Page 304 - Atlas of Histology with Functional Correlations

A higher magnification with the transmission electron micrograph illustrates the

sarcomeres in a contracted skeletal muscle. During muscle contraction, the
sarcomere shortens, the Z lines (2, 6) are drawn closer together, and the thick
and thin filaments slide past each other. This action narrows the I bands (7) and
H bands (8), whereas the A band (1) remains unchanged. In the middle of the
sarcomere is the dense-staining M band (4). The tubules or the cisternae of the

sarcoplasmic reticulum surround each sarcomere of every myofibril (see Fig.
8.5). At the A band (1) and I band (7) junction (A–I junctions), the sarcoplasmic
reticulum tubules expand into terminal cisternae. To allow synchronous

stimulation and contraction of all sarcomeres, tiny tubular invaginations of the
sarcolemma, called the T tubules (3), penetrate every myofibril, and are located
at the A–I junctions (1, 7). Here, one T tubule (3) is surrounded on each side by
the expanded terminal cisternae of the sarcoplasmic reticulum and forms a triad
(5). In mammalian skeletal muscles, the triads (5) are located at the A–I

junctions. The stimulus for muscle contraction, delivered via a nerve, is then
disseminated to each sarcomere of each myofibril through the T tubules (3) in
the triads (5).

FIGURE 8.6 ■ Ultrastructure of sarcomeres, T tubules, and triads in skeletal

muscle. Courtesy of Carter Rowley, Ft. Collins, CO. ×50,000.

FIGURE 8.7 | Skeletal Muscles, Nerves, and

Motor Endplates

A group of skeletal muscle fibers (6, 7) have been teased apart and stained to
illustrate nerve terminations or myoneural junctions on individual muscle fibers.
The characteristic cross-striations (2, 8) are visible in each muscle fiber (6, 7).

The dark-stained, stringlike structures between the separated muscle fibers (6, 7)

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