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Muscle Contraction
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Muscle Tissue
Muscle is a tissue built of specialized contractile cells, called muscle
cells ormuscle fibers ormyocytes ormyofibers.
There are two main categories of muscle:
1) Striated muscle tissue has alternating light and dark bands (which
come from the organization of the contractile proteins), giving it a
striated appearance. In vertebrates, the striated muscle makes up the
skeletal (most) and cardiac muscle.
2)Smooth muscle tissue also uses contractile proteins, but they are
not organized in the same fashion, so it doesnt have a striated
appearance. In vertebrates, smooth muscle lines the gut, respiratory
tract, blood vessels, etc.
Tissue built to generate contractile force
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Neuromuscular Junction
Where neurons and muscles meet
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Neuromuscular Junction
Fast chemical transmission at the NMJ
Step-by-step:
1) AP depolarizes terminal
2) Voltage-gated Ca++ channels
open
3) ACh is released into the synapse
4) ACh binds to nictotinic receptors
5) Nicotinic recepors allow Na+ and
K+ ion flow, depolarizing the muscle
cell
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Neuromuscular Junction
Fast chemical transmission at the NMJ
6) The muscular AP propagates to all
parts of the muscle, perhapsresulting in contraction
7) Acetylcholinesterase hydrolyzes
ACh into acetate and choline
8) Choline is actively transported
back into the motor neuron terminal.
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Neuromuscular Junction
Muscle Contraction
When the current at the muscle is
sufficiently large, the muscle cell
contracts. Exocytosis of each vesicle of
ACh releases a quantum of ACh
(analogous to an EPSP). Approx. 100
quanta are required to initiate an AP in
the muscle. That number can be
released in response to a single AP
reaching the NMJ.
Although each fiber is only innervated by one nerve, there may be
multiple synapses from that nerve, which promotes more synchronous
depolarization of the fiber (which can be 15 cm long in humans).
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Skeletal Muscle Organization
Skeletal Muscles are Organized Hierarchically
Skeletal muscle consists of multiple bundles
of muscle fibers
Myofibers are long, multinucleate, cylindrical
cells, organized in parallel
Each myofiber consists of many parallel
myofibril subunits
Each myofibril is composed of repeatedsarcomere subunits
The sarcomere is the fundemental unit of
contraction, which consists ofactin and
myosin filaments
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Skeletal Muscle Organization
Skeletal Muscles are Organized Hierarchically
Skeletal muscle consists of multiple bundles of muscle fibers
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Skeletal Muscle Organization
Skeletal Muscles are Organized Hierarchically
Myofibers are long, multinucleate, cylindrical cells, organized in parallel
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Skeletal Muscle Organization
Skeletal Muscles are Organized Hierarchically
Each myofiber consists of many parallel myofibril subunits
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Skeletal Muscle Organization
Skeletal Muscles are Organized Hierarchically
Each myofibril is composed of repeated sarcomere subunits
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Skeletal Muscle Organization
Skeletal Muscles are Organized Hierarchically
The sarcomere is the fundemental unit of contraction, which consists of
actin and myosin filaments
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Sarcomeres
Skeletal muscle fibers are subdivided into repeated 2 m long striatedsarcomeres with a z disk at each end.
Actin (thin) filaments: ~2000 per disk. Attached at their midpoint to z-
disks and project to either side
Myosin (thick) filaments: ~1000 per sarcomere.At the center of each
sarcomere and attach to actin at each end.
Titin filaments: 10% of total muscle mass. Function as elastic bands.
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Actin FilamentsThe thin filaments
Each actin filament is
composed of:
1) Two twisted, bended
polymer chains of globularactin molecules
2) Two strands oftropomyosin molecules that lie from end to end in
the grooves formed by the actin chains
3)Troponin molecules attached at intervals to tropomyosin strands
Tropomyosin and troponin act to control whether myosin cross-bridges
can interact with the thin filaments
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Myosin FilamentsThe thick filaments
Each myosin filament has ~200
myosin heavy chain molecules
arranged in helical pairs.
Each myosin molecule has aglobular head with an actin binding
domain (actin attachment site), a
nucleotide pocket (ATP hydrolysis
site), and a flexible neck with two
light chains of myosin.
The head-neck segment tilts to a
smaller angle when the molecule
interacts with actin.
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Muscle Contraction
The Sliding Filament Theory
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Muscle Contraction
The Sliding Filament Theory
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Muscle Contraction
Molecular interactions of contraction
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Muscle Contraction
1) The rigor state
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Muscle Contraction
2) ATP binding
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Muscle Contraction
3) ATP hydolysis
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Muscle Contraction
4) Rebinding to actin
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Muscle Contraction
5) Phosphate release and filament sliding(power stroke)
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Muscle Contraction
6) ADP release (back to rigor)
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Muscle ContractionMolecular interactions of contraction
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Muscle ContractionMolecular movement requires ATP and Ca2+ ions
No Calcium Calcium ions present
Ca2+ bound to
troponin
In the absence of Ca2+, tropomyosin blocks the myosin binding
sites on the actin filaments
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Muscle ContractionMolecular movement requires ATP and Ca2+ ions
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T-tubules and the SRTransverse Tubules and the Sarcoplasmic Rectulum membrane systems
The sarcolemma (the myocyte cell
membrane) formsdeep transverse
invaginations into the myocyte, called
transverse tubules (orT-tubules).
The T-tubule system is continuous with
the ECF and thus contains extremely
high Ca2+ concentrations.
The sarcoplasmic recticulum orSR is
a network of longitudinal membrane-bound tubules contained entirely within
the myocyte between two T-tubules.
Protein receptor channels join the T-
tubules and SR.
T-tubu
le
The SR
NMJ
Muscle cell
Axon
terminal
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Excitation-Contraction CouplingSarcolemma Depolarization
1) ACh release at the NMJ
2) Opening of ACh gated ion
channels
3) AP propagates down the T-tubules
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Excitation-Contraction CouplingCa2+ efflux from the SR
4) Depolarization reaches the DHPR
receptors, opening the associated
RyR receptors and releasing Ca2+
from the SR
5) Ca2+ ions diffuse into the cytosol
and bind to troponin, enablingfilament sliding
6) Cross bridges go through several
cycles of movement while the Ca2+
is present.
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Excitation-Contraction CouplingAP termination
7) Meanwhile at the NMJ,
acetylcholinesterase (AChE)
hydrolizes ACh to terminate the AP
8) Depolarization ceases at the T-
tubules and RyR channels close
9) Ca2+ ATPases actively transport
Ca2+ ions back into the SR
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Excitation-Contraction CouplingCa2+ efflux mediates filament sliding
Relaxed Contracted
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