Transcript Three Types of Muscle Tissue

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
9
Muscles and
Muscle
Tissue: Part A
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Muscle Functions
1. Movement of bones or fluids (e.g., blood)
2. Maintaining posture and body position
3. Stabilizing joints
4. Heat generation (esp. skeletal muscle)
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Overview of Muscle Tissue
Three Types of Muscle Tissue
1. Skeletal muscle tissue:
•
Attached to bones and skin
•
Striated
•
Voluntary
•
Powerful
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Three Types of Muscle Tissue
2. Cardiac muscle tissue:
•
Walls of heart
•
Striated and Involuntary
•
Contains intercalated discs
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Three Types of Muscle Tissue
3. Smooth muscle tissue:
•
In walls of hollow organs (e.g., stomach,
urinary bladder, etc.)
•
Not striated
•
Involuntary
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Table 9.3
Skeletal Muscle Structure
• Each muscle is served by one artery, one nerve,
and one or more veins
• Three Layers of connective tissue sheaths
surround skeletal muscle:
• Epimysium: dense regular CT surrounding entire
muscle
• Perimysium: fibrous CT surrounding fascicles (groups
of muscle fibers)
• Endomysium: fine areolar CT surrounding each
muscle fiber
• Tendon – muscles CT wrappings extend beyond
the muscle as a ropelike tendon which connects
it to bone
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Epimysium
Epimysium
Perimysium
Endomysium
Tendon
(b)
Muscle fiber
in middle of
a fascicle
Fascicle
(wrapped by perimysium)
Endomysium
(between individual
muscle fibers)
Perimysium Fascicle
(a)
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Muscle fiber
Figure 9.1
Muscle Action
• Action – muscles main function
• Attachments
• Origin- fixed/immovable point of muscle
attachment
• Insertion – muslces attachment on the
movable bone
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Muscle Action
• Functional Classification
• Prime mover – has the major responsibility for
producing a specific movement
• Synergist - helps the prime mover by adding
a little extra force to the same movement or
reducing undesirable movements that might
occur as the prime mover contracts
• Antagonist – muscles that oppose a
movement
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Skeletal Muscle Fiber Structure
• Myofibrils
• A muscle fiber is composed of thousands of
stacked, cylindrical, contractile structures called:
myofibrils
• Each myofibril consists of smaller contractile units
called: sarcomeres
• Each sarcomere contains an orderly arrangement
of rod-like protein filaments called: myofilaments
• Thick filaments – contain myosin
• Thin filaments – contain actin
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Microscopic Anatomy
• Sarcoplasm – cytoplasm of a muscle cell
• Myoglobin – red, oxygen storing pigment
• Sarcolemma- plasma membrane of muscle
cell
• T (transverse) tubules – areas of the
sarcolemma that penetrate the cells interior at
regular intervals
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Sarcolemma
Mitochondrion
Myofibril
Dark A band Light I band Nucleus
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Microscopic Anatomy
Sarcoplasmic Reticulum – Smooth ER of the
muscle cell
• Surround each myofibril
• Stores and regulates intracellular calcium
• Most tubules of SR run longitudinally along
myofibril, others form perpendicular cross
channels
• Terminal cisternae- paired cross channels
with a T-tubule running between them form
a triad
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Myofibril
I band
A band
I band
Z disc
H zone
Z disc
M line
Sarcolemma
Triad:
• T tubule
• Terminal
cisternae
of the SR (2)
Tubules of
the SR
Myofibrils
Mitochondria
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Figure 9.5
Sarcomere Structure
• Sarcomere – smallest contractile unit of a muscle
fiber; the region of a myofibril between two
successive Z discs
• Myofilments- filaments that constitute myofibrils
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Sarcomere Structure
• A sarcomere extends from Z disc to Z disc
• Z line (band): proteins that anchor thin filaments;
connects myofibrils to one another
• Sarcomere regions
• H zone (band)- lighter mid region within the A band
where filaments don’t overlap
• M line- middle dark line of sarcomere; holds
adjacent thick filaments together
• A band – dark band; contains thick and thin
filaments
