Download Muscle Tissue

MUSCLE TISSUE
I. Skeletal Muscles
Introduction
• Generally, both ends of a muscle are attached to
bone by tough tendons
• When a muscle contracts, it shortens.
a.This places tension on tendons connecting it to a bone.
b.This moves the bone at a joint.
c.The bone that moves is attached at the muscle insertion.
d.The bone that does not move is attached at the muscle
origin.
e.Movement is toward the insertion
• The main muscle responsible for movement in a
given direction is the agonist or prime mover
• Flexors and extensors that act on the same joint to
produce opposite actions are antagonists
• Ex. Bicep and Tricep
Skeletal Muscle Actions
Structure of Skeletal Muscle
• Connective tissue
components
a.Skeletal muscles are
surrounded by a fibrous
epimysium
b.Connective tissue called
perimysium subdivides the
muscle into fascicles
c.Each fascicle is subdivided into
muscle fibers (myofibers)
surrounded by endomysium
Muscle Fiber Structure
a.Have many of the organelles found
in other cells
b.Have plasma membranes called
sarcolemma
c.Are multinucleated; form a
syncytium
d.Are striated
1)I bands: light bands
2)A bands: dark bands
3)Z-lines (discs): dark lines in the
middle of the I bands
Skeletal Muscle Fibers
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Nuclei
Muscle fiber
© Ed Reschke
Motor Unit
• A motor unit is a single motor neuron and all the muscle
fibers it innervates; all the muscle fibers in a motor unit
contract at once
• Graded contractions – varied contraction strength due to
different numbers of motor units being stimulated
• Neuromuscular junction: site where a motor neuron
stimulates a muscle fiber
• Motor end plate: area of the muscle fiber sarcolemma
where a motor neuron stimulates it using the
neurotransmitter, acetylcholine
Motor Unit
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Somatic motor neuron
Motor unit
Spinal cord
Motor unit
Somatic motor neuron
(a)
Motor unit
Skeletal
muscle fibers
(b)
Neuromuscular
junctions
Somatic
motor axon
Control of Motor Units
• Contraction strength comes from motor unit
recruitment
• Finer muscle control requires smaller motor units
(fewer muscle fibers
1)The eye muscles may have ~23 muscle fibers/motor units.
2)Larger, stronger muscles may have motor units with thousands
of muscle fibers.
3)Control and strength are trade-offs
II. Mechanisms of Contraction
Muscle Fiber Binding
• Each fiber has densely packed subunits called myofibrils
that run the length of the muscle fiber
• Stacked in register so that the dark and light bands align
• Composed of thick and thin myofilaments
Sarcomere
• Unit of contraction
• From one Z line to the next Z line
• Contains overlapping of thick and thin filaments
Arrangement of Thick and Thin
Filaments
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Sarcomere
A band
I band
H band
I band
Thin
filament
Thick
filament
(a)
Z disc
Z disc
Myofibril
Sarcomere
Copyright by Dr. R.G. Kessel and R.G. Kardon, Tissues and Organs: A Text-Atlas of Scanning
Electron Microscopy, W.H. Freeman, 1979
Arrangement of Thick and Thin
Filaments
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Z disc
Z disc
(b)
M
SR
Myofibril
M
Copyright by Dr. R.G. Kessel and R.G. Kardon, Tissues and Organs: A Text-Atlas of Scanning
Electron Microscopy, W.H. Freeman, 1979
Cross Bridges
a.More about myofilaments
1)Thick: composed of the protein myosin
a) Each protein has two globular heads with actin-binding sites
and ATP-binding sites.
2)Thin: composed of the protein actin
a) Have proteins called tropomyosin and troponin that prevent
myosin binding at rest.
Action of Sliding
1)Sliding is produced by several cross bridges that form
between myosin and actin.
a)The myosin head serves as a myosin ATPase enzyme, splitting
ATP into ADP + Pi.
b)This allows the head to bind to actin when the muscle is
stimulated.
Cont’d
2)Release of Pi upon binding cocks the myosin head,
producing a power stroke that pulls the thin
filament toward the center.
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Actin
1
ADP
Pi
2
Pi
Power
stroke
Cont’d
3)After the power stroke, ADP is released and a new ATP
binds.
a)This makes myosin release actin.
b)ATP is split.
4)The myosin head straightens out and rebinds to actin
farther back.
