Introduction to Muscles

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Introduction to Muscles

• Microanatomy

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Muscles and Forces Muscle cells convert
chemical energy into force Three major
types Skeletal attached to bony skeleton
Smooth surrounds hollow organs and tubes,
stomach, intestinal tract, blood vessels
Cardiac muscle pumping muscle of the heart
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Muscle such as biceps made up of muscle cells,
bound together by connective tissue and attached
to bone at either end by tendon
Three layers of connective tissue
epimysium around entire muscle perimysium-
divides muscle into clumps of muscle fibers
endomysium- surrounds each muscle fiber
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Skeletal Muscle Structure Muscle cells are
gigantic cells, 10 to 100 µm in diameter, up to
20 cm long Adult muscle cell formed by fusion
of undifferentiated mononucleate cells
(myoblasts) to form a multinucleate cell.
Damaged muscle cells generally do not regenerate
some replacement by undifferentiated satellite
cells associated with endomysium
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Skeletal Muscle Structure Muscle cells are
activated by motor neurons. One motor neuron (in
spinal cord) will innervate (connect to) many
muscle fibers. The collection of muscle fibers
innervated by one motor neuron is called a motor
unit it is the contractile unit of a
muscle.Typically there are many motor units in a
single muscle.
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Muscle Ultrastructure In the light
microscope, a series of light and dark bands can
be observed in histological slides and in
polarized light in living muscle striations,
hence striated muscle. Within a muscle cell there
are many cylindrical - elements (1-2 µm in
diameter) called myofibrils, that have
striations. Each striation is called a
sarcomere
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Muscle Ultrastructure Each sarcomere is
composed of two major proteins Myosin, a long
shaft with two cross bridges (lollipops) at one
end. Myosin molecules bundled together so that
the cross bridges stick out at regular intervals
around the bundles circumference (15 nm
diameter). Individual myosin molecules connected
together at the center of the sarcomere by a
specialized protein at the M line one bundle
faces to the left, the other to the right.
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Muscle Ultrastructure The bundle of myosin
molecules is called a thick filament. An array
of thick filaments makes up the region of the
sarcomere called the A band. Actin, a major
cytoskeletal protein, is composed of globular
subunits that hook together like beads on a
necklace to form a long filament. Two actin
filaments wrap around each other to form a thin
filament (5 nm). On the surface of the thin
filament a long protein, tropomyosin, wraps
around it, attached at regular intervals by the
protein troponin.
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Muscle Ultrastructure The thin filaments are
bound to a protein lattice called the Z disk at
either end of the sarcomere the thin filament
region is called the I band. The thick filaments
are also attached to the Z disk proteins by a
protein called titin. Thick and thin filaments
are interdigitated with six thin filaments around
a thick filament.
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