keletal muscles allow us to move. They can
shorten (contract), thereby generating the
force for movement. Muscles can also relax
and return to their original length. The spe-
cific structure and arrangement of skeletal muscle al-
lows its muscle cells to work the way they do. Let’s take
a closer look.
Muscles Are Composed of Bundles
of Muscle Fibers
Muscles consist of bundles of muscle fibers or muscle cells
wrapped in connective tissue (Figure 6.2). (Note that we
use the terms
muscle fibers
and
mus-
cle cells
interchangeably; they mean
the same
thing.)
Each wrapped
bundle of fibers is called a fascicle.
The individual muscle cells are
fascicle
(FAS-i-kul) A
small bundle or clus-
ter of skeletal muscle
fibers.
b. Each muscle fiber (cell)
consists of a plasma
membrane surrounding
bundles of cylindrical
m yofibrils, which are
enveloped by sarcoplasmic
reticulum. The sarcolemma
(plasma membrane) regularly
invaginates at the transverse
tubule. Fibers contain
mitochondria and numerous
nuclei.
Sarcolemma
Myofibril
Sarcoplasm
Nucleus
rThick filament
Thin filament
Transverse
tubule
Terminal —
cisterns
Mitochondrion
Details of a muscle fiber
Sarcomere
Z disc
c.
Each myofibril consists of overlapping patterns of
thick and thin filam ents subdivided into regular
compartments called sarcomeres. Sarcomeres have
alternating dark bands (A-bands) and light bands
(I-bands). At the center of each A-band is a lighter
H-zone. In the middle of each I-band is a Z-disc or
Z-line. A sarcomere stretches from Z-disc to Z-disc.
Z disc
Z disc
Sarcomere
. Z disc
H zone —
]
A thick filament and a myosin molecule
<— I band — ][----------------------A band---------------------- ]f— I band — ►
Actin
Troponin
Tropomyosin
Myosin-binding site (covered by tropomyosin)
Portion of a thin filament
Details of filaments and Z discs
d. Thick filaments are made out of golf club-shaped proteins called myosin
that are twisted together. Thin filaments are made of twisted strands of
bead-like proteins called
actin
and are anchored to the Z-discs. The
grooves of the actin strands are covered with a protein called tropomyosin.
Each tropomyosin molecule has two troponin proteins on one end.
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