Download Muscle Tissue A

Muscles and Muscle Tissue
Part A
Prepared by Janice Meeking & W. Rose.
Figures from Marieb & Hoehn 8th ed.
Portions copyright Pearson Education
Types of Muscle Tissue
1. Skeletal
•
•
•
Striated
Voluntary
Attached to bone (& skin)
2. Cardiac
•
•
•
Striated
Heart only
Involuntary
3. Smooth
•
•
Involuntary
Walls of hollow organs
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Table 9.3
Muscle Characteristics
• Excitability: ability to receive and respond to
stimuli
• Contractility: ability to shorten when stimulated
• Elasticity: ability to stretch and “bounce back”
Muscle Functions
• Move bones and fluids (e.g., blood)
• Maintain posture and body position (counteract
gravity)
• Stabilize joints
• Generate heat
Muscles attach directly to bone, or
indirectly via tendon or aponeurosis
Epimysium
Bone Epimysium
Perimysium
Endomysium
Tendon
(b)
Perimysium Fascicle
(a)
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Muscle fiber
in middle of
a fascicle
Blood vessel
Fascicle
(wrapped by perimysium)
Endomysium
(between individual
muscle fibers)
Muscle fiber
(cell)
Figure 9.1
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Table 9.1
Microscopic Anatomy of a
Skeletal Muscle Fiber
• Cylindrical cell 10 to 100 m in diameter, up to
30 cm (!) long
• Multiple peripheral nuclei; many mitochondria
• Glycogen granules for energy storage,
myoglobin for O2 transport & storage
• Myofibrils (80% of volume, striated),
sarcoplasmic reticulum, T tubules
Sarcolemma
Mitochondrion
Myofibril
Dark A band Light I band Nucleus
(b) Diagram of part of a muscle fiber showing the myofibrils.
In this diagram, one myofibril sticks out from the cut end of the fiber.
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Sarcomere
• Smallest contractile unit (functional unit) of a
muscle fiber
• The region of a myofibril between two
successive Z discs
• Composed of thick and thin myofilaments
made of contractile proteins
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Thin (actin)
filament
Thick (myosin)
filament
Z disc
I band
H zone
A band
Sarcomere
Z disc
I band
M line
(c) Small part of one myofibril enlarged to show the myofilaments
responsible for the banding pattern. Each sarcomere extends from
one Z disc to the next.
Sarcomere
Z disc
M line
Z disc
Thin (actin)
filament
Elastic (titin)
filaments
Thick
(myosin)
filament
(d) Enlargement of one sarcomere (sectioned lengthwise). Notice the
myosin heads on the thick filaments.
Copyright © 2010 Pearson Education, Inc.
Figure 9.2c, d
Longitudinal section of filaments
within one sarcomere of a myofibril
Thick filament
Thin filament
In the center of the sarcomere, the thick
filaments lack myosin heads. Myosin heads
are present only in areas of myosin-actin overlap.
Thick filament
Thin filament
Each thick filament consists of many
A thin filament consists of two strands
myosin molecules whose heads protrude of actin subunits twisted into a helix
at opposite ends of the filament.
plus two types of regulatory proteins
(troponin and tropomyosin).
Portion of a thick filament
Portion of a thin filament
Myosin head
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
Sarcoplasmic Reticulum (SR)
• Network of smooth ER around each myofibril
• Terminal cisternae at each end
• Stores & releases Ca2+ to regulate contraction
T tubules
• Continuous with sarcolemma
• Penetrate cell’s interior
• Associate with the paired terminal cisternae to
form triads that encircle each sarcomere
Part of a skeletal
muscle fiber (cell)
Myofibril
I band
A band
I band
Z disc
H zone
Z disc
M line
Sarcolemma
Sarcolemma
Triad:
• T tubule
• Terminal
cisternae
of the SR (2)
Tubules of
the SR
Myofibrils
Mitochondria
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Figure 9.5
Triads
• T tubules conduct electrical impulses
(action potentials) deep into muscle fiber
• Proteins embedded in membranes of T
tubules & SR terminal cisternae connect
with one another in precise arrangement
• T tubule proteins: voltage sensors
• SR foot proteins: SR Ca2+ release
channels
Sliding Filament Model
• In relaxed state, thin and thick filaments overlap only
slightly
• During contraction, myosin heads bind to actin, detach,
and bind again, pulling thin filaments along the thick
filaments, increasing the length over which thick & thin
overlap
• Filaments themselves do not change length significantly
• Sarcomeres shorten, muscle cells shorten, and whole
muscle shortens*
* Muscle “contraction” causes shortening iff inward force
exceeds outward force
Z
Z
H
A
I
I
1 Fully relaxed sarcomere of a muscle fiber
Z
I
Z
A
I
2 Fully contracted sarcomere of a muscle fiber
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Figure 9.6
Requirements for
Skeletal Muscle Contraction
1. Activation: neural stimulation at
neuromuscular junction
2. Excitation-contraction coupling:
•
Generation and propagation of an action
potential along sarcolemma, which causes…
•
Final trigger: a brief rise in intracellular Ca2+
Neuromuscular Junction
• Situated midway along length of muscle fiber
• Axon terminal and muscle fiber are separated by the
synaptic cleft
• Synaptic vesicles of axon terminal contain
neurotransmitter acetylcholine (ACh)
• Sarcolemma under the nerve terminal has ACh
receptors
Events at neuromuscular junction
• Somatic motor neurons (in spinal cord usually*)
fire to stimulate muscles
• Action potentials travel along axons, in nerves,
from spinal cord* to muscles
• Each axon forms several branches as it enters a
muscle
• Each axon ending forms a neuromuscular
junction with a single muscle fiber
* Somatic motor neurons for most muscles of the head are in the brain
Events at neuromuscular junction (cont.)
