Download Muscle Fiber

UNIT 3
Chapters 9 and 10
Muscle and Nerves
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Three Types of Muscle Tissue
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Table 9.3
Muscle Functions
1. Movement of
bones or fluids
(e.g., blood)
2. Maintaining
posture and body
position
3. Stabilizing joints
4. Heat generation
(especially
skeletal muscle)
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Special Characteristics of Muscle Tissue
• Excitability
(responsiveness or
irritability): ability to
receive and respond to
stimuli
• Contractility: ability to
shorten when stimulated
• Extensibility: ability to be
stretched
• Elasticity: ability to recoil
to resting length
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Layers of the muscle**
Epimysium
Bone Epimysium
Perimysium
Endomysium
Tendon
(b)
Muscle fiber
in middle of
a fascicle
Fascicle
(wrapped by perimysium)
Endomysium
(between individual
muscle fibers)
Perimysium Fascicle
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Muscle fiber
(Muscle cell)
Figure 9.1
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Table 9.1
Muscle Fiber/ Cell
Sarcolemma
Mitochondrion
Myofibril
Dark A band
Light I band
Nucleus
Myofibril
• Densely packed, rodlike elements
• ~80% of cell volume
• Exhibit striations: perfectly aligned repeating series of dark A bands
and light I bands
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Sarcomere**
• Smallest contractile unit
(functional unit) of a muscle
fiber
Thin (actin)
filament
Z disc
H zone
Z disc
• The region of a myofibril
between two Z discs
• Composed of thick (Myosin)
and thin (Actin)
myofilaments
• Thick filaments: run the
entire length of an A band
• Thin filaments: run the
length of the I band and
partway into the A band
• Z disc: anchors the thin
filaments and connects
myofibrils to one another
• H zone: lighter midregion
where filaments do not
overlap
• M line: line of protein
myomesin that holds
adjacent thick filaments
together
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Thick (myosin)
filament
I band
A band
Sarcomere
I band
M line
(c) myofibril
Sarcomere
Z disc
M line
Z disc
Thin (actin)
filament
Thick
(myosin)
filament
(d) sarcomere
Figure 9.2c, d
Longitudinal section of filaments
within one sarcomere of a myofibril
Thick filament
Thin filament
Each thick filament consists of many
myosin molecules whose heads protrude
at opposite ends of the filament.
Portion of a thick filament
Myosin head
A thin filament consists of two strands
of actin subunits twisted into a helix
plus two types of regulatory proteins
(troponin and tropomyosin).
Portion of a thin filament
Tropomyosin
Troponin
Actin
Actin-binding sites
ATPbinding
site
Heads
Tail
Flexible hinge region
Myosin molecule
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Active sites
for myosin
attachment
Actin
subunits
Actin subunits
Figure 9.3
Part of a skeletal
muscle fiber (cell)
I band
A band
I band
Z disc
H zone
Z disc
Myofibril
M line
Sarcolemma
Triad:
• T tubule
• Terminal
cisternae
of the SR (2)
Sarcolemma
Tubules of
the SR
Myofibrils
Mitochondria
Sarcoplasmic Reticulum (SR)
T Tubules
• Network of smooth endoplasmic
reticulum membrane surrounding
each myofibril
• Continuous with the sarcolemma
• Functions in the regulation of
intracellular Ca2+ levels
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• Penetrate the cell’s interior at each A
band–I band junction
• Associate with the paired terminal
cisternae to form triads that encircle
each sarcomere
Figure 9.5
Sliding Filament Model of Contraction
• In the relaxed state, thin
and thick filaments
overlap only slightly
• During contraction,
myosin heads bind to
actin, detach, and bind
again, to propel the thin
filaments toward the M
line
• As H zones shorten and
disappear, sarcomeres
shorten, muscle cells
shorten, and the whole
muscle shortens
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Z
Z
H
A
I
I
Fully relaxed sarcomere of a muscle fiber
Z
Z
I
A
I
Fully contracted sarcomere of a muscle fiber
Figure 9.6
Setting the stage
Axon terminal
of motor neuron
Action potential
Synaptic cleft
is generated
ACh
Sarcolemma
Terminal cisterna of SR
Motor Neuron
↓
Synapse
↓
Muscle Cell
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Muscle fiber Ca2+
Triad
One sarcomere
Figure 9.11, step 1
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
Action
potential (AP)
Nucleus
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated
Ca2+
channels
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Axon terminal
of motor neuron
3 Ca2+ entry causes some
Fusing synaptic
vesicles
synaptic vesicles to release
their contents (acetylcholine)
by exocytosis.
