| 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. |
|
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.
Deep invaginations in the
sarcolemma and sarcoplasm exist. These invaginations are called the
Transverse tubules. The transverse tubules terminate between two adjacent
sarcoplasmic reticulum. The Transverse tubules, sometimes called the T
tubules, allow the depolarization (resulting from an action potential) to
propagate down the sarcolemma and to pass within close proximity of the
sarcolasmic reticulum, thereby eliminating the need for the depolarization
to travel through the sarcoplasm. This is advantageous for muscular
contraction because the process of propagation through the sarcoplasm is
much slower than propagation down the sarcolemma. |
| 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. |