• Nerve impulse arrives at axon terminal
• ACh is released and binds with receptors on
sarcolemma
• If enough ACh is released fast enough, action
potential will be generated
• ACh effects are temporary because it is
destroyed by the enzyme acetylcholinesterase
PLAY
A&P Flix™: Events at the Neuromuscular Junction
Action
potential (AP)
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Nucleus
Sarcolemma of
the muscle fiber
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
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic
cleft
Fusing synaptic
vesicles
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Figure 9.8
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.
Copyright © 2010 Pearson Education, Inc.
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
Generating an Action Potential
1. Local depolarization (end plate potential):
• Before this occurs, quiet muscle is -70 mV inside.
Lot of K inside, Na outside.
• ACh binds to ACh-receptor channels in sarcolemma
and makes them open.
• The open channels are like an electrical short circuit
across the membrane: positive current flows in to
the negative cell.
• Net result: + charge enters cell, so voltage inside
cell rises (becomes less negative), i.e.
depolarization (end plate potential) to ~0 mV.
Generating an Action Potential
2.Generation and propagation:
•
End plate potential spreads to adjacent
membrane areas
•
Voltage-regulated Na+ channels open,
causing Na+ influx and even more
depolarization
•
If threshold is reached, an action potential is
generated: local depolarization spreads
•
Voltage-regulated Na+ channels open in
adjacent patch, causing it to depolarize to
threshold
Generating an Action Potential
3.Repolarization
•
Na channels close and voltage-gated K
channels open
•
K+ flows out thru the open K channels
•
The outflow (loss) of + charge causes cell to
return to a negative voltage
•
Fiber cannot be stimulated and is in a
refractory period until repolarization is
complete
•
Ionic conditions of resting state are restored
by Na+/K+ pump
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
Copyright © 2010 Pearson Education, Inc.
Closed Na+ Open K+
Channel
Channel
Na+
K+
3 Repolarization
Figure 9.9
Depolarization
due to Na+ entry
Na+
channels
open
Na+ channels
close, K+ channels
open
Repolarization
due to closure of Na + channels
and to K+ leaving cell through
open K + channels
Threshold
K+ channels
close
Copyright © 2010 Pearson Education, Inc.
Figure 9.10
Excitation-Contraction (E-C) Coupling
• Sequence of events by which an action
potential on the sarcolemma causes
contraction (sliding of filaments)
• Latent period
• Time when E-C coupling events occur
• Time between action potential initiation and
beginning of force development
Copyright © 2010 Pearson Education, Inc.
Events of Excitation-Contraction (E-C)
Coupling
• AP is propagated along sarcomere to T
tubules, down T tubules to triads
• Voltage-sensitive proteins in T tubules cause
Ca-release channels in SR to open and
release Ca++ from SR
• Ca++ level in cytoplasm rises and leads to
contraction
Copyright © 2010 Pearson Education, Inc.
Setting the stage
Axon terminal
of motor neuron
Action potential
Synaptic cleft
is generated
ACh
Sarcolemma
Terminal cisterna of SR
Muscle fiber Ca2+
Triad
One sarcomere
Copyright © 2010 Pearson Education, Inc.
Figure 9.11, step 1
Steps in E-C Coupling
1. Action potential is
propagated along the
sarcolemma and down
the T tubules.
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
2a. Voltage-sensitive
proteins in T tubule
change shape, cause Carelease channels in SR to
open.
2b. Calcium ions are
released from SR.
Ca2+
release
channel
Terminal
cisterna
of SR
Ca2+
Copyright © 2010 Pearson Education, Inc.
Figure 9.11, step 4
Actin
Ca2+
Troponin
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
4 Contraction begins
Myosin
cross
bridge
The aftermath
Copyright © 2010 Pearson Education, Inc.
