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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
ATP
hydrolysis
ADP
ADP
Thick filament
Pi
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
4 As ATP is split into ADP and Pi, the myosin
head is energized (cocked into the high-energy
conformation).
ATP
ATP
Myosin head
(low-energy
configuration)
3 As new ATP attaches to the myosin head, the link between
myosin and actin weakens, and the cross bridge detaches.
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Figure 9.12
Sequential Events of Contraction
As seen at: http://www.sci.sdsu.edu/movies/actin_myosin_gif.html
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Contraction of Skeletal Muscle Fibers
Contraction – refers to the activation of myosin’s
cross bridges (force-generating sites)
Shortening occurs when the tension generated by
the cross bridge exceeds forces opposing
shortening
Contraction ends when cross bridges become
inactive, the tension generated declines, and
relaxation is induced
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Contraction of Skeletal Muscle (Organ Level)
Contraction of muscle fibers (cells) and muscles
(organs) is similar
The two types of muscle contractions are:
Isometric contraction – increasing muscle tension
(muscle does not shorten during contraction), e.g.
load is not moved
Isotonic contraction – decreasing muscle length
(muscle shortens during contraction), e.g. load is
moved
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Motor Unit: The Nerve-Muscle Functional Unit
Each muscle is served by at least one motor nerve (which
may contain hundreds of motor neuron axons)
An axon branches as it enters the muscle with each branch
terminal forming a neuromuscular junction w/ a single
muscle fiber
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Figure 9.13a
Motor Unit: The Nerve-Muscle Functional Unit
A motor unit is a motor neuron and all the muscle
fibers it supplies
The number of muscle fibers per motor unit can
vary from four to several hundred
Muscles that control fine movements (fingers,
eyes) have small motor units with small axon
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Motor Unit: The Nerve-Muscle Functional Unit
Large weight-bearing muscles (thighs, hips) have
large motor units with large axon
Muscle fibers from a motor unit are spread
throughout the muscle; therefore, contraction of a
single motor unit causes weak contraction of the
entire muscle
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Muscle Twitch
A muscle twitch is the response of a muscle to a
single, brief threshold stimulus
There are three phases to a muscle twitch
Latent period: the time following stimulation
Period of contraction: time when cross bridges are
active
Period of relaxation: time when cross bridges are
inactive
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Phases of a Muscle Twitch
Latent period – first few
msec after stimulus; EC
coupling taking place
Period of contraction –
cross bridges form;
muscle shortens
Period of relaxation –
Ca2+ reabsorbed; muscle
tension goes to zero
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Figure 9.14a
Muscle Twitch Comparisons
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Figure 9.14b
Graded Muscle Responses
Graded muscle responses are:
Long lasting, controlled contraction
Responses are graded by:
Changing the frequency of stimulation
Changing the strength of the stimulus
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Muscle Response and Stimulation Frequency
A single stimulus results in a single contractile response – a
muscle twitch
Frequently delivered stimuli (muscle does not have time to
completely relax) increases contractile force-wave summation
(2nd contraction occurs before muscle has relaxed)
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Figure 9.15
Muscle Response and Stimulation Frequency
More rapidly delivered stimuli result in
incomplete tetanus
If stimuli are given quickly enough, complete
tetanus results
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Figure 9.15
Muscle Response: Stimulation Strength
Threshold stimulus – the stimulus strength at
which the first observable muscle contraction
occurs
Beyond threshold, muscle contracts more
vigorously as stimulus strength is increased
Force of contraction is precisely controlled by
multiple motor unit summation
This phenomenon, called recruitment, brings more
and more muscle fibers into play
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Stimulus Intensity and Muscle Tension
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Figure 9.16
Size Principle
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Figure 9.17
Treppe: The Staircase Effect
Staircase – increased contraction in response to
multiple stimuli of the same strength
Contractions increase because:
There is increasing availability of Ca2+ in the
sarcoplasm
Muscle enzyme systems become more efficient
because heat is increased as muscle contracts
E.g. warm-ups for athletes
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Muscle Tone
Muscle tone:
Is the constant, slightly contracted state of all muscles,
which does not produce active movements
Keeps the muscles firm, healthy, and ready to respond to
stimulus
Stabilizes joints
Maintains posture
Spinal reflexes account for muscle tone by:
Activating one motor unit and then another
Responding to activation of stretch receptors in muscles
and tendons
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Isotonic Contractions
In isotonic contractions, the muscle changes in
length (decreasing the angle of the joint) and
moves the load
The two types of isotonic contractions are
concentric and eccentric
Concentric contractions – the muscle shortens
while generating force
Eccentric contractions – the muscle elongates
while under tension
In both types the thin filaments are sliding
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Isotonic Contractions
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Figure 9.19a
Isotonic Contractions
Concentric contractions
Muscles shorten while generating force
E.g. bicep curl
Eccentric contractions
Muscles lengthen while generating force
E.g. bicep “negative” curl
Greater damage done in eccentric contractions
The muscles are also stronger during eccentric contractions
How the myosin heads move along actin in eccentric
contractions is not known.
