Chapter 9
Muscles and Muscle Tissue
281
9
area where thick and thin filaments overlap. Notice that a hex-
agonal arrangement of six thin filaments surrounds each thick
filament, and three thick filaments enclose each thin filament.
Molecular Composition of Myofilaments
Muscle contraction
depends on the myosin- and actin-containing myofilaments.
As noted earlier, thick filaments (about 16 nm in diameter) are
composed primarily of the protein
myosin
. Each myosin mol-
ecule consists of two heavy and four light polypeptide chains,
and has a rodlike tail attached by a flexible hinge to two globular
heads
(Figure 9.3)
. Te tail consists of two intertwined helical
polypeptide heavy chains.
Te globular heads, each associated with two light chains,
are the “business end” of myosin. During contraction, they link
the thick and thin filaments together, forming
cross bridges
(Figure 9.4
on p. 284), and swivel around their point of at-
tachment. As we will explain shortly, these cross bridges act as
motors to generate force.
Each thick filament contains about 300 myosin molecules
bundled together, with their tails forming the central part of
the thick filament and their heads facing outward at the end
of each thick filament (Figure 9.3). As a result, the central
portion of a thick filament (in the H zone) is smooth, but its
ends are studded with a staggered array of myosin heads. Te
heads bear actin and A±P-binding sites and also have intrinsic
A±Pase activity that splits A±P to generate energy for muscle
contraction.
Te thin filaments (7–8 nm thick) are composed chiefly
of the protein
actin
(blue in Figure 9.3). Actin has kidney-
shaped polypeptide subunits, called
globular actin
or
G actin
,
which bear the active sites to which the myosin heads attach
during contraction. In the thin filaments, G actin subunits are
polymerized into long actin filaments called
filamentous
, or F,
actin
. ±wo intertwined actin filaments, resembling a twisted
double strand of pearls, form the backbone of each thin fila-
ment (Figure 9.3).
Tin filaments also contain several regulatory proteins.
Polypeptide strands of
tropomyosin
(tro
0
po-mi
9
o-sin),
a rod-shaped protein, spiral about the actin core and help
stiffen and stabilize it. Successive tropomyosin molecules are
arranged end to end along the actin filaments, and in a re-
laxed muscle fiber, they block myosin-binding sites on actin
so that myosin heads on the thick filaments cannot bind to
the thin filaments.
Troponin
(tro
9
po-nin), the other major protein in thin fila-
ments, is a globular three-polypeptide complex (Figure 9.3).
One of its polypeptides (±nI) is an inhibitory subunit that
binds to actin. Another (±n±) binds to tropomyosin and helps
position it on actin. Te third (±nC) binds calcium ions.
Both troponin and tropomyosin help control the myosin-actin
interactions involved in contraction. Several other proteins help
form the structure of the myofibril.
Te
elastic filament
we referred to earlier is composed of
the giant protein
titin
(Figure 9.2d). ±itin extends from the
Z disc to the thick filament, and then runs within the thick
filament (forming its core) to attach to the M line. It holds
Myofibrils contain the contractile elements of skeletal muscle
cells, the sarcomeres, which contain even smaller rodlike struc-
tures called
myofilaments
. ±able 9.1 (bottom three rows; p. 283)
summarizes these structures, which we discuss next.
Striations, Sarcomeres, and Myofilaments
Striations
, a re-
peating series of dark and light bands, are evident along the
length of each myofibril. In an intact muscle fiber, the dark
A
bands
and light
I bands
are nearly perfectly aligned, giving the
cell its striated appearance.
As illustrated in Figure 9.2c:
Each dark A band has a lighter region in its midsection called
the
H zone
(H for
helle
; “bright”).
Each H zone is bisected vertically by a dark line called the
M line
(
M
for middle) formed by molecules of the protein
myomesin.
Each light I band also has a midline interruption, a darker
area called the
Z disc
(or Z line).
Te region of a myofibril between two successive Z discs is
a
sarcomere
(sar
9
ko-mĕr; literally, “muscle segment”). Aver-
aging 2 μm long, a sarcomere is the smallest contractile unit
of a muscle fiber—the
functional unit
of skeletal muscle. It
contains an A band flanked by half an I band at each end.
Within each myofibril, the sarcomeres align end to end like
boxcars in a train.
If we examine the banding pattern of a myofibril at the mo-
lecular level, we see that it arises from orderly arrangement of
even smaller structures within the sarcomeres. Tese smaller
structures, the
myofilaments
or
filaments
, are the muscle
equivalents of the actin- or myosin-containing microfilaments
described in Chapter 3. As you will recall, the proteins actin
and myosin play a role in motility and shape change in virtually
every cell in the body. Tis property reaches its highest develop-
ment in the contractile muscle fibers.
Te central
thick filaments
containing myosin (red) extend
the entire length of the A band (Figure 9.2c and d). Tey are con-
nected in the middle of the sarcomere at the M line. Te more
lateral
thin filaments
containing actin (blue) extend across the
I band and partway into the A band. Te Z disc, a coin-shaped
sheet composed largely of the protein alpha-actinin, anchors
the thin filaments. We describe the third type of myofilament
illustrated in Figure 9.2d, the
elastic filament
, in the next section.
Intermediate (desmin) filaments (not illustrated) extend from
the Z disc and connect each myofibril to the next throughout
the width of the muscle cell.
Looking at the banding pattern more closely, we see that the
H zone of the A band appears less dense because the thin fila-
ments do not extend into this region. Te M line in the center of
the H zone is slightly darker because of the fine protein strands
there that hold adjacent thick filaments together. Te myofila-
ments are connected to the sarcolemma and held in alignment
at the Z discs and the M lines.
A longitudinal view of the myofilaments (Figure 9.2d) is a
bit misleading because it looks as if each thick (red) filament
interdigitates with only four thin (blue) filaments. Te cross sec-
tion of a sarcomere on the far right in Figure 9.2e shows an
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