
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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
PLAY
Events at the Neuromuscular Junction
Interactive Physiology® Animation: Muscular System:
Events at the Neuromuscular Junction
Changes in a Sarcomere During Contraction
I band
Myosin
Z
Actin
Z
A band
Muscle
relaxed
Z
Half of
I band
Sarcomere shortens
with contraction
Z
M
H zone
H
A band constant
Z
Half of
I band
Z
A band
Half of
I band
M
line
Z
line
Z
line
Muscle
contracted
I
H
I
H zone and I band both shorten
Figure 12-8
Sliding Filament Theory
PLAY
Interactive Physiology® Animation: Muscular System:
Sliding Filament Theory
The Molecular Basis of Contraction
Troponin
G-Actin
TN
Myosin head
Tropomyosin
blocks binding
site on actin
Pi
ADP
(a) Relaxed state. Myosin head cocked.
Figure 12-9a
The Molecular Basis of Contraction
1
Cytosolic Ca2+
3 Tropomyosin shifts,
exposing binding
site on actin
2
TN
5
ADP
Power stroke 4
Pi
(b) Initiation of contraction
Actin
moves
1 Ca2+ levels increase
in cytosol.
2 Ca2+ binds to
troponin (TN).
3 Troponin-Ca2+
complex pulls
tropomyosin
away from actin’s
myosin-binding site.
4 Myosin binds to
actin and completes
power stroke.
5 Actin filament
moves.
Figure 12-9b
The Molecular Basis of Contraction
G-actin molecule
Myosin
binding sites
1 ATP binds to myosin.
Myosin releases actin.
Myosin
filament
Tight binding in the rigor state
ATP
ADP
2 Myosin hydrolyses ATP. Myosin
head rotates and binds to actin.
4 Myosin releases ADP.
Contractionrelaxation
Actin filament moves
toward M line.
Sliding filament
Pi
Ca2+
ADP
Pi
signal
3 Power stroke
Relaxed state with myosin heads cocked
Figure 12-10
The Molecular Basis of Contraction
G-actin molecule
Myosin
binding sites
Myosin
filament
Tight binding in the rigor state
Figure 12-10, step 0
The Molecular Basis of Contraction
G-actin molecule
Myosin
binding sites
Myosin
filament
1 ATP binds to myosin.
Myosin releases actin.
Tight binding in the rigor state
ATP
Figure 12-10, steps 0–1
The Molecular Basis of Contraction
1 ATP binds to myosin.
Myosin releases actin.
ATP
2 Myosin hydrolyses ATP. Myosin
head rotates and binds to actin.
ADP
Pi
Relaxed state with myosin heads cocked
Figure 12-10, steps 1–2
The Molecular Basis of Contraction
2 Myosin hydrolyses ATP. Myosin
head rotates and binds to actin.
Ca2+
signal
3 Power stroke
Actin filament moves
toward M line.
ADP
Pi
Pi
Relaxed state with
myosin heads cocked
Figure 12-10, steps 2–3
The Molecular Basis of Contraction
3 Power stroke
4 Myosin releases ADP.
Actin filament moves
toward M line.
Pi
ADP
Figure 12-10, steps 3–4
Excitation-Contraction Coupling
Axon terminal of
somatic motor neuron
1
1
Muscle fiber
2
ACh
Somatic motor neuron releases
ACh at neuromuscular junction.
2 Net entry of Na+ through ACh
receptor-channel initiates a
muscle action potential
Na+
Motor end plate
RyR
T-tubule
Sarcoplasmic
reticulum
Ca2+
DHP
Z disk
Troponin
Actin
Tropomyosin
M line
Myosin head
Myosin thick filament
(a) Initiation of muscle action potential
KEY
DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel
Figure 12-11a
Excitation-Contraction Coupling
Axon terminal of
somatic motor neuron
1
1
Muscle fiber
ACh
Somatic motor neuron releases
ACh at neuromuscular junction.
Motor end plate
RyR
T-tubule
Sarcoplasmic
reticulum
Ca2+
DHP
Z disk
Troponin
Actin
Tropomyosin
M line
Myosin head
Myosin thick filament
(a) Initiation of muscle action potential
KEY
DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel
Figure 12-11a, step 1
Excitation-Contraction Coupling
Axon terminal of
somatic motor neuron
1
1
Muscle fiber
2
ACh
Somatic motor neuron releases
ACh at neuromuscular junction.
2 Net entry of Na+ through ACh
receptor-channel initiates a
muscle action potential
Na+
Motor end plate
RyR
T-tubule
Sarcoplasmic
reticulum
Ca2+
DHP
Z disk
Troponin
Actin
Tropomyosin
M line
Myosin head
Myosin thick filament
(a) Initiation of muscle action potential
KEY
DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel
Figure 12-11a, steps 1–2
Excitation-Contraction Coupling
3
3
4 DHP receptor opens RyR Ca2+
release channels in sarcoplasmic
reticulum and Ca2+ enters
cytoplasm.
4
5
7
5
Ca2+ binds to troponin, allowing
actin-myosin binding.
Ca2+ released
6 Myosin heads execute power
stroke.
6
Myosin thick filament
Distance actin moves
(b) Excitation-contraction coupling
Action potential in t-tubule alters
conformation of DHP receptor.
7
Actin filament slides toward center
of sarcomere.
KEY
DHP = dihydropyridine L-type calcium channel
RyR = ryanodine receptor-channel
Figure 12-11b
Electrical and Mechanical Events in Muscle
Contraction
• A twitch is a single contraction-relaxation
cycle
Muscle fiber
+30
Action potential
from CNS
Neuron
membrane
potential
in mV
-70
Motor
Recording
end plate electrodes
+30
Axon
Muscle fiber
terminal
membrane
potential
Muscle action
in mV
potential
-70
Time
2
msec
Time
Development
of tension
during one
muscle twitch
Tension
Latent Contraction Relaxation
period phase
phase
10–100 msec
Time
Figure 12-12
Phosphocreatine
1.
2.
3.
Creatine phosphate
Glycolysis
Krebs cycle
Figure 12-13
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