Essential Cell Biology 5th edition

Muscle Contraction
601
plus end
minus end
minus end
myosin-ll
plus end
The myosin filament is like a double-headed arrow, with the two sets of
myosin heads pointing outward, away from the middle. One set binds to
actin filaments in one orientation and moves the filaments one way; the
other set binds to other actin filaments in the opposite orientation and
moves the filaments in the opposite direction. As a result, a myosin filament
slides sets of oppositely oriented actin filaments past one another
ECB5 E17.39/17.39
(Figure 17–39). Thus, if actin filaments and myosin filaments are organized
together in a bundle, the bundle can generate a strong contractile
force. This is seen most clearly in muscle contraction, but it also occurs in
the much smaller contractile bundles of actin filaments and myosin-II filaments
(see Figure 17−29B) that assemble transiently in nonmuscle cells,
and in the contractile ring that pinches a dividing cell in two by contracting
and pulling inward on the plasma membrane (see Figure 17–29D).
Actin Filaments Slide Against Myosin Filaments During
Muscle Contraction
In most animals, skeletal muscle fibers are huge, multinucleated individual
cells formed by the fusion of many separate smaller cells. The
nuclei of the contributing cells are retained in the muscle fiber and lie just
beneath the plasma membrane. The bulk of the cytoplasm is made up of
myofibrils, the contractile elements of the muscle cell. These cylindrical
structures are 1–2 μm in diameter and may be as long as the muscle cell
itself (Figure 17–40A).
A myofibril consists of a chain of identical tiny contractile units, or
sarcomeres. Each sarcomere is about 2.5 μm long, and the repeating
pattern of sarcomeres gives the vertebrate myofibril a striped appearance
(Figure 17–40B). Sarcomeres are highly organized assemblies of
two types of filaments—actin filaments and myosin filaments composed
Figure 17–39 A small, bipolar myosin-II
filament can slide two actin filaments
of opposite orientation past each
other. Similar sliding movement mediates
the contraction of interacting actin and
myosin-II filaments in both muscle and
nonmuscle cells. As with myosin-I, a
myosin-II head group walks toward the
plus end of the actin filament with which
it interacts. Note that multiple myosin
molecules are required to generate
movement: when one myosin head releases
the filament to reposition itself, other
myosins must remain attached so the
structure does not fall apart.
QUESTION 17–8
If both the actin and myosin
filaments of muscle are made
up of subunits held together by
weak noncovalent bonds, how is it
possible for a human being to lift
heavy objects?
(A)
(B)
nucleus plasma membrane myofibril
Figure 17–40 A skeletal muscle cell is packed with myofibrils.
(A) In an adult human, these huge, multinucleated cells (also
called muscle fibers) are typically 50 μm in diameter, and they can
be several centimeters long. They contain numerous myofibrils,
in which actin filaments and myosin-II filaments are arranged in
a highly organized structure, giving each myofibril—and skeletal
muscle cell—a striated or striped appearance; for this reason,
skeletal muscle is also called striated muscle. (B) Low-magnification
electron micrograph of a longitudinal section through a skeletal
muscle cell of a rabbit, showing that each myofibril consists of
a repeating chain of sarcomeres, the contractile units of the
myofibrils. (B, courtesy of Roger Craig.)
sarcomere ~2.5 µm
sarcomere
two myofibrils