Download Skeletal muscles are composed of striated subunits called

Skeletal muscles are composed of striated subunits called sarcomeres,
which are composed of the myofilaments actin and myosin.
LEARNING OBJECTIVE [ edit ]
Illustrate the structure of a skeletal muscle fiber
KEY POINTS [ edit ]
Muscles are composed of long bundles of myocytes or muscle fibers.
Myofibrils, the basic rod­like subunit of muscle cells, are composed of long proteins that lie
parallel to the muscle fibers.
Sarcomeres are repeating subunits of thin and thick protein fibers within myofibrils, from one Z
line to the next Z line.
Myofilaments are composed of thick (myosin) and thin (actin) filaments, which interact to cause
muscle contractions.
TERMS [ edit ]
myofibril
cylindrical organelles, found within muscle cells, that are the contractile unit of muscles
sarcomere
the contractile unit of the myofibril of a striated muscle
myocyte
a single muscle fiber cell
Give us feedback on this content: FULL TEXT [ edit ]
Skeletal Muscle Fiber Structure
Skeletal muscle is a form of striated muscle tissue that is controlled voluntarily. It is one of
three major muscle types, along
with cardiac and smooth muscle. As their
name suggests, most skeletal muscles are
attached to bones by bundles
of collagen fibers known as tendons. The
term muscle refers to multiple bundles of
muscle fibers held together by connective
tissue.
Myocytes
Skeletal muscle tissue is made up of
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individual muscle cellscalled myocytes,
sometimes colloquially called "muscle fibers." They are formed from the fusion of
developmental myoblasts (a type of embryonic progenitor cell that gives rise to a muscle cell)
in a process known as myogenesis. Myocytes are long, cylindrical, multinucleated cells that
run parallel to each other within a muscle.
Myocyte: skeletal muscle cell
A skeletal muscle cell is surrounded by a plasma membrane called the sarcolemma with a cytoplasm
called the sarcoplasm. A muscle fiber is composed of many fibrils, packaged into orderly units.
These cells are incredibly large, with diameters of up to 100 µm and lengths of up to 30 cm.
The plasma membrane of a skeletal muscle fiber, the sarcolemma, is the site of action
potential conduction, which triggers muscle contraction.
Myofibrils
Within each muscle fiber or myoctye are myofibrils: long cylindrical structures that lie
parallel to the muscle fiber. Myofibrils run the entire length of the muscle fiber. Because they
are only approximately 1.2 µm in diameter, hundreds to thousands can be found inside one
muscle fiber. They attach to the sarcolemma at their ends, so that as myofibrils shorten, the
entire muscle cell contracts.
Sarcomeres
Myofibrils are composed of long proteins such as actin,myosin, and titin, while other
proteins hold them together. These proteins are organized into thin filaments and thick
filaments, which repeat along the length of the myofibril in sections called sarcomeres.
Muscles contract by sliding the thin (actin) and thick (myosin) filaments along each other.
Dark "A" bands and light "I" bands repeat along myofibrils. The alignment of myofibrils in
the cell causes the entire cell to appear striated or banded.
Each I band has a dense line running vertically through the middle called a Z disc or Z line.
The Z discs mark the border of units called sarcomeres, which are the functional units of
skeletal muscle. One sarcomere, the space between two consecutive Z discs, contains one
entire A band and two halves of an I band, one on either side of the A band. A myofibril is
composed of many sarcomeres running along its length. As the sarcomeres individually
contract, the myofibrils and muscle cells shorten.
Sarcomere
A sarcomere is the region from one Z line to the next Z line. Many sarcomeres are present in a myofibril,
resulting in the striation pattern characteristic of skeletal muscle.
Myofilaments (Thick and Thin Filaments)
Myofibrils are composed of smaller structures called myofilaments. There are two main types
of filaments: thick filaments and thin filaments. Thick filaments occur only in the A band of a
myofibril and are composed of the protein myosin. The tail of a myosin molecule connects
with other myosin molecules to form the central region of a thick filament near the M line,
whereas the heads align on either side of the thick filament where the thin filaments overlap.
I
Band
H
Zone
I
Band
CapZ
Titin
Z­disk
Myosin
head
Relaxed
Myosin
tail
Contracted
Actin
filament
M­line
Thin and thick filaments of a sarcomere
The thin (actin) filaments slide past the thick (myosin) filaments to produce a muscle contraction. Thick
filaments are found only on the A band, while thin filaments are found on the I band and part of the A
band.
Thin filaments are composed of actin, tropomyosin, andtroponin. Thin flilaments, attached
to a protein in the Z disc called alpha­actinin, occur across the entire length of the I band and
part way into the A band. Thin filaments do not extend all the way into the A bands, leaving a
central region of the A band that contains only thick filaments, called the H zone. The middle
of the H zone has a vertical line called the M line, at which accessory proteins hold together
thick filaments. Both the Z disc and the M line hold myofilaments in place to maintain the
structural arrangement and layeringof the myofibril. Myofibrils are connected to each other
by intermediate, or desmin, filaments that attach to the Z disc.