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Lauralee Sherwood
Hillar Klandorf
Paul Yancey
Chapter 8
Muscle Physiology
Sections 8.4-8.8
Kip McGilliard • Eastern Illinois University
8.5 Skeletal Muscle Metabolism and Fiber Types
 Phosphagens
• ATP is required for muscle contraction, but storage of
ATP is limited
• Creatine phosphate (vertebrates) and arginine
phosphate (nonvertebrates) are the first energy
storehouse tapped at the onset of contractile activity
• Phosphogens contain a high-energy phosphate group
that can be quickly donated to ADP
creatine
kinase
Creatine phosphate + ADP <——> creatine + ATP
• Vertebrate muscle contains 5x as much creatine
phosphate as ATP
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8.5 Skeletal Muscle Metabolism and Fiber Types
 Oxidative phosphorylation
• Takes place in muscle mitochondria
• Requires oxygen
• Fueled by fatty acids or glucose
• Rich yield (~30 ATP per glucose)
• Multistep pathway requires more time
• Used during light to moderate (aerobic) activity
• Myoglobin stores oxygen in muscle fibers
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8.5 Skeletal Muscle Metabolism and Fiber Types
 Glycolysis
• Takes place in muscle cytoplasm
• Can form ATP in the absence of oxygen
• Fueled by glucose
• Insects also use trehalose, a nonreducing sugar
• Low yield (2 ATP per glucose)
• Proceeds more rapidly than oxid-phos
• Used during high-intensity (anaerobic) activity
• Produces lactate and accompanying acidosis
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8.5 Skeletal Muscle Metabolism and Fiber Types
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Biceps
contracts
Biceps
relaxes
Blood Liver
glucose glycogen
During
contraction
Muscle
glycogen
Glucose
Blood
During
rest
Muscle fiber
Contraction
Myosin
ATPase
Relaxation
Ca2+ pump of
sarcoplasmic
reticulum
(Main source
when O2 not
present)
Glycolysis
Lactate
Fatty
acids
Pyruvate
Oxidative
phosphorylation
Protein
Amino Rare
acids
(Main source
when O2
present)
Creatine
(Immediate
source)
During
rest
Creatine
Phosphate
Creatine
kinase
During
contraction
Fat
stores
Figure 8-21 p360
8.5 Skeletal Muscle Metabolism and Fiber Types
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 Fatigue
• Decreased contractile response of exercising
muscle to stimulation
• Causes of fatigue
• Local increase in ADP and Pi
• Accumulation of lactate
• Accumulation of extracellular K+
• Depletion of glycogen energy reserves
• Central fatigue involves a decrease in CNS
stimulation of motor neurons
8.5 Skeletal Muscle Metabolism and Fiber Types
 Oxygen deficit
• An animal must continue to breathe deeply
and rapidly after exhaustive activity.
• Oxygen is needed for recovery of energy
systems through oxidative phosphorylation
• Replenishment of creatine phosphate
• Conversion of lactate into pyruvic acid and
pyruvic acid into glucose
• Replenishment of glycogen stores
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8.5 Skeletal Muscle Metabolism and Fiber Types
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 Skeletal muscle fiber types
• Slow-oxidative fibers (Type I)
• 60 - 100 msec to peak tension
• Lower myosin-ATPase activity
• High resistance to fatigue
• Fast-oxidative fibers (Type IIa)
• 20 - 40 msec to peak tension
• Higher myosin-ATPase activity
• Intermediate resistance to fatigue
• Fast-glycolytic fibers (Type IIb, IId, or IIx)
• Similar to fast-oxidative fibers in speed and myosinATPase activity
• Low resistance to fatigue
8.5 Skeletal Muscle Metabolism and Fiber Types
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8.5 Skeletal Muscle Metabolism and Fiber Types
 Adaptation of muscle fibers
• Skeletal muscle has a high degree of plasticity
• Regular endurance activities improve oxidative
capacity
• Increase in number of mitochondria
• Increase in number of capillaries
• Regular high-intensity activity stimulates
hypertrophy (increased diameter) of fastglycolytic fibers
• Increased synthesis of myosin and actin filaments
• Increased muscle strength
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8.5 Skeletal Muscle Metabolism and Fiber Types
 Adaptation of muscle fibers
• Hormones influence muscle size and strength
• Testosterone and growth hormone/IGF-I promote
synthesis of myosin and actin filaments
• Myostatin is a negative regulator of muscle growth