5)Continues until the sarcomere has shortened
Activation of the Myosin Head
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Troponin
Actin
Tropomyosin
Myosin binding site
Thin
filament
Actin-binding
sites
Myosin head
1
ATP-binding
site
ATP
Myosin tail
Thick
filament
ADP
2
Pi
Cross Bridges
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1 Resting fiber; cross bridge is
not attached to actin
6 ATP is hydrolyzed and
phosphate binds to
myosin, causing cross
bridge to return to its
Myosin head
original orientation
Thin
filament
ADP
Pi
Cross bridge
Thick filament
2 Cross bridge
binds to actin
ATP
5 A new ATP binds to
myosin head, allowing
it to release from actin
3 Pi is released from myosin
head, causing conformational
change in myosin
4 Power stroke causes
filaments to slide; ADP
is released
Regulation of Contraction
• F-actin is made of 300-400 G-actin subunits, arranged
in a double row and twisted to form a helix
• Tropomyosin physically blocks cross bridges
Role of Calcium
• When muscle cells are stimulated, Ca2+ is released
inside the muscle fiber.
• Some attaches to troponin C, causing a
conformational change in troponin and tropomyosin.
• Myosin is allowed access to form cross bridges with
actin.
Role of Calcium
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Actin
Tropomyosin
Relaxed
muscle:
tropomyosin
blocks the
binding site
Troponin
ADP
Pi
Binding
site
Cross bridge
Myosin
Ca2+
Contracting
muscle:
myosin
head binds
to actin
Ca2+
ADP
Ca2+
Pi
Excitation-Coupling Contraction
• Sarcoplasmic reticulum (SR)
a.SR is modified endoplasmic reticulum that stores Ca2+
when muscle is at rest.
b.Most is stored in terminal cisternae.
c.When a muscle fiber is stimulated, Ca2+ diffuses out of
calcium release channels (ryanodine receptors).
d.At the end of a contraction, Ca2+ is actively pumped back
into the SR.
Sarcoplasmic Reticulum
Stimulating a Muscle Fiber
• Acetylcholine is released from the motor neuron
• End plate potentials are produced
• Action potentials are generated (All-or-none event)
• Voltage-gated calcium channels in transverse
tubules change shape and cause calcium channels
in SR to open
• Calcium is released and can bind to troponin C
Stimulating a Muscle Fiber
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Axon terminal
Sarcolemma
2
1
Ca2+
3
Transverse
tubule
4
Sarcoplasmic
reticulum
1 Nicotinic acetylcholine receptor
2 Skeletal muscle voltage-gated
sodium channel
3 Transverse tubule voltage-gated
calcium channel
4 Sarcoplasmic reticulum calcium
release channel
Muscle Relaxation
• Action potentials cease
• Calcium release channels close
• Ca2+-ATPase pumps move Ca2+ back into SR (active
transport)
• No more Ca2+ is available to bind to troponin C
• Tropomyosin moves to block the myosin heads
from binding to actin
III. Contractions of Skeletal
Muscles
Types of Muscle Contractions
• Force Velocity Curve
a.For muscles to contract, they must generate force that is
greater than the opposing forces.
b.The greater the force, the slower the contraction.
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Velocity of shortening
vmax
•
•
0
•
•
•
Contraction at
this point is
isometric
Force
(load opposing contraction)
Isotonic Contractions
• Isotonic contraction: Muscle fibers shorten when
the tension produced is just greater than the load
• Concentric contraction: A muscle fiber shortens
when force is greater than load
• Eccentric contraction: A muscle may actually
lengthen, despite contraction, if the load is too
great
• Allows you to lower a weight gently after a full
concentric contraction
Isometric Contraction
• Muscles can’t shorten because the load is too great
Series Elastic Component
• Noncontractile parts of the muscle and tendons
must be pulled tight when muscles contract.
• Tendons are elastic, resist distension, and snap
back to resting length.
• Tendons absorb some of the tension as muscles
contract.
Length-Tension Relationship
• Muscle strength is determined by:
•
•
•
•
Number of fibers recruited to contract
Frequency of stimulation
Thickness of each muscle fiber (thicker is stronger)
Initial length of the fiber at rest
Length-Tension Relationship
• Tension is maximal when sarcomeres are at normal
resting length
• Increasing sarcomere length decreases muscle
tension
• There are fewer interactions between myosin and actin
• At a certain point, no tension can be generated
• Decreasing sarcomere length decreases muscle
tension because the fiber gets shorter and thicker
Length-Tension Relationship
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Relative tension
2.0 μm
2.25 μm
1.65 μm
1.0
0.5
1.25 μm
0
60
3.65 μm
80
100
120
140
160
Percentage rest length
1.65 μm
2.25 μm
3.65 μm
IV. Energy Requirements of
Skeletal Muscles
Where do muscles get energy?
• At rest and for mild exercise: from the aerobic respiration
of fatty acids
• For moderate exercise: from glycogen stores
• For heavy exercise: from blood glucose
a.As exercise intensity and duration increase, GLUT4 channels
are inserted into the sarcolemma to allow more glucose into
cells.
