287
CHAPTER NINE
Muscular System
f brils may be thought oF as sarcomeres joined end to end.
(f g. 9.4
a
). Muscle f bers, and in a way muscles themselves,
are basically collections oF sarcomeres, discussed later in this
chapter as the Functional units oF muscle contraction.
The striation pattern oF skeletal muscle has two main
parts. The First, the
I bands
(the light bands), are com-
posed oF thin actin f
laments held by direct attachments to
structures called
Z lines,
which appear in the center oF the
I bands. The second part oF the striation pattern consists oF
the
A bands
(the dark bands), composed oF thick myosin f l-
aments overlapping thin actin f
laments (f
g. 9.4
b
).
The A band consists not only oF a region where thick
and thin f laments overlap, but also a slightly lighter central
region (
H zone
) consisting only oF thick f laments. The A band
includes a thickening known as the
M line,
which consists oF
proteins that help hold the thick f laments in place (f g. 9.4
b
).
The myosin f laments are also held in place by the Z lines but
are attached to them by a large protein called
titin
(connectin)
(f g. 9.5)
. A sarcomere extends From one Z line to the next.
Thick Filaments are composed oF many molecules oF
myosin. Each myosin molecule consists oF two twisted pro-
tein strands with globular parts called
cross-bridges
(heads)
that project outward along their lengths. Thin f
laments con-
sist oF double strands oF actin twisted into a helix. Actin mol-
ecules are globular, and each has a binding site to which the
cross-bridges oF a myosin molecule can attach
(f g. 9.6)
.
Two other types oF protein,
troponin
and
tropomyo-
sin,
associate with actin Filaments. Troponin molecules
have three protein subunits and are attached to actin.
Tropomyosin molecules are rod-shaped and occupy the lon-
gitudinal grooves oF the actin helix. Each tropomyosin is
held in place by a troponin molecule, Forming a troponin-
tropomyosin complex (see f
g. 9.6).
The Fascia associated with each individual organ oF the
muscular system is part oF a complex network oF Fasciae that
extends throughout the body. The portion oF the network
that surrounds
the muscles is called
deep fascia.
It is contin-
uous with the
subcutaneous fascia
that lies just beneath the
skin, Forming the subcutaneous layer described in chapter 6
(p. 172). The network is also continuous with the
subserous
fascia
that Forms the connective tissue layer oF the serous
membranes covering organs in various body cavities and lin-
ing those cavities (see chapter 5, p. 163).
Skeletal Muscle Fibers
Recall From chapter 5 (p. 164) that a skeletal muscle Fiber
is a single muscle cell (see f g. 5.28). Each f ber Forms From
many undiFFerentiated cells that Fuse during development.
The resulting multinucleated muscle f ber is a thin, elongated
cylinder with rounded ends that attach to the connective tis-
sues associated with a muscle. Just beneath the muscle cell
membrane (
sarcolemma
), the cytoplasm (
sarcoplasm
) oF the
f ber contains many small, oval nuclei and mitochondria. The
sarcoplasm also has abundant, parallel, threadlike structures
called
myof
brils
(mi
o-f
-brilz)
(f g. 9.4
a
)
.
The myoFibrils play a Fundamental role in the muscle
contraction mechanism. They consist oF two types oF protein
f laments: thick f laments composed oF the protein
myosin
(mi
o-sin), and thin f laments composed primarily oF the pro-
tein
actin
(ak
tin). (Two other thin f lament proteins, troponin
and tropomyosin, will be discussed later.) The organization oF
these f laments produces the alternating light and dark stria-
tions characteristic oF skeletal muscle (and cardiac muscle)
f bers. The striations Form a repeating pattern oF units called
sarcomeres
(sar
ko-me
¯rz) along each muscle f ber. The myo-
Fascicle
Perimysium
Endomysium
Muscle fiber
Nucleus
Myofibrils
FIGURE 9.3
Scanning electron micrograph of a fascicle (fasciculus) surrounded by its connective tissue sheath, the perimysium. Muscle F
bers
within the fascicle are surrounded by endomysium (320×).
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