Muscles and Muscle Tissue
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
depends on the myosin- and actin-containing myoﬁlaments.
As noted earlier, thick ﬁlaments (about 16 nm in diameter) are
composed primarily of the protein
. 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
. 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
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
Te thin ﬁlaments (7–8 nm thick) are composed chieﬂy
of the protein
(blue in Figure 9.3). Actin has kidney-
shaped polypeptide subunits, called
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
, or F,
. ±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
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.
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.
we referred to earlier is composed of
the giant protein
(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-
. ±able 9.1 (bottom three rows; p. 283)
summarizes these structures, which we discuss next.
Striations, Sarcomeres, and Myoﬁlaments
, a re-
peating series of dark and light bands, are evident along the
length of each myoﬁbril. In an intact muscle ﬁber, the dark
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
Each H zone is bisected vertically by a dark line called the
for middle) formed by molecules of the protein
Each light I band also has a midline interruption, a darker
area called the
(or Z line).
Te region of a myoﬁbril between two successive Z discs is
ko-mĕr; literally, “muscle segment”). Aver-
aging 2 μm long, a sarcomere is the smallest contractile unit
of a muscle ﬁber—the
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
, 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.
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
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
, 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