Figure 9.11, step 7
Role of calcium in contraction
When intracellular Ca2+ concentration is low:
• Tropomyosin blocks the active sites on actin
• Myosin heads cannot attach to actin
• Muscle fiber relaxes
When intracellular Ca2+ concentration is high:
• Ca2+ binds to troponin
• Troponin changes shape and moves tropomyosin
away from active sites
• Cross bridge cycling occur, over and over, as long as
Ca is high and ATP is available
• When nervous stimulation ceases, Ca2+ is pumped
back into SR and contraction ends
Cross Bridge Cycle
1. Cross bridge forms: “cocked” (i.e. full of
potential energy) myosin head attaches to thin
filament
2. Working (power) stroke: myosin head pivots
and pulls thin filament toward M line
3. Cross bridge detachment: ATP attaches to
myosin head and the cross bridge detaches
4. “Cocking” of the myosin head: energy from
hydrolysis of ATP cocks the myosin head into
the high-potential-energy state
Excitation-Contraction Coupling Video
http://www.youtube.com/watch?v=CepeYFvqmk4
College or Department name here
Actin
ADP
Myosin
cross bridge
Pi
Myosin
Thick
filament
1
Cross bridge formation
ADP
ATP
hydrolysis
Pi
4
2
Cocking of myosin head
Power (working) stroke
ATP
ATP
ATP
3
Cross bridge detachment
Figure 9.12
Actin
Ca2+
Myosin
cross bridge
Thin filament
ADP
Pi
Thick filament
Myosin
1 Cross bridge formation.
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Figure 9.12, step 1
ADP
Pi
ADP + Pi are released
2 The power (working) stroke.
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Figure 9.12, step 3
ATP
ATP binds (must bind
for release to occur)
3 Cross bridge detachment.
Copyright © 2010 Pearson Education, Inc.
Figure 9.12, step 4
ADP
ATP
Pi
hydrolysis
ATP is hydrolyzed,
releasing energy
4 Cocking of myosin head.
Copyright © 2010 Pearson Education, Inc.
Figure 9.12, step 5
Cross Bridge Cycle videos
1. Action Potential focus on EC coupling, cavemanl29
2. Muscle Contraction includes role of Ca, cavemanl29
Sliding Filament by Sara Egner. A good animation.
www.sci.sdsu.edu/movies/actin_myosin_gif.html
Color Atlas of Physiology, Agamemnon Despopoulos, Stefan
Silbernagl, Thieme Medical Publishers, Inc. , 1991, New York
STOP
Skip triad slides if short on time
Ultrastructure of a skeletal muscle triad
Wagenknecht, T., Samso, M. (2002).
Three-dimensional reconstruction of
ryanodine receptors. Frontiers in
Bioscience 7: D1464-D1474.
T-tubule voltage sensor protein (DHPR) embedded in
T-tubule membrane.
SR calcium-release channel (RyR1, RyR3) has large
cytoplasmic domain (“foot”) that spans gap between
SR and T-tubule, and small domain in SR membrane.
Some skeletal muscles lack RyR3. RyR1, but not RyR3,
required for excitation-contraction coupling.
Clara Franzini-Armstrong (2004), Biol Res 37: 507-512.
Department of Health, Nutrition, and Exercise Sciences
Skip triad slides if short on time
Ultrastructure of a skeletal muscle triad
View of proteins at interface
of T tubule and sarcoplasmic
reticulum (SR), “looking
down” onto double layer of a
T tubule membrane and an
SR membrane at the triad.
The membranes themselves
are transparent, and we only
see the proteins embedded in
the membranes.
Black circles: voltage sensor proteins in T-tubules.1
White circles: SR calcium-release channel proteins.2
1. Dihydropyridine receptors
2. Ryanodine receptors
SR calcium-release channels (white circles) form precise arrays in SR junctional
membrane. Their cytoplasmic portions (“feet”) bridge the gap between SR and
T-tuble. T-tubule voltage sensors (black circles) are located exactly across from
alternating groups of SR calcium-release channels.
Clara Franzini-Armstrong (2004), Biol Res 37: 507-512.
Department of Health, Nutrition, and Exercise Sciences
Skip triad slides if short on time
Comparison of triad ultrastructure in skeletal,
cardiac, and mutant skeletal muscle
Views of proteins at interface of T
tubule and sarcoplasmic reticulum
(SR), “looking down” onto double
layer of a T tubule membrane and
an SR membrane at the triad. The
membranes themselves are
transparent, and we only see the
proteins embedded in the
membranes.
Black circles: voltage sensor
proteins in T-tubules.1
White circles: SR calciumrelease channel proteins.2
1. Dihydropyridine receptors
2. Ryanodine receptors
Skeletal muscle: Voltage sensor proteins (DHP receptors) are aligned with SR Ca-release channel proteins
(ryanodine receptors).
Cardiac muscle: Voltage sensors NOT aligned with SR Ca-release channels.
Dyspedic (“footless”) skeletal muscle: Ca-release channel gene has been knocked out, and voltage sensors
are disorganized.
Clara Franzini-Armstrong (2004), Biol Res 37: 507-512.
Skip triad slides if short on time
Triad ultrastructure in skeletal, cardiac, and mutant skeletal muscle
Views “looking down” onto
transparent double layer of a
T tubule membrane and an
SR membrane.
Skeletal muscle: Voltage sensor proteins (DHP receptors) aligned with SR Ca-release channel
proteins (ryanodine receptors).
Cardiac muscle: Voltage sensors NOT aligned with SR Ca-release channels.
Dyspedic (“footless”) skeletal muscle: Ca-release channel gene knocked out; voltage sensors are
disorganized. Clara Franzini-Armstrong (2004), Biol Res 37: 507-512.
Department of Health, Nutrition, and Exercise Sciences