ACh
4 Acetylcholine, a
neurotransmitter, diffuses across
the synaptic cleft and binds to
receptors in the sarcolemma.
Na+
K+
channels that allow simultaneous
passage of Na+ into the muscle
fiber and K+ out of the muscle
fiber.
by its enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase.
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Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
5 ACh binding opens ion
6 ACh effects are terminated
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic
cleft
Ach–
Degraded ACh
Na+
Acetylcholinesterase
Postsynaptic membrane
ion channel opens;
ions pass.
Postsynaptic membrane
ion channel closed;
ions cannot pass.
K+
Figure 9.8
Axon terminal
Open Na+
Channel
Na+
Synaptic
cleft
Closed K+
Channel
ACh
ACh
Na+ K+
Na+ K+
++
++ +
+
K+
Action potential
+
+ +++
+
2 Generation and propagation of
the action potential (AP)
1 Local depolarization:
generation of the end
plate potential on the
sarcolemma
Sarcoplasm of muscle fiber ecc movie
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Closed Na+ Open K+
Channel
Channel
Na+
K+
3 Repolarization
Figure 9.9
Events at Muscle Cell Membrane movie gap
Depolarization
due to Na+ entry
Na+ channels
close, K+ channels
open
Repolarization
due to K+ exit
Na+
channels
open
Threshold
K+ channels
close
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Figure 9.10
Pap movie
1 Action potential is
Steps in
E-C Coupling:
propagated along the
sarcolemma and down
the T tubules.
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
Ca2+
release
channel
Terminal
cisterna
of SR
2 Calcium
ions are
released.
Ca2+
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Figure 9.11, step 4
• At low intracellular Ca2+ concentration:
Actin
Ca2+
Troponin
•
Tropomyosin blocks the active sites on actin
•
Myosin heads cannot attach to actin
•
Muscle fiber relaxes
Tropomyosin
blocking active sites
Myosin
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin.
Active sites exposed and•
ready for myosin binding
At higher intracellular Ca2+ concentrations:
•
Ca2+ binds to troponin
•
Troponin changes shape and moves
tropomyosin away from active sites
•
Events of the cross bridge cycle occur
•
When nervous stimulation ceases,
Ca2+ is pumped back into the SR and
contraction ends
4 Contraction begins
Myosin
cross
bridge
The aftermath
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Cbc movieFigure 9.11, step 7
Spinal cord
Motor Motor
unit 1 unit 2
Nerve
Motor neuron
cell body
Motor
Muscle
neuron
axon
Axon terminals at
neuromuscular junctions
• Motor unit = a motor
neuron and all (four to
several hundred) muscle
fibers it supplies
• Small motor units in
muscles that control fine
movements (fingers, eyes)
• Large motor units in large
weight-bearing muscles
(thighs, hips)
Muscle • Muscle fibers from a motor
fibers
unit are spread throughout
the muscle so that a single
motor unit causes weak
contraction of entire
muscle
• Motor units in a muscle
usually contract
asynchronously; helps
prevent fatigue
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Figure 9.13a
Graded Muscle Responses
• Variations in the
degree of muscle
contraction
Responses are
graded by:
1. Changing the
frequency of
stimulation
2. Changing the
strength of the
stimulus
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Muscle Metabolism:
Energy for Contraction
• ATP is the only source
used directly for
contractile activities
• Available stores of ATP
are depleted in 4–6
seconds
• ATP is regenerated by:
1. Direct
phosphorylation of
ADP by creatine
phosphate (CP)
2. Anaerobic pathway
(glycolysis)
3. Aerobic respiration
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• At 70% of maximum contractile
activity:
• Bulging muscles compress
blood vessels
(b)
Anaerobic pathway
Glycolysis and lactic acid formation
Energy source: glucose
Glucose (from
glycogen breakdown or
delivered from blood)
• Oxygen delivery is impaired
• Pyruvic acid is converted into
lactic acid
Glycolysis
in cytosol
• Lactic acid:
• Diffuses into the bloodstream
• Used as fuel by the liver,
kidneys, and heart
• Converted back into pyruvic
acid by the liver
2
O2
ATP
Pyruvic acid
net gain
O2
Released
to blood
Lactic acid
Oxygen use: None
Products: 2 ATP per glucose, lactic acid
Duration of energy provision:
60 seconds, or slightly more
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Figure 9.19b
• Produces 95%
of ATP during
rest and light to
moderate
exercise
• Fuels: stored
glycogen, then
bloodborne
glucose, pyruvic
acid from
glycolysis, and
free fatty acids
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(c)
Aerobic pathway
Aerobic cellular respiration
Energy source: glucose; pyruvic acid;
free fatty acids from adipose tissue;
amino acids from protein catabolism
Glucose (from
glycogen breakdown or
delivered from blood)
O2
Pyruvic acid
Fatty
acids
O2
Aerobic respiration
in mitochondria
Amino
acids
32
CO2
H2O
ATP
net gain per
glucose
Oxygen use: Required
Products: 32 ATP per glucose, CO2, H2O
Duration of energy provision: Hours
Figure 9.19c
Large
number of
muscle
fibers
activated
Large
muscle
fibers
High
frequency of
stimulation
Muscle and
sarcomere
stretched to