• Interconversion between fast-glycolytic and fastoxidative fibers takes place with specific forms of
regular exercise
• Unused muscle loses mass and strength (disuse
atrophy)
• When muscle is damaged, limited repair is possible
due to ability to form new myoblasts
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8.6 Adaptations for Flight: Continuous High Power
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at High Contraction Frequencies
 High-force and high-frequency operation
are both needed for flight
• Increased body temperature allows more rapid
ATP synthesis and increases activity of Ca2+
pumps
• Birds have higher body temperature than mammals
• Insects must warm up before flight
• Mitochondrial structure is altered in birds and
insects for higher O2 consumption
• Synchronous muscle contractions power flight
muscle of hummingbirds and large insects
8.6 Adaptations for Flight: Continuous High Power
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at High Contraction Frequencies
 Asynchronous contractions as adaptations
to high-frequency flight
• Occurs in most insects
• A single Ca2+ pulse maintains muscle in an activated
state for successive cycles
• Flight muscles are attached to the walls of the
thorax rather than to the wings
• Contraction is triggered by stretch and deactivated by
shortening in the presence of elevated myoplasmic
Ca2+
• Reduction of Ca2+ cycling reduces ATP demand
8.6 Adaptations for Flight: Continuous High Power
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at High Contraction Frequencies
Base
of
wing
Wing
hinge
Cuticle
Figure 8-22a p367
Longitudinal
muscles
relaxed
Wing
Pivot
point
Dorsal–ventral muscles contracted
Longitudinal muscles contracted
Dorsal–ventral muscles relaxed
Figure 8-22b p367
8.7 Control of Motor Movement
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 Motor inputs controlling motor neuron output
• Afferent neurons
• Spinal reflexes are important for maintaining
posture and basic protective movements
• Primary motor cortex
• Fibers of pyramidal cells descend directly to motor
neurons (corticospinal motor system)
• Fine, discrete movements of hands and fingers
• Brain stem
• Part of multineuronal (extrapyramidal) motor
system
• Regulation of overall body posture involving
involuntary movements of trunk and limbs
8.7 Control of Motor Movement
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ANIMATION: Nervous system and muscle
contraction
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8.7 Control of Motor Movement
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 Muscle proprioreceptors monitor changes in
muscle length and tension
• Muscle length is monitored by muscle spindles
• Bundles of specialized intrafusal fibers lying
within spindle-shaped connective tissue capsules
• Changes in muscle tension are detected by
Golgi tendon organs
• Endings of afferent fibers entwined within bundles
of connective tissue fibers in the tendon
• Frequency of firing is directly related to tension
developed in the muscle
• Afferent information reaches the level of
conscious awareness of muscle tension
8.7 Control of Motor Movement
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Capsule
Alpha motor
neuron axon
Intrafusal (spindle)
muscle fibers
Gamma motor
neuron axon
Afferent neuron
axons
Two types of afferent
sensory endings that
serve as stretch
receptors in muscle
spindle
Contractile end
portions of intrafusal
fiber
Noncontractile
central portion
of intrafusal
fiber
Extrafusal (“ordinary”)
muscle fibers
Figure 8-24a p371
Skeletal muscle
Afferent fiber
Golgi tendon organ
Collagen
Tendon
Bone
(b) Golgi tendon organ
Figure 8-24b p371
8.7 Control of Motor Movement
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 Muscle spindles play a key role in stretch
reflexes (e.g. patellar tendon or knee-jerk
reflex).
• When a muscle is passively stretched, intrafusal
fibers in muscle spindles increase firing of afferent
neurons
• Afferent neurons directly synapse on alpha motor
neurons in the spinal cord, resulting in contraction
of the muscle that was stretched
• Gamma motor neurons initiate contraction of
muscular end regions of intrafusal fibers to adjust
tension in muscle spindles
• The primary purpose of the stretch reflex is to resist
the tendency for passive stretch of extensor
muscles by gravity (maintains upright posture).
