Skeletal muscle
cells develop when hundreds of individual embryonic cells called
myoblasts fuse. The resulting cells' cell membrane is
called the sarcolemma and the cytoplasm is called
sarcoplasm. Some of the unique characteristics of skeletal
muscle cells (fibers) are:
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Skeletal
muscles are very large. Their diameter is about
100 microns (about 13 times the diameter of a red blood
cell) and they can be as long as 30 to 40 cm (10 to 16
in.). |
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Skeletal
muscles are multinucleate. The nuclei of the
myoblasts that fuse to form the cell remain within the
cell. Some myoblasts do not fuse but remain associated
with the fiber as satellite cells that can divide
and differentiate to repair damaged cells. |
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There
are internal tubular extensions of the sarcolemma into
the sarcoplasm called transverse tubules or T
tubules. |
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Myofibrils and Myofilaments
Within
each skeletal muscle cell or fiber there are cylindrical
structures called myofibrils. The myofibrils are
about 1/100 the diameter of the cell but extend the full
length of the cell and attach to the cell membrane at
either end. Contraction of the myofibrils result in
contraction of the entire cell. Transverse tubules form
loops around myofibrils along their entire length. In
between the loops of transverse tubule, there is a
network of tubules of smooth endoplasmic reticulum
called the sarcoplasmic reticulum. The
sarcoplasmic reticulum surrounds the myofibril like a
sleeve. The sarcoplasmic reticulum on either side of the
loops of transverse tubules fuse to form expanded
chambers called terminal cisternae. The
combination of a transverse tubule flanked by two
terminal cisternae is called a triad.
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The energy for muscle contraction is supplied by the
numerous mitochondria that surround the
myofibrils. Glycogen granules that also surround
the myofibrils as a ready source of fuel.
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The myofibrils
are made-up of myofilaments. The myofilaments are
organized in repeating units along the length of the myofibril
called sarcomeres.
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Sarcomere Organization
The primary
microfilaments in myofibrils are actin (thin) and
myosin (thick) filaments. Within each sarcomere
there is a precise arrangement of these filaments. The
sarcomeres of neighboring myofibrils are in alignment
and they create the banding pattern visible with the
light microscope. |
The difference
in the size and density of the thick and thin filaments
is responsible for the banding pattern. The center of
the sarcomere has a narrow band called the H band
where there are only thick filaments. The H band is in
the center of the A band that includes a zone of
overlap between the thick and thin filaments. In the
zone of overlap, each thick filament is surrounded by
six thin filaments and each thin filament is
surrounded by three thick filaments. The I band is
where the thin filaments do not overlap with the thick
filaments at either end of the sarcomere, and straddling
the Z line. |
The thick
filaments are linked in the center of the sarcomere by
proteins that form the M line. At either end of
the sarcomere, the actin filaments are attached to one
another by proteins that form the Z line. The Z
lines mark the boundaries of each sarcomere. |
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Thin Filaments
The thin
filament is composed of 300-400 G-actin protein
molecules that form a linear molecule called F-actin.
The F-actin molecules are held together by another
protein called nebulin. Each G-actin molecule has
an active site that binds to a site on the myosin
molecule. This binding is prevented by another protein
called tropomyosin that covers the active site on
the actin. Tropomyosin is held in position by a protein
called troponin. When calcium ions bind to
troponin, the position of tropomyosin changes to uncover
the binding site and permits the binding of actin to
myosin. |
At either end of
the sarcomere the actin filaments are attached to the Z
line or disc by a protein
called actinin. |
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Thick Filaments
Thick filaments
are composed of a bundle of about 500 myosin molecules.
Each myosin molecule consists of two strands, of which
each has a tail that twists around the other, and a
head. The head projects outward toward the thin
filaments and are called cross-bridges because
they attach to the thin filaments during contraction. |
The thick
filaments are associated with a protein called titin.
Titin contributes to the normal alignment of the
thick and thin filaments and has elastic properties that
restores the sarcomere to its original resting position
after being stretched. |
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