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Isometric Contractions
Tension increases to the muscle’s capacity, but the
muscle neither shortens nor lengthens
Occurs if the load is greater than the tension that
the muscle is capable of
E.g. holding an object in your hand
Here, the thin filaments are not sliding
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Isometric Contractions
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Figure 9.19b
Muscle Metabolism: Energy for Contraction
Providing energy for contraction
ATP is the only source used directly for contractile
activity
As soon as available stores of ATP are hydrolyzed
(4-6 seconds), they are regenerated by:
The interaction of ADP with creatine phosphate
(CP) (via creatine kinase)
Anaerobic glycolysis
Aerobic respiration
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Muscle Metabolism: Energy for Contraction
Creatin kinase
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Figure 9.20
Muscle Metabolism: Anaerobic Glycolysis
When muscle contractile activity reaches 70% of
maximum:
Bulging muscles compress blood vessels
Oxygen delivery is impaired
Pyruvic acid is converted into lactic acid
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Muscle Metabolism: Anaerobic Glycolysis
The lactic acid:
Diffuses into the bloodstream
Is picked up and used as fuel by the liver, kidneys,
and heart
Is converted back into pyruvic acid or glucose by
the liver and is released back into the bloodstream
for muscle use or convert it to glycogen for storage
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Muscle Metabolism: Anaerobic Glycolysis
Thus, Anaerobic glycolysis:
Results in fast production of ATP
But poor efficiency of ATP production (5%
compared to aerobic pathway)
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Muscle Metabolism: Aerobic Respieration
Aerobic Respiration:
Occurs in mitochondria
Requires oxygen
Glucose + O2
38 ATP/Glucose
CO2 + H2O + ATP
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Muscle Metabolism: Aerobic Respieration
Initially, muscle glycogen provides fuel
Afterward, blood borne glucose, pyruvic acid and
free fatty acids are the major sources of fuel
After 30 minutes, fatty acids become the major
fuel source
Because of the many steps involved in ATP
production, production of ATP is slow
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Muscle Fatigue
Muscle fatigue – the muscle is in a state of
physiological inability to contract
Muscle fatigue occurs when:
ATP production fails to keep pace with ATP use
There is a relative deficit of ATP, causing
contractures (permanent contraction)
Lactic acid accumulates in the muscle
Ionic imbalances are present
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Muscle Fatigue
Intense exercise produces rapid muscle fatigue
(with rapid recovery)
Na+-K+ pumps cannot restore ionic balances
quickly enough
Low-intensity exercise produces slow-developing
fatigue
SR is damaged and Ca2+ regulation is disrupted
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Oxygen Debt
Vigorous exercise causes dramatic changes in
muscle chemistry
For a muscle to return to a resting state:
Oxygen reserves must be replenished
Lactic acid must be converted to pyruvic acid
Glycogen stores must be replaced
ATP and CP reserves must be resynthesized
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Oxygen Debt
Oxygen debt – the extra amount of O2 needed for
the above restorative processes
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Heat Production During Muscle Activity
Only 40% of the energy released in muscle activity
is useful as work
The remaining 60% is given off as heat
Dangerous heat levels are prevented by radiation
of heat from the skin and sweating
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Force of Muscle Contraction
The force of contraction is affected by:
The number of muscle fibers contracting
the more motor fibers in a muscle, the
stronger the contraction
The relative size of the muscle – the bulkier the
muscle, the greater its strength
Degree of muscle stretch – muscles contract
strongest when muscle fibers are 80-120% of their
normal resting length
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Force of Muscle Contraction
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Figure 9.21a–c
Length Tension Relationships
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Figure 9.22
Muscle Fiber Type: Functional Characteristics
Speed of contraction – determined by speed in
which ATPases split ATP
The two types of fibers are slow and fast
ATP-forming pathways
Oxidative fibers – use aerobic pathways
Glycolytic fibers – use anaerobic glycolysis
These two criteria define three categories – slow
