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Muscle is one of the primary tissue types.
Skeletal muscle perform six major functions.
Skeletal muscle produces skeletal movement by
contracting and pulling on the bones of the skeleton.
Skeletal muscles maintain posture and body position by
maintaining tension, thereby allowing you to hold your head
while sitting and reading a book or balancing your body
when you walk or stand.
Skeletal muscles support soft tissues, such as abdominal
muscles and pelvic floor muscles, supporting the organs
of the abdominal-pelvic cavity.
Skeletal muscles guard entrances and exits by
surrounding the openings of the
digestive and urinary tracts.
Skeletal muscles maintain body temperature by releasing heat
when they are working.
And finally, skeletal muscles can store nutrient reserves
because the protein in muscles can be broken down into amino
acids, which can then be used to produce energy.
Each muscle is composed of muscle cells
called muscle fibers.
These muscle fibers are contained in
bundles called fascicles.
Muscles have connective tissue that's associated with the
entire muscle, with the fascicle or muscle bundle, and
with the individual muscle fiber.
Epimysium is a dense layer of collagen fibers that surrounds
the entire muscle.
It separates the muscle from nearby tissues and organs.
The perimysium divides the skeletal muscle into a series
of compartments.
Each compartment contains a muscle fascicle.
The perimysium contains blood vessels and nerves.
Within each fascicle, the endomysium is more delicate
and surrounds each individual muscle fiber.
The endomysium contains capillary blood vessels, nerve
fibers, and myosatellite cells, which are stem cells,
that help to repair damaged muscle tissue.
The collagen fibers of the epimysium, the perimysium and
the endomysium come together to form either a bundle, known
as the tendon, or a broad sheet, called an aponeurosis.
Tendons and aponeuroses usually attach skeletal
muscles to bones.
Skeletal muscle cells or fibers are very different from
the typical cells we've seen so far.
One obvious difference is that these skeletal muscle fibers
are much larger than other cells.
A muscle fiber from the thigh muscle could have a length up
to 12 inches.
A second difference is that skeletal muscle contains
hundreds of nuclei just internal to the cell membrane.
These nuclei are needed to produce enzymes and structural
proteins that are required for normal muscle contractions.
The cell membrane of a muscle fiber is known as the
sarcolemma and the cytoplasm is known as the sarcoplasm.
Inside each muscle fiber are hundreds to thousands of
structures called myofibrils.
These structures are cylindrical in shape.
They can actively shorten in shape and are responsible for
skeletal muscle fiber contraction.
Myofibrils consist of protein filaments called myofilaments.
These myofilaments are either thin filaments composed
primarily of actin or thick filaments composed
primarily of myosin.
The Myofibrils are anchored to each end of the muscle fiber,
which is connected to its tendon.
As a result, when the myofibrils contract, the
entire cell shortens and pulls on the tendon.
Transverse tubules or T-tubules are narrow tubes
that are continuous with the sarcolemma and extend deep
into the sarcoplasm.
They are filled with extra cellular fluid and form
passageways through the muscle fiber, like a network of
tunnels through a mountain.
Electrical impulses called action potentials travel along
the T-tubules into the cell interior.
These action potentials trigger muscle fiber
contraction.
Branches of the T-tubules surround each myofibril.
The sarcoplasmic reticulum is similar to the smooth
endoplasmic reticulum of other cells.
The sarcoplasmic reticulum fits over each individual
myofibril like a lacy shirt sleeve.
Where a T-tubule encircles the myofibril, the sarcoplasmic
tubule expands, and these chambers are
called terminal cisternae.
The terminal cisternae contains stored calcium.
And sometimes the concentration of calcium
inside the cisternae is a thousand times higher than the
levels inside the sarcoplasm.
The thick and thin filaments of the myofibril are organized
into repeating functional units called sarcomeres.
Sarcomeres are the smallest functional units
of the muscle fiber.
Interactions between the thick and thin filaments of
sarcomeres are responsible for muscle contraction.
One myofibril consists of approximately 10,000
sarcomeres end to end.
A sarcomere contains thick filaments, thin filaments,
proteins that stabilize the positions of the thick and
thin filaments, and proteins that regulate the interactions
between the thick and thin filaments.
The thick and thin filaments are different in size,
density, and distribution.
These differences account for the banded
appearance of each myofibril.
The thick filaments are at the center of each sarcomere.
Proteins of the M-line connect the central portion of each
thick filament to neighboring thick filaments.
M stands for middle.
The thin filaments are located between the thick filaments in
an area called the zone of overlap.
A single thin filament contains two rows of 300 to
400 individual globular molecules.
Each of these molecules contains an active site that
can bind to myosin.
Under resting conditions, a complex called the
tropomyosin-troponin molecule covers the active sites and
prevents the binding of myosin to the active site.
Tropomyosin is a double-stranded, rope-like
structure that covers the active sites
on the actin molecules.
Troponin is a molecule that locks the tropomyosin molecule
to the actin molecule, thereby preventing the exposure of the
actin molecule's active site.
A thick filament contains about 300 myosin molecules.
The myosin molecules are twisted around each other.
Each myosin molecule has a long tail and a head which
projects outward toward the nearest thin filament.
When the myosin heads interact with thin filaments during a
contraction, they are known as cross-bridges.
The connection between the head and the tail of the
myosin acts as a hinge that lets the head pivot.
When they head pivots, it swings toward the M-line or
the center of the sarcomere.
All the myosin molecules are arranged with their tails
pointing towards the M-line.
When the myosin head forms a cross-bridge with the actin
molecule's active site and the head pivots, causing the thin
filaments to slide toward the center of each sarcomere, this
is known as the sliding filament theory.
During a contraction, sliding occurs in every sarcomere
along the myofibril.
As a result, the myofibril gets shorter.
When myofibrils get shorter, so does the muscle fiber.
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Ch 10 Muscle Tissue mp4

1812 タグ追加 保存
Cheng-Hong Liu 2014 年 11 月 20 日 に公開
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