| Skeletal muscle is composed of single muscle cells called
fibers. Each fiber is composed of myofibrils.
MACROSTRUCTURE
Skeletal muscle shape and organization is maintained by three pieces
of connective tissue:
The Epimysium: The outermost layer of connective tissue,
the epimysium wraps around the entire muscle and is continuous with the
tendon.
The Perimysium: The middle layer of connective tissue,
the perimysium attaches one bundle of muscle fibers to another bundle of
muscle fibers. A bundle of muscle fibers is termed a fasciculus.
The Endomysium: The innermost layer of connective
tissue, the enomysium wraps each individual muscle fiber and allows them
to be bound into a fasciculus. |
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MICROSTRUCTURE
A myofibril can be divided into
two myofilaments: Actin and Myosin. Actin is a thin protein,
while Myosin is a thick protein. Actin and myosin make up two of
the four contractile proteins.
The myofilaments are suspended
in a gel-like substance
called the sarcoplasm. The sarcoplasm is the cytoplasm of a muscle
cell. Mitochondria are also embedded in the sarcoplasm, along with
a structure called the sarcoplasmic reticulum. The sarcoplasmic
reticulum is a specialized form of the endoplasmic reticulum, and stores
calcium ions. Hence, the highest concentration of calcium in the
cell is located within the sarcoplasmic
reticulum.
The sarcoplasm is wrapped by the
sarcolemma, the membrane of the muscle cell.
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STRIATIONS / SARCOMERE STRUCTURE
Actin and myosin are arranged in a fashion that
causes visible striations, or alternating regions of light and
dark. The smallest functional unit of muscular contraction, called
a sarcomere, is used to demonstrate the different bands and lines ,
which have been assigned names: the I band, A band, Z line, and H
band.
The I band stands for isotropic band, or
"light passing" band because a beam of light can be shined
through it. The light can pass through this area because it
contains only the thin filament, actin, and does not contain the thick
filament myosin. The I band is divided in half by the Z line, the
point at which actin is anchored. A sarcomere is defined as one Z
line to another Z line, meaning that half of the I band lies in one
sarcomere while the other half lies in another sarcomere. The I
band can be found at each end of a sarcomere.
The A band stands for anisotropic band, or
"no light passing" because the area is darker and light does
not pass as freely through it. The light does not pass as freely
because the A band contains both the thick filament, myosin, and
the thin filament, actin. The H zone is the zone that contains
only myosin filaments, and is located midway through the A band.
During skeletal muscle contraction the Z lines
become closer together and the I band shortens. The H zone also
shortens until it eventually disappears completely. The A band
stays the same length.
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