Page 297 - Atlas of Histology with Functional Correlations

SECTION 1 Skeletal Muscle

There are three types of muscle tissues in the body: skeletal muscle, cardiac

muscle, and smooth muscle. These muscles can be identified by their structure
and function, with each muscle type showing morphologic and functional
similarities as well as differences. All muscle tissues consist of elongated cells
called fibers. The cytoplasm of muscle cells is called sarcoplasm, and the
surrounding cell membrane or plasmalemma is called sarcolemma.

Skeletal muscle fibers are long, cylindrical, multinucleated cells, with
peripheral nuclei because of the fusion of numerous mesenchymal cells called
myoblasts during embryonic development. Each muscle fiber is composed of
smaller subunits called myofibrils that extend the entire length of the fiber. The

myofibrils, in turn, are composed of tiny myofilament units formed by the
contractile thin protein actin and the thick protein myosin.

In the sarcoplasm of each skeletal muscle, the arrangement of actin and
myosin filaments is very regular, forming the distinct cross-striation patterns
seen under a light microscope as lighter-staining I bands and dark-staining A

bands. Because of these cross-striations, skeletal muscle is also called striated
muscle. Transmission electron microscopy illustrates the internal organization of
the contractile proteins in each myofibril. These high-resolution images show

that each light I band is bisected by a dense transverse Z line (disc or band).
Between the two adjacent Z lines is found the smallest structural and functional
contractile unit of the muscle, the sarcomere. Sarcomeres are the repeating
contractile units seen along the entire length of each myofibril and are highly
characteristic features of the sarcoplasm of skeletal and cardiac muscle fibers.

The center and the dark-staining part of each sarcomere contains the thick
(myosin) filaments, which form the A band. The peripheries and the light-
staining portion of the sarcomere contain the light-staining, thin actin filaments.
Actin and myosin filaments are precisely aligned and stabilized within individual

myofibrils and sarcomeres by accessory proteins. The thin actin filaments are
bound to the protein α-actinin, which binds them to the dense Z line (band,
disc). The thick myosin filaments are anchored to the Z line by the very large
protein called titin that positions and centers the myosin filaments on the Z line.

Titin acts like a spring between the end of the myosin filament and the Z line.
Another large protein, nebulin, extends the length of the thin filaments actin,

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