slightly over 100%
of resting length
Contractile force
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Figure 9.21
Effects of Exercise
• Aerobic (endurance) exercise leads to increased:
•
Results in greater endurance, strength, and
resistance to fatigue
• Resistance exercise (typically anaerobic) results
in:
•
Muscle hypertrophy (due to increase in fiber
size)
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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Muscle Fiber Type: Speed and Metabolism
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Table 9.2
SMOOTH MUSCLE
Longitudinal layer
of smooth muscle
(shows smooth
muscle fibers in
cross section)
Small
intestine
Mucosa
Circular layer of
smooth muscle
(shows longitudinal
views of smooth
muscle fibers)
• Found in walls of most hollow organs (except heart)
• Usually in two layers (longitudinal and circular)
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Figure 9.26
Peristalsis
• Alternating
contractions and
relaxations of smooth
muscle layers that mix
and squeeze
substances through
the lumen of hollow
organs
• Longitudinal layer
contracts; organ
dilates and shortens
• Circular layer
contracts; organ
constricts and
elongates
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Microscopic Structure:
Smooth Muscle
• 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
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Table 9.3
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Table 9.3
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Table 9.3
Varicosities
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Smooth
muscle
cell
• Autonomic nerve
fibers innervate
smooth muscle at
diffuse junctions
• Varicosities
(bulbous swellings)
of nerve fibers store
and release
neurotransmitters
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Synaptic
vesicles
Mitochondrion
Varicosities release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
Figure 9.27
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+
• 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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Figure 9.28a
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
• Sliding filament mechanism
• Final trigger is  intracellular Ca2+
• Ca2+ is obtained from the SR and extracellular space
• 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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Extracellular fluid (ECF)
Ca2+
Plasma membrane
Cytoplasm
1 Calcium ions (Ca2+)
enter the cytosol from
the ECF via voltagedependent or voltageindependent Ca2+
channels, or from
the scant SR.
Ca2+
Sarcoplasmic
reticulum
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Figure 9.29, step 1
2 Ca2+ binds to and
activates calmodulin.
Ca2+
Inactive calmodulin
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Activated calmodulin
Figure 9.29, step 2
3 Activated calmodulin
activates the myosin
light chain kinase
enzymes.
Inactive kinase
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Activated kinase
Figure 9.29, step 3
4 The activated kinase enzymes
catalyze transfer of phosphate
to myosin, activating the myosin
ATPases.
ATP
ADP
Pi
Pi
Inactive
myosin molecule
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Activated (phosphorylated)
myosin molecule
Figure 9.29, step 4
5 Activated myosin forms cross
bridges with actin of the thin
filaments and shortening begins.
Thin
filament
Thick
filament
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Figure 9.29, step 5
Regulation of Contraction
Neural regulation:
• Neurotransmitter binding
  [Ca2+] in sarcoplasm
• Response depends on
neurotransmitter
released and type of
receptor molecules
Hormones and local
chemicals:
• May bind to G protein–
linked receptors
• May either enhance or
inhibit Ca2+ entry
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Special Features of Smooth Muscle
Contraction
Stress-relaxation response:
• Responds to stretch only briefly, then
adapts to new length
• Retains ability to contract on demand
• Enables organs such as the stomach
and bladder to temporarily store
contents
Length and tension changes:
• Can contract when between half and
twice its resting length
Hyperplasia:
• Smooth muscle cells can divide and
increase their numbers
• Example:
• estrogen effects on uterus at puberty
and during pregnancy
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Types of Smooth Muscle
Single-unit (visceral) smooth
muscle:
• Sheets contract rhythmically
as a unit (gap junctions)
• Often exhibit spontaneous
action potentials
• Arranged in opposing
sheets and exhibit stressrelaxation response
Multiunit smooth muscle:
• Located in large airways,
large arteries, arrector pili
muscles, and iris of eye
• Gap junctions are rare
• Arranged in motor units
• Graded contractions occur
in response to neural stimuli
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Muscular Dystrophy
• Group of inherited muscledestroying diseases
• Muscles enlarge due to fat
and connective tissue
deposits
• Muscle fibers atrophy
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
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