The main contents of this chapter include all of the following: Force of muscle contraction, avelocity and duration of contraction, muscle fiber type, effects of exercise, effects of resistance exercise, the overload principle, smooth muscle, peristalsis, microscopic structure, innervation of smooth muscle,...and other contents.
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prepared by Janice Meeking, Mount Royal College
C H A P T E R
Copyright © 2010 Pearson Education, Inc.
9
Muscles and Muscle
Tissue: Part C
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Force of Muscle Contraction
• The force of contraction is affected by:
• Number of muscle fibers stimulated
(recruitment)
• Relative size of the fibers - hypertrophy of
cells increases strength
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Force of Muscle Contraction
• The force of contraction is affected by:
• Frequency of stimulation - frequency allows
time for more effective transfer of tension to noncontractile components
• Length-tension relationship - muscles
contract most strongly when muscle fibers are 80–120% of their normal resting length
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Large number of
muscle fibers activated
Contractile force
High frequency of stimulation
Large muscle fibers
Muscle and sarcomere stretched to slightly over 100%
of resting length
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Sarcomeres greatly shortened
Sarcomeres at resting length
Sarcomeres excessively
stretched
170%
Optimal sarcomere operating length (80%–120% of resting length)
100%
75%
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Velocity and Duration of Contraction
Influenced by:
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Muscle Fiber Type
Classified according to two characteristics:
1 Speed of contraction: slow or fast,
according to:
• Pattern of electrical activity of the motor
neurons
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Muscle Fiber Type
2 Metabolic pathways for ATP synthesis:
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Muscle Fiber Type
Three types:
• Slow oxidative fibers
• Fast oxidative fibers
• Fast glycolytic fibers
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fibers Small load
Contractile velocity
Contractile duration
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Effects of Resistance Exercise
• Resistance exercise (typically anaerobic) results in:
size)
glycogen stores, and connective tissue
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The Overload Principle
• Forcing a muscle to work hard promotes increased muscle strength and endurance
• Muscles adapt to increased demands
• Muscles must be overloaded to produce further gains
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Trang 16Copyright © 2010 Pearson Education, Inc. Figure 9.26
Small intestine
(a) (b) Cross section of the
intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other.
Mucosa
Longitudinal layer
of smooth muscle
(shows smooth muscle fibers in cross section)
Circular layer of smooth muscle
(shows longitudinal views of smooth muscle fibers)
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Peristalsis
• Alternating contractions and relaxations of
smooth muscle layers that mix and squeeze substances through the lumen of hollow
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Microscopic Structure
• Spindle-shaped fibers: thin and short
compared with skeletal muscle fibers
• Connective tissue: endomysium only
• SR: less developed than in skeletal muscle
• Pouchlike infoldings (caveolae) of
sarcolemma sequester Ca2+
• No sarcomeres, myofibrils, or T tubules
Trang 19Copyright © 2010 Pearson Education, Inc. Table 9.3
Trang 20Copyright © 2010 Pearson Education, Inc. Table 9.3
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Innervation of Smooth Muscle
• Autonomic nerve fibers innervate smooth muscle at diffuse junctions
• Varicosities (bulbous swellings) of nerve fibers store and release neurotransmitters
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Smooth muscle cell
Varicosities release
their neurotransmitters into a wide synaptic cleft (a diffuse junction).