Aerobic Respiration
• Provides 95% of the ATP
• Mitochondria absorbs oxygen, ADP, phosphate ions
from cytoplasm
• Molecules in the Krebs Cycle  oxidation of acetly
coA into C02 and ATP
Metabolism of Skeletal Muscle
• Anaerobic for the first 45-90 seconds of moderate
to heavy exercise
• Does not require oxygen
• Breaks down glycogen reserves
• Allows time to increase oxygen supply
What determines light or heavy
exercise?
• Aerobic Capacity
• Also called aerobic capacity, or VO2 max
• Determines whether a given exercise is light, moderate, or
heavy for a given person
• Greater for males and younger people
• Ranges from 12 ml O2/minute/kg body weight to 84 ml
O2/minute/kg body weight
• Lactate Threshold
• Also called anaerobic threshold
• Another way to determine exercise intensity for a given
person
• % of maximal oxygen uptake at which a rise in blood lactate
levels occurs
• Occurs at about 50−70% VO2 max
Cont’d
• Need for glucose increases during exercise
• More GLUT4 receptors in plasma membrane
• Blood glucose levels drop
• Liver provides more glucose through hydrolysis of
glycogen and through gluconeogenesis
Oxygen Debt
a.When a person exercises,
oxygen is withdrawn from
reserves in hemoglobin and
myoglobin.
1)To create cross bridges in muscle
contraction and pump calcium
back into SR at rest
2)To metabolize lactic acid
b.Breathing rate continues to be
elevated after exercise to repay
this debt.
Phosphocreatine
a.ATP may be used faster than it can be created through
cellular respiration.
b.ADP is combined with Pi from phosphocreatine.
1)Creatine is produced by the liver and kidneys or obtained in
the diet.
2)Phosphocreatine stores are replenished at rest.
c.Creatine supplements can increase muscle
phosphocreatine and aid short-term high-energy
exercise, but long-term use may damage the liver.
Slow/Fast Twitch Fibers
• Slow-twitch (type I): slower contraction speed; can
sustain contraction for long periods without
fatigue; rich capillary supply; more mitochondria;
more respiratory enzymes; more myoglobin
a.Said to have high oxidative capacity, so are called slow
oxidative fibers
b.Due to high myoglobin content (which has a red
pigment), these are also called red fibers
c.Found in postural muscles
Slow/Fast Twitch Fibers
• Fast (type IIx): faster contraction speed, fatigue
fast, fewer capillaries, mitochondria, respiratory
enzymes, and less myoglobin
• Also called white fibers
• Have more glycogen stores and are called fast glycolytic
fibers
• Found in stronger muscles
Muscle Fatigue
• Reduced ability to generate force
• Due to:
a.Accumulation of extracellular K+, reducing membrane potential
b.Short duration
c. Depletion of stored glycogen
d.Reduced SR calcium release
e.Lactic acid accumulation and lower pH
f. Increased concentration of PO4 due to phosphocreatine breakdown
g.Lack of ATP
h.Buildup of ADP
i. Fatigue of upper motor neurons (in the CNS), called central fatigue
Adaptation to Strength Training
• Hypertrophy: Type II muscle fibers become thicker
due to increased amount of actin and myosin (more
sarcomeres).
• Thicker fibers can split into two fibers, which can also
increase in size.
• Also requires titin, nebulin, and obscurin
V. Cardiac and Smooth Muscles
Introduction
• Cardiac and smooth muscles are:
a.Involuntary
b.Regulated by autonomic nervous system
c.Like skeletal muscle, contraction is due to myosin/actin cross
bridges stimulated by calcium
Cardiac Muscles
• Striated
• Myosin and actin filaments form sarcomeres.
• Contraction occurs by means of sliding thin
filaments.
• Unlike skeletal muscle fibers, these fibers are short,
branched, and connected via gap junctions called
intercalated discs (electrical synapses that permit
impulses to be conducted cell to cell).
Cardiac Muscle
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Gap junctions
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Myocardial
cells
Nucleus
Intercalated discs
© Ed Reschke
Myocardium
a.A myocardium is a mass of cardiac muscle cells connected to
each other via gap junctions.
b.Action potentials that occur at any cell in a myocardium can
stimulate all the cells in the myocardium.
c.It behaves as a single functional unit.
d.The atria of the heart compose one myocardium, and the
ventricles of the heart compose another myocardium.
Pacemaker Potential
a.Cardiac muscle can produce action potentials
automatically (without innervation).
1)Begin in a region called the pacemaker
b.Heart rate is influenced by autonomic innervation and
hormones.
Smooth Muscle
• Found in blood vessel walls, bronchioles, digestive
organs, urinary and reproductive tracts
a.Produce peristaltic waves to propel contents of these organs
b.No sarcomeres, but still contain large amounts of actin and
myosin
c. Long actin filaments attached to dense bodies
d.Myosin filaments are stacked vertically and can form cross
bridges with actin its entire length
e.Arrangement allows contraction even when greatly stretched
Comparison of Skeletal, Cardiac, and Smooth Muscle