8.7 Control of Motor Movement
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Descending pathways
coactivating alpha and
gamma motor neurons
Afferent input from
sensory endings of
muscle spindle fiber
Stretch reflex pathway
Alpha motor neuron
output to regular
skeletal muscle fiber
Extrafusal
skeletal
muscle fiber
Intrafusal
muscle
spindle fiber
Spinal
cord
Gamma motor neuron
output to contractile end
portions of spindle fiber
(a) Pathways involved in monosynaptic stretch reflex and coactivation of alpha
and gamma motor neurons
Figure 8-25a p372
8.7 Control of Motor Movement
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Extensor muscle of
knee (quadriceps
Muscle
femoris)
spindle
Patellar tendon
Alpha motor
neuron
Figure 8-26 p373
ANIMATION: Stretch reflex
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8.8 Smooth Muscle and Cardiac Muscle
 Smooth muscle
• Mostly in walls of hollow organs and tubes
• Fibers are smaller than skeletal muscle fibers and
spindle-shaped, with a single nucleus
• Fibers are arranged in sheets
• Three types of filaments
• Thick myosin filaments
• Thin actin filaments anchored at dense bodies
• Intermediate filaments form a scaffold for dense
bodies
• Diagonal arrangement of filaments -- no striations
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8.8 Smooth Muscle and Cardiac Muscle
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Smooth muscle cells
Nucleus
Figure 8-27a p376
Smooth muscle cells
Dense bodies
Figure 8-27b p376
8.8 Smooth Muscle and Cardiac Muscle
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Dense
body Bundle of thick
and thin filaments
Plasma
membrane Thin
filament
One relaxed contractile unit
extending from side to side
One contracted
contractile unit
Thick
filament
Thin
filament
Thick
filament
(a) Relaxed smooth muscle cell
(b) Contracted smooth muscle cell
Figure 8-28 p377
Dense
body
Bundle of
thick and thin
filaments
Plasma
membrane
One relaxed contractile unit
extending from side to side
Thick
Thin
filament filament
Thin
filament
Thick
filament
(a) Relaxed smooth muscle cell
Figure 8-28a p377
One contracted
contractile unit
(b) Contracted smooth muscle cell
Figure 8-28b p377
8.8 Smooth Muscle and Cardiac Muscle
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 Mechanism of smooth muscle contraction
• During excitation, cytosolic Ca2+ is increased
• Ca2+ binds with calmodulin
• Ca2+-calmodulin complex binds to and activates
myosin light chain kinase (MLC kinase)
• MLC kinase phosphorylates myosin light chains
• Allows myosin heads to interact with actin and
cross-bridge cycling begins
8.8 Smooth Muscle and Cardiac Muscle
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8.8 Smooth Muscle and Cardiac Muscle
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Smooth muscle
Skeletal muscle
Muscle excitation
Muscle excitation
Rise in cytosolic Ca2+
(mostly from
extracellular fluid)
Rise in cytosolic Ca2+
(entirely from
intracellular
sarcoplasmic reticulum)
Series of
biochemical events
Phosphorylation of
myosin cross bridges
in thick filament
Binding of actin and
myosin at cross
bridges
Physical
repositioning of
troponin and
tropomyosin
Uncovering of crossbridge binding sites on
actin in thin filament
Binding of actin and
myosin at cross
bridges
Pi
Contraction
Contraction
Figure 8-30 p378
8.8 Smooth Muscle and Cardiac Muscle
 Classification of smooth muscle
• Phasic vs. tonic
• Phasic smooth muscle contracts in bursts
triggered by action potentials that cause
increased cytosolic Ca2+
• Tonic smooth muscle is partially contracted
at all times; varies its contraction according to
cytosolic Ca2+ level
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8.8 Smooth Muscle and Cardiac Muscle
 Multiunit vs. single-unit smooth muscle
• Multiple units must be separately stimulated by
nerves to contract
• Contractile activity is neurogenic and phasic
• Can be initiated by the autonomic nervous system
• Single-unit muscle fibers are self-excitable and
contract as a single unit
• Gap junctions electrically link neighboring cells
(functional syncytium)
• Contractile activity is myogenic and may be phasic
(pacemaker potentials) or tonic (slow-wave
potentials)
• Modified by the autonomic nervous system
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8.8 Smooth Muscle and Cardiac Muscle
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Figure 8-31a p379
Figure 8-31b p379
8.8 Smooth Muscle and Cardiac Muscle
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Mitochondrion
Vesicle containing
neurotransmitter
Axon of
postganglionic
autonomic
neuron
Varicosity
Neurotransmitter
Varicosities
Smooth
muscle cell
Figure 8-32 p380
8.8 Smooth Muscle and Cardiac Muscle
 Smooth muscle contracts more slowly and
uses less energy than skeletal muscle.
• Lower myosin ATPase activity results in
slower contraction
• Slower rate of Ca2+ removal results in slower
relaxation
• Latch state (vertebrates) or catch state
(nonvertebrates) maintains tension for long
periods with very low ATP consumption
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8.8 Smooth Muscle and Cardiac Muscle
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8.8 Smooth Muscle and Cardiac Muscle
 Cardiac muscle
• Found only in the heart
• Cells form a branching network
• Similarity to skeletal muscle
•
•
•
•
Striated
Length-tension relationship
Abundance of mitochondria and myoglobin
T tubules and sarcoplasmic reticulum
• Similarity to smooth muscle
• Self-excitation
• Interconnected by gap junctions
• Innervated by the autonomic nervous system
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