oxidative fibers, fast oxidative fibers, and fast
glycolytic fibers
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Muscle Fiber Type: Speed of Contraction
Slow oxidative fibers contract slowly, have slow
acting myosin ATPases, and are fatigue resistant
Fast oxidative fibers contract quickly, have fast
myosin ATPases, and have moderate resistance to
fatigue
Fast glycolytic fibers contract quickly, have fast
myosin ATPases, and are easily fatigued
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Table 9.2
Effects of Aerobic Exercise
Aerobic exercise results in an increase of:
Muscle capillaries
Number of mitochondria
Myoglobin synthesis
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Effects of Resistance Exercise
Resistance exercise (typically anaerobic) results in:
Muscle hypertrophy
Increased mitochondria, myofilaments, and
glycogen stores
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The Overload Principle
Forcing a muscle to work promotes increased
muscular strength
Muscles adapt to increased demands
Muscles must be overloaded to produce further
gains
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Smooth Muscle
Composed of spindle-shaped fibers with a
diameter of 2-10 µm and lengths of several
hundred µm
Lack the coarse connective tissue sheaths of
skeletal muscle, but have fine endomysium
Organized into two layers (longitudinal and
circular) of closely apposed fibers
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Smooth Muscle
Found in walls of hollow organs (except the heart)
Have essentially the same contractile mechanisms
as skeletal muscle
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Smooth Muscle
Microscopic structure of smooth muscle fibers
Spindle shaped
One central nucleus
Much smaller than skeletal muscle fiber
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Figure 9.24
Smooth Muscle
Lack connective tissue sheaths found in skeletal muscle
Organized into sheets of closely opposed fibers
Occur in blood vessels and hollow organs of:
Respiratory
Digestive
Urinary
And reproductive tracts
2 sheets with fibers oriented at right angles
Longitudinal layers: fibers run parallel to long axis
Contraction shortens the organ
Circular layer: fibers run around the circumference of the organ
Contraction constricts the lumen and elongates the organ
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Peristalsis
When the longitudinal layer contracts, the organ
dilates and contracts
When the circular layer contracts, the organ
elongates
Peristalsis – alternating contractions and
relaxations of smooth muscles that mix and
squeeze substances through the lumen of hollow
organs
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Innervation of Smooth Muscle
Smooth muscle lacks neuromuscular junctions
Innervating nerves have bulbous swellings called
varicosities
Innervated via the autonomic nervous system
Varicosities release neurotransmitters into wide
synaptic clefts called diffuse junctions
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Innervation of Smooth Muscle
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Figure 9.25
Microscopic Anatomy of Smooth Muscle
SR is less developed than in skeletal muscle and
lacks a specific pattern
T tubules are absent
Ca++ entry comes partly from the S.R., but mainly
via Ca++ channels directly from the extracellular
space (caveolae where extracellular fluid and Ca++
are sequestered)
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Microscopic Anatomy of Smooth Muscle
Ca2+ is sequestered in caveoli allows rapid influx
when channels are opened
Contraction ends when Ca++ is pumped out
There are no visible striations and no sarcomeres
Thin and thick filaments are present
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Proportion and Organization of Myofilaments
in Smooth Muscle
Ratio of thick to thin filaments is much lower than in
skeletal muscle
Smooth muscle
1:13
Skeletal muscle
1:2
Thick filaments have actin-gripping heads along their entire
length
There is no troponin complex
Contraction is initiated by a calcium-regulated
phosphorylation of myosin, rather than a calciumactivated troponin system
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Proportion and Organization of Myofilaments
in Smooth Muscle
Thick and thin filaments spiral along the long axis of the
cell, causing smooth muscle to contract in a corkscrew
manner
Noncontractile intermediate filament bundles attach to
dense bodies (analogous to Z discs) at regular intervals
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Proportion and Organization of Myofilaments
in Smooth Muscle
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Figure 9.26
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