• I band – Light band; contains thin filaments only
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Thin (actin)
filament
Thick (myosin)
filament
(c)Part
Z disc
I band
H zone
A band
Sarcomere
Z disc
I band
M line
of one myofibril
Sarcomere
Z disc
M line
Z disc
Thin (actin)
filament
Elastic (titin)
filaments
Thick
(myosin)
filament
(d)
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Figure 9.2c, d
The Neuromuscular Junction
• Motor Neurons – Located in the brain or
spinal cord
• Skeletal Muscle is innervated by: somatic
motor neurons
• Each muscle is served by: one artery, one
nerve and one/more veins
• Neuron axons: travel from origin all the way to
the skeletal muscle it innervates
• Motor unit: one motor neuron and all of the
skeletal muscle fibers it innervates
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The Neuromuscular Junction
• Neuromuscular Junction includes axonal
endings, synaptic cleft and junctional folds of
sarcolemma
• Each axon terminal forms: a neuromuscular
junction with a single muscle fiber
• Synaptic Cleft –the small space between the
axon terminal and the sarcolemma
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The Neuromuscular Junction
• Axon Terminal
• Synaptic vesicles – small membranous sacs
that store the neurotransmitter acetylcholine
• Acetylcholine (Ach) –
• When a nerve impulse reaches the axon
terminal Ca2+ floods into the axon terminal
• This causes synaptic vesicles fuse to axon
terminal leading to
• Exocytosis of: acetylcholine into synaptic
cleft
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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
The Neuromuscular Junction
• The Motor End Plate –
• Contains Ach receptors
• Highly-excitable region of the sarcolemma
• Responsible for initiation of action potentials
• End plate potential:a local electrical event that
ignites the action potential
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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
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Figure 9.11, step 1
Resting Membrane Potential
• Sarcolemma – is polarized prior to a nerve
impulse = cells interior is negative relative to
the outside
• The difference between the charge inside and
outside the membrane is called: resting
membrane potential
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Action Potential Generation
• Steps:
• Ach
• Is released from axon terminal
• Diffuses across synaptic cleft
• Binds to Ach receptors on junctional folds of
sarcolemma
• Binding of Ach to receptors causes:
chemically gated cation channels to open
• This allows Na+ and K+ to pass
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Action Potential Generation
• Depolarization
• Chemically gated cation channels open:
allowing Na+ and K+ to pass
• More Na+ diffuses in than K+ diffuses out
• This produces a local change in the membrane
potential or depolarization = end plate potential
• This causes the interior of the muscle fiber
(just below sarcolemma) to become less
negative
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Axon terminal
Open Na+
Channel
Na+
Synaptic
cleft
Closed K+
Channel
ACh
ACh
Na+ K+
Na+
K+
K+
++
++ +
+
Action potential
+
+ +++
+
1 Local depolarization: generation of the
end plate potential on the sarcolemma
Sarcoplasm of muscle fiber
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Figure 9.9, step 1
Action Potential Generation
• Propagation
• The depolarization wave spreads to: areas adjacent
to the neuromuscular junction
• Depolarization travels down the sarcolemma:
opening voltage-gated sodium channels, so Na+
enters
• Once a threshold voltage is reached the AP is
initiated
• The AP propagates (spreads) along the length of
the sarcolemma, opening more voltage-gated Na+
channels
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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
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Figure 9.9, step 2
Action Potential Generation
• Repolarization –
• Restores the sarcolemma to its initial polarized state
• Repolarization follows the depolarization wave
• Due to the closing of voltage gated Na+ channels and
opening of voltage-gated K+ channels
• K+ diffuses out (K+efflux) of muscle fiber
• Absolute refractory period- is the interval from the
beginning of the AP until the fiber is able to conduct
another AP
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Closed Na+
Channel
Open K+
Channel
Na+
K+
3 Repolarization
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Figure 9.9, step 3
