Page 303 - Atlas of Histology with Functional Correlations

section of the skeletal muscle is shown. This transmission electron micrograph

illustrates the organization of the myofibrils and myofilaments in a partially
contracted skeletal muscle. Each myofibril consists of repeating units called
sarcomeres, the contractile elements in striated muscles. A sarcomere (5) is
located between two electron-dense Z lines. Located in each sarcomere (5) are
the light-staining thin actin and the dark-staining thick myosin myofilaments.

The thin actin filaments extend from the Z lines and form the light-staining I
bands. In the center of each sarcomere (5) is the dark-staining A band
composed mainly of the thick myosin filaments overlapping the thin actin

filaments. Each A band is bisected by a denser M band where the adjacent
myosin filaments are linked. On each side of the M band are smaller lighter H
bands (2, 3) that consist only of myosin filaments. Surrounding each sarcomere
are the tubules of the sarcoplasmic reticulum (4) and mitochondria (1). During
muscle contraction, the length of the thick and thin filaments remains

unchanged, whereas the size of each sarcomere (5) decreases (see Fig. 8.6).

FIGURE 8.5 ■ Ultrastructure of myofibrils in skeletal muscle. Courtesy of
Carter Rowley, Ft. Collins, CO. ×33,500.

FIGURE 8.6 | Ultrastructure of Sarcomeres,

Tubules, and Triads in Skeletal Muscle

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