A myofibril is a basic rod-like unit of a muscle cell. Muscles are composed of
tubular cells called myocytes, known as muscle fibers in striated muscle, and
these cells in turn contain many chains of myofibrils. They are created during
embryonic development in a process known as myogenesis.
Myofibrils are composed of long proteins including actin, myosin, and titin, and
other proteins that hold them together. These proteins are organized into thick
and thin filaments called myofilaments, which repeat along the length of the
myofibril in sections called sarcomeres. Muscles contract by sliding the thick
and thin filaments along each other. Structure
The filaments of myofibrils, myofilaments, consist of two types,
thick and thin: Thin filaments consist primarily of the
protein actin, coiled with nebulin filaments. Actin, when polymerized into
filaments, forms the "ladder" along which the myosin filaments "climb" to
generate motion Thick filaments consist primarily of the
protein myosin, held in place by titin filaments. Myosin is responsible for
force generation. It is composed of a globular head with both ATP and actin
binding sites, and a long tail involved in its polymerization into myosin
filaments. The protein complex composed of actin
and myosin is sometimes referred to as "actinomyosin."
In striated muscle, such as skeletal and cardiac muscle, the actin and myosin
filaments each have a specific and constant length on the order of a few
micrometers, far less than the length of the elongated muscle cell. The filaments
are organized into repeated subunits along the length of the myofibril. These
subunits are called sarcomeres. The muscle cell is nearly filled with
myofibrils running parallel to each other on the long axis of the cell. The
sarcomeric subunits of one myofibril are in nearly perfect alignment with those
of the myofibrils next to it. This alignment gives rise to certain optical
properties which cause the cell to appear striped or striated. In smooth
muscle cells, this alignment is absent, hence there are no apparent striations
and the cells are called smooth. Exposed muscle cells at certain angles, such as
in meat cuts, can show structural coloration or iridescence due to this
periodic alignment of the fibrils and sarcomeres.
Formation A study of the developing leg muscle in
a 12-day chick embryo using electron microscopy proposes a mechanism for the
development of myofibrils. Developing muscle cells contain thick filaments
that are 160–170 Å in diameter and thinfilaments that are 60–70 Å in
diameter. Young myofibers contain a 7:1 ratio of thin to thick filaments. Along
the long axis of the muscle cells in subsarcolemmal locations, free
myofilaments become aligned and aggregate into hexagonally packed
arrays. These aggregates form regardless of the presence of Z band or M band
material. Aggregation occurs spontaneously because the tertiary
structures of actin and myosin monomers contain all the "information" with the
ionic strength and ATP concentration of the cell to aggregate into the
filaments. Appearance
The names of the various sub-regions of the sarcomere are based on their
relatively lighter or darker appearance when viewed through the light
microscope. Each sarcomere is delimited by two very dark colored bands called
Z-discs or Z-lines. These Z-discs are dense protein discs that do not easily
allow the passage of light. The T-tubule is present in this area. The area
between the Z-discs is further divided into two lighter colored bands at either
end called the I-bands, and a darker, grayish band in the middle called the A
band. The I bands appear lighter because these
regions of the sarcomere mainly contain the thin actin filaments, whose smaller
diameter allows the passage of light between them. The A band, on the other
hand, contains mostly myosin filaments whose larger diameter restricts the
passage of light. A stands for anisotropic and I for isotropic,
referring to the optical properties of living muscle as demonstrated with
polarized light microscopy. The parts of the A band that abut the I
bands are occupied by the both actin and myosin filaments. Also within the A band
is a relatively brighter central region called the H-zone in which there is no
actin/myosin overlap when the muscle is in a relaxed state. Finally, the H-zone
is bisected by a dark central line called the M-line.
Action The myosin heads form cross bridges with
the actin myofilaments; this is where they carry out a 'rowing' action along
the actin. When the muscle fibre is relaxed, the myosin head has ADP and
phosphate bound to it. When a nerve impulse arrives, Ca2+ ions
cause troponin to change shape; this moves the troponin + tropomyosin complex
away, leaving the myosin binding sites open.
The myosin head now binds to the actin myofilament. Energy in the head of the
myosin myofilament moves the head, which slides the actin past. ADP is thereby
released. ATP presents itself, and the myosin
heads disconnect from the actin to grab the ATP. The ATP is then broken down
into ADP and phosphate. Energy is released and stored in the myosin head
to utilize for later movement. The myosin heads now return to their upright
relaxed position. If calcium is present, the process is repeated.
When a muscle contracts, the actin is pulled along myosin toward the center of
the sarcomere until the actin and myosin filaments are completely overlapped. The
H zone becomes smaller and smaller due to the increasing overlap of actin and
myosin filaments, and the muscle shortens. Thus when the muscle is fully
contracted, the H zone is no longer visible. Note that the actin and myosin
filaments themselves do not change length, but instead slide past each
other. This is known as the sliding filament theory of muscle contraction.
See also Motor protein
References External links
Muscle Physiology Primer Animation of sarcomere contraction