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
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Closed Na+ Open K+
Channel
Channel
Na+
K+
3 Repolarization
Figure 9.9
Destruction of Acetylcholine
• Effects are quickly terminated by: the enzyme
acetylcholinesterase
• Prevents: continued muscle fiber contraction
in the absence of additional stimulation
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Sliding Filament Mechanism
• When a muscle fiber contracts: myosin heads
bind to actin, detach, and bind again,
propelling thin filaments toward M line
• In the relaxed state: thin and thick filaments
slightly overlap
• During a contraction: thin filaments slide past
thick ones so that they overlap to a greater
degree until the H zone disappears;
sarcomeres & muscle cells shorten, so whole
muscle shortens
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Z
I
1
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I
Fully relaxed
Z
2
Z
H
A
I
Z
A
Fully contracted
I
Figure 9.6
Ultrastructure of Myofilaments
• The structure of actin and myosin play a major
role in the sliding filament‖ mechanism
• Thick Filament (Myosin) Structure - Composed of
myosin tails and heads
• The head of each myosin molecule: bear actin and ATPbinding sites and contain ATPase enzymes
• Myosin heads are attracted to: actin of thin filaments
• Myosin has the ability to: split ATP to generate energy for
muscle contraction
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Ultrastructure of Myofilaments
• Thin Filament Structure
• Two actin strands are: twisted around one
another
• Troponin (protein) molecules: regulatory
protein of the thin filament that binds calcium;
joined to tropomyosin
• Normally, the tropomyosin strand: covers the
myosin binding sites on actin
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Longitudinal section of filaments
within one sarcomere of a myofibril
Thick filament
Thin filament
Thick filament
Thin filament
Portion of a thick filament
Myosin head
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
Steps in the Sliding filament Mechanism
• In a relaxed state, the actin filaments: are not
bound by myosin heads
• ATP binds to the myosin head and: hydrolyzes
ATP
• The energy released is stored and the myosin
head is energized (“cocked “) position
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ADP
ATP
Pi
hydrolysis
4 Cocking of myosin head.
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Figure 9.12, step 5
Steps in the Sliding filament Mechanism
• An action potential travels down the
sarcolemma and down the T-tubules
• The T-tubules carry the impulse into the
interior of the cell
• The impulse causes the SR to release calcium
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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
Steps in the Sliding filament Mechanism
• Ca2+ ions: are bound by troponin
• Troponin and tropomyosin complex: troponin
changes shape causing tropomyosin to move
away from myosin binding sites on actin
• The active sites for binding of the myosin head
are now uncovered
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Actin
Ca2+
Troponin
Tropomyosin
blocking active sites
Myosin
The aftermath
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Figure 9.11, step 5
Steps in the Sliding filament Mechanism
• Myosin heads bind to actin
• Once each myosin head binds to actin: a
cross bridge is formed
• As the myosin head tilts inward: it pulls the
actin filament towards the M line
• This inward pulling of the actin is called the
power stroke.
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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
2 The power (working) stroke.
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Figure 9.12, step 3
Steps in the Sliding filament Mechanism
• Once the power stroke has occurred: myosin
binds ATP
• Binding of the ATP: causes myosin to detach
from the actin
• The newly attached ATP: is hydrolyzed and
the cycle starts again
• This process continues in a ratchet-like
manner until the actin filaments are
overlapping
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ATP
3 Cross bridge detachment.
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Figure 9.12, step 4
Thin filament
Actin
Ca2+
Myosin
cross bridge
ADP
Pi
Thick
filament
Myosin
Cross
bridge
formation.
1
ADP
ADP
Pi
Pi
ATP
hydrolysis
2 The power (working)
stroke.
4 Cocking of myosin head.
ATP
ATP
3 Cross bridge
detachment.
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Figure 9.12