Synaptic vesicles
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Myofilaments in Smooth Muscle
• Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)
• Thick filaments have heads along their entire length
• No troponin complex; protein calmodulin
binds Ca2+
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Myofilaments in Smooth Muscle
• Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner
• Dense bodies: proteins that anchor
noncontractile intermediate filaments to
sarcolemma at regular intervals
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Trang 26Copyright © 2010 Pearson Education, Inc. Figure 9.28b
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Contraction of Smooth Muscle
• Slow, synchronized contractions
• Cells are electrically coupled by gap junctions
• Some cells are self-excitatory (depolarize
without external stimuli); act as pacemakers for sheets of muscle
• Rate and intensity of contraction may be
modified by neural and chemical stimuli
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Contraction of Smooth Muscle
• Sliding filament mechanism
• Final trigger is intracellular Ca2+
• Ca2+ is obtained from the SR and extracellular space
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Role of Calcium Ions
• Ca2+ binds to and activates calmodulin
• Activated calmodulin activates myosin (light chain) kinase
• Activated kinase phosphorylates and
activates myosin
• Cross bridges interact with actin
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Trang 32Copyright © 2010 Pearson Education, Inc. Figure 9.29
Calcium ions (Ca 2+ ) enter the cytosol from the ECF via voltage- dependent or voltage- independent Ca 2+
channels, or from the scant SR.
Inactive calmodulin
Inactive kinase
Inactive myosin molecule
Activated (phosphorylated) myosin molecule
Activated kinase Activated calmodulin
Cytoplasm
Ca 2+ binds to and activates calmodulin.
Activated calmodulin activates the myosin light chain kinase enzymes.
The activated kinase enzymes catalyze transfer of phosphate
to myosin, activating the myosin ATPases.
Activated myosin forms cross bridges with actin of the thin filaments and shortening begins.
Thin filament
Thick filament
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enter the cytosol from the ECF via voltage- dependent or voltage-
channels, or from the scant SR.
Extracellular fluid (ECF)
Ca 2+
Ca 2+
Plasma membrane
Sarcoplasmic reticulum
Cytoplasm
1
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Ca 2+
Ca 2+ binds to and activates calmodulin.
2
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Inactive kinase Activated kinase
Activated calmodulin activates the myosin light chain kinase enzymes.
3
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Activated (phosphorylated) myosin molecule
The activated kinase enzymes catalyze transfer of phosphate
to myosin, activating the myosin ATPases.
4
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Activated myosin forms cross bridges with actin of the thin
filaments and shortening begins.
Thin filament
Thick filament
5
Trang 38Copyright © 2010 Pearson Education, Inc. Figure 9.29
Calcium ions (Ca 2+ ) enter the cytosol from the ECF via voltage- dependent or voltage- independent Ca 2+
channels, or from the scant SR.
Inactive calmodulin
Inactive kinase
Inactive myosin molecule
Activated (phosphorylated) myosin molecule
Activated kinase Activated calmodulin
Cytoplasm
Ca 2+ binds to and activates calmodulin.
Activated calmodulin activates the myosin light chain kinase enzymes.
The activated kinase enzymes catalyze transfer of phosphate
to myosin, activating the myosin ATPases.
Activated myosin forms cross bridges with actin of the thin filaments and shortening begins.
Thin filament
Thick filament
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Contraction of Smooth Muscle
• Very energy efficient (slow ATPases)
• Myofilaments may maintain a latch state for prolonged contractions
Relaxation requires:
• Ca2+ detachment from calmodulin
• Active transport of Ca2+ into SR and ECF
• Dephosphorylation of myosin to reduce
myosin ATPase activity
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Regulation of Contraction
Neural regulation:
• Neurotransmitter binding [Ca2+] in
sarcoplasm; either graded (local) potential or action potential
• Response depends on neurotransmitter
released and type of receptor molecules
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Regulation of Contraction
Hormones and local chemicals:
• May either enhance or inhibit Ca 2+ entry
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Special Features of Smooth Muscle
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Types of Smooth Muscle
Single-unit (visceral) smooth muscle:
junctions)
stress-relaxation response
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Types of Smooth Muscle: Multiunit
Multiunit smooth muscle:
• Located in large airways, large arteries, arrector pili muscles, and iris of eye
neural stimuli
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Muscular Dystrophy
Duchenne muscular dystrophy (DMD):
• Most common and severe type
• Inherited, sex-linked, carried by females and
expressed in males (1/3500) as lack of dystrophin
• Victims become clumsy and fall frequently; usually die
of respiratory failure in their 20s
• No cure, but viral gene therapy or infusion of stem
cells with correct dystrophin genes show promise