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Each myofibril is composed of numerous sarcomeres, the functional contracile region of a striated muscle.Sarcomeres are composed of myofilaments of myosin and actin which interact using the sliding filament model and cross bridge cycle to contract.
Myocytes, sometimes colloquially termed “muscle fibers” form
the bulk of muscle tissue. They are bound together by perimysium, a sheath of
connective tissue, into bundles termed fascicles; which are in turn bundled
together to form muscle tissue. Myocytes contain numerous specialized cellular
structures which facilitate their contraction, and therefore that of the muscle as a whole.
The highly specialized structure of myocytes has led to the creation
of terminology which differentiates them from generic animal cells.
Myocytes can be incredibly large, with diameters of up to
100 µm and lengths of up to 30 cm. The sarcoplasm is rich with glycogen and myoglobin
which store the glucose and oxygen required for energy generation; and is
almost completely filled with myofibrils, the long fibers composed of
myofilaments that facilitate muscle contraction.
The sarcolemma of myocytes contains numerous invaginations
(pits) termed transverse tubules which are usually perpendicular to the length
of the myocyte. Transverse tubules play an important role in supplying the
myocyte with Ca+ ions which are key for muscle contraction.
Each myocyte contains multiple nuclei, due to their
derivation from multiple myoblasts (a progenitor cell that gives rise to
myocytes), which are located to the periphery of the myocyte and flattened so
as not to impact myocyte contraction.
Each myocyte can contain many thousands of myofibrils.
Myofibrils run parallel to the myocyte and typically run for its entire length
attaching to the sarcolemma at either end. Each myofibril is surrounded by the
sarcoplasmic reticulum which is closely associated with the transverse tubules.
The sarcoplasmic reticulum acts as a sink of Ca+ ions, which are
released upon signalling from the transverse tubules.
Myofibrils are composed of long myofilaments of actin and
myosin and other associated proteins. These proteins are organized into regions
termed sarcomeres which are the functional contractile region of the myocyte.
Within the sarcomere actin and myosin myofilaments are interlaced with each
other and via the sliding filament model of contraction slide over each other.
The regular organisation of these sarcomeres is what gives skeletal and cardiac
muscle their distinctive striated appearance.
Myofibrils are composed of smaller structures called
myofilaments. There are two main types of filaments: thick filaments and thin
filaments. Thick filaments occur are composed predominately of myosin proteins,
the tails of which bind together leaving the heads exposed to the interlaced
thin filaments. Thin filaments are composed predominately of actin, tropomyosin,
and troponin. The molecular model of contraction which describes the
interaction between actin and myosin myofilaments is termed the cross bridge