Chapter 9
Muscles and Muscle Tissue
279
9
of an average body cell—and their length is phenomenal, some
up to 30 cm long. Teir large size and multiple nuclei are not
surprising once you learn that hundreds of embryonic cells fuse
to produce each fiber.
Sarcoplasm
, the cytoplasm of a muscle cell, is similar to the
cytoplasm of other cells, but it contains unusually large amounts
of
glycosomes
(granules of stored glycogen that provide glucose
during muscle cell activity) and
myoglobin
, a red pigment that
stores oxygen. Myoglobin is similar to hemoglobin, the pigment
that transports oxygen in blood.
In addition to the usual organelles, a muscle cell contains
three structures that are highly modified: myofibrils, sarcoplas-
mic reticulum, and ± tubules. Let’s look at these structures more
closely because they play important roles in muscle contraction.
Myofibrils
A single muscle fiber contains hundreds to thousands of rod-
like
myofibrils
that run parallel to its length (Figure 9.2b). Te
myofibrils, each 1–2 μm in diameter, are so densely packed in
the fiber that mitochondria and other organelles appear to be
squeezed between them. Tey account for about 80% of cellular
volume.
In
indirect attachments
, the muscle’s connective tissue
wrappings extend beyond the muscle either as a ropelike
tendon
(Figure 9.1a) or as a sheetlike
aponeurosis
(ap
0
o-
nu-ro
9
sis). Te tendon or aponeurosis anchors the muscle to
the connective tissue covering of a skeletal element (bone or
cartilage) or to the fascia of other muscles.
Indirect attachments are much more common because of
their durability and small size. ±endons are mostly tough colla-
gen fibers which can withstand the abrasion of rough bony pro-
jections that would tear apart the more delicate muscle tissues.
Because of their relatively small size, more tendons than fleshy
muscles can pass over a joint—so tendons also conserve space.
Before moving on to microscopic anatomy, you may want to
review the top three rows of ±able 9.1.
Microscopic Anatomy of a Skeletal
Muscle Fiber
Each skeletal muscle fiber is a long cylindrical cell with multiple
oval nuclei just beneath its
sarcolemma
or plasma membrane
(Figure 9.2b)
. Skeletal muscle fibers are huge cells. Teir diam-
eter typically ranges from 10 to 100 μm—up to ten times that
Bone
Perimysium
Endomysium
(between individual muscle fibers)
Muscle fiber
Perimysium wrapping a fascicle
Epimysium
Tendon
Epimysium
Muscle fiber
in middle of
a fascicle
Blood vessel
Perimysium
Endomysium
(a)
Fascicle
(b)
Figure 9.1
Connective tissue sheaths of skeletal muscle: epimysium, perimysium,
and endomysium.
(b)
Photomicrograph of a cross section of part of a skeletal muscle (30
3
).
(For a related image, see
A Brief Atlas of the Human Body
, Plate 29.)
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