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EBUNILO KRYSTAL CHIAMAKA
14/MHS02/021
NURSING SCIENCE
200 LEVEL
ANA 203 ASSIGNMENT
LINES AND BANDS OF MUSCLE TISSUE
A sarcomere is the basic unit of striated muscle tissue. Skeletal muscles are composed
of tubular muscle cells (myocytes called muscle fibers) which are formed in a process known as
myogenesis. Muscle fibers are composed of tubular myofibrils. Myofibrils are composed of
repeating sections of sarcomeres, which appear under the microscope as dark and light bands.
Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a
muscle contracts or relaxes. Two of the important proteins are myosin, which forms the thick
filament, and actin, which forms the thin filament. Myosin has a long, fibrous tail and a globular
head, which binds to actin. The myosin head also binds to ATP, which is the source of energy for
muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by
calcium ions. Actin molecules are bound to the Z line, which forms the borders of the sarcomere.
Other bands appear when the sarcomere is relaxed.]A muscle fiber from a biceps muscle may
contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into
sarcomeres.
Bands
Muscle contraction based on sliding filament hypothesis
The sarcomeres are what give skeletal and cardiac muscles their striated appearance.
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A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs, or
Z bodies). In electron micrographs of cross-striated muscle, the Z-line (from the German
"Zwischenscheibe", the disc in between the I bands) appears as a series of dark lines.
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Surrounding the Z-line is the region of the I-band (for isotropic). I-band is the zone of
thin filaments that is not superimposed by thick filaments.
Following the I-band is the A-band (for anisotropic). Named for their properties under a
polarizing microscope. An A-band contains the entire length of a single thick filament.
Within the A-band is a paler region called the H-zone (from the German "heller",
brighter). Named for their lighter appearance under a polarization microscope. H-band is
the zone of the thick filaments that is not superimposed by the thin filaments.
Inside the H-zone is a thin M-line (from the German "Mittelscheibe", the disc in the
middle of the sarcomere) formed of cross-connecting elements of the cytoskeleton.
The relationship between the proteins and the regions of the sarcomere are as follows:
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Actin filaments, the thin filaments, are the major component of the I-band and extend into
the A-band.
Myosin filaments, the thick filaments, are bipolar and extend throughout the A-band.
They are cross-linked at the centre by the M-band.
The giant protein titin (connectin) extends from the Z-line of the sarcomere, where it
binds to the thick filament (myosin) system, to the M-band, where it is thought to interact
with the thick filaments. Titin (and its splice isoforms) is the biggest single highly
elasticated protein found in nature. It provides binding sites for numerous proteins and is
thought to play an important role as sarcomeric ruler and as blueprint for the assembly of
the sarcomere.
Another giant protein, nebulin, is hypothesised to extend along the thin filaments and the
entire I-Band. Similar to titin, it is thought to act as a molecular ruler along for thin
filament assembly.
Several proteins important for the stability of the sarcomeric structure are found in the Zline as well as in the M-band of the sarcomere.
Actin filaments and titin molecules are cross-linked in the Z-disc via the Z-line protein
alpha-actinin.
The M-band proteins myomesin as well as C-protein crosslink the thick filament system
(myosins) and the M-band part of titin (the elastic filaments).
The interaction between actin and myosin filaments in the A-band of the sarcomere is
responsible for the muscle contraction (sliding filament model).
Contraction
Upon muscle contraction, the A-bands do not change their length (1.85 micrometer in
mammalian skeletal muscle), whereas the I-bands and the H-zone shorten. This causes the Z
lines to come closer together.
The protein tropomyosin covers the myosin binding sites of the actin molecules in the muscle
cell. To allow the muscle cell to contract, tropomyosin must be moved to uncover the binding
sites on the actin. Calcium ions bind with troponin-C molecules (which are dispersed throughout
the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the crossbridge binding site on the actin.
The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a
unique form of endoplasmic reticulum in the sarcoplasm. Muscle contraction ends when calcium
ions are pumped back into the sarcoplasmic reticulum, allowing the contractile apparatus and,
thus, muscle cell to relax.
During stimulation of the muscle cell, the motor neuron releases the neurotransmitter
acetylcholine, which travels across the neuromuscular junction (the synapse between the terminal
bouton of the neuron and the muscle cell). Acetylcholine binds to a post-synaptic nicotinic
acetylcholine receptor. A change in the receptor conformation allows an influx of sodium ions
and initiation of a post-synaptic action potential. The action potential then travels along T
(transverse) tubules until it reaches the sarcoplasmic reticulum. Here, the depolarized membrane
activates voltage-gated L-type calcium channels, present in the plasma membrane. The L-type
calcium channels are in close association with ryanodine receptors present on the sarcoplasmic
reticulum. The inward flow of calcium from the L-type calcium channels activate ryanodine
receptors to release calcium ions from the sarcoplasmic reticulum. This mechanism is called
calcium-induced calcium release (CICR). It is not understood whether the physical opening of
the L-type calcium channels or the presence of calcium causes the ryanodine receptors to open.
The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites,
permitting muscle contraction.
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Rest
At rest, the myosin head is bound to an ATP molecule in a low-energy configuration and is
unable to access the cross-bridge binding sites on the actin. However, the myosin head can
hydrolyze ATP into adenosine diphosphate (ADP) and an inorganic phosphate ion. A portion of
the energy released in this reaction changes the shape of the myosin head and promotes it to a
high-energy configuration. Through the process of binding to the actin, the myosin head releases
ADP and an inorganic phosphate ion, changing its configuration back to one of low energy. The
myosin remains attached to actin in a state known as rigor, until a new ATP binds the myosin
head. This binding of ATP to myosin releases the actin by cross-bridge dissociation. The ATPassociated myosin is ready for another cycle, beginning with hydrolysis of the ATP.
The A-band is visible as dark transverse lines across myofibers; the I-band is visible as lightly
staining transverse lines, and the Z-line is visible as dark lines separating sarcomeres at the lightmicroscope level.