the muscle (this is depicted in Figure 9–5). Thus, the ability
of a muscle ﬁ ber to generate force and movement depends on
the interaction of the contractile proteins actin and myosin.
An actin molecule is a globular protein composed of a
single polypeptide that polymerizes with other actins to form
two intertwined, helical chains (
). These chains
make up the core of a thin ﬁ lament. Each actin molecule
contains a binding site for myosin. The myosin molecule, on
the other hand, is composed of two large polypeptide
and four smaller
combine to form a molecule that consists of two globular heads
(containing heavy and light chains) and a long tail formed by
the two intertwined heavy chains (see Figure 9–7). The tail of
each myosin molecule lies along the axis of the thick ﬁ lament,
and the two globular heads extend out to the sides, forming
the cross-bridges. Each globular head contains two binding
sites, one for actin and one for ATP. The ATP binding site also
serves as an enzyme—an ATPase that hydrolyzes the bound
ATP, harnessing its energy for contraction.
The myosin molecules in the two ends of each thick
lament are oriented in opposite directions, so that their tail
ends are directed toward the center of the ﬁ lament. Because of
this arrangement, the power strokes of the cross-bridges move
the attached thin ﬁ laments at the two ends of the sarcomere
toward the center during shortening (see Figure 9–6).
The sequence of events that occurs between the time a
cross-bridge binds to a thin ﬁ lament, moves, and then is set
to repeat the process is known as a
cycle consists of four steps: (1) attachment of the cross-bridge
to a thin ﬁ lament, (2) movement of the cross-bridge, producing
tension in the thin ﬁ lament, (3) detachment of the cross-bridge
from the thin ﬁ lament, and (4) energizing the cross-bridge so
it can again attach to a thin ﬁ lament and repeat the cycle. Each
cross-bridge undergoes its own cycle of movement indepen-
dently of the other cross-bridges. At any instant during con-
traction, only a portion of the cross-bridges are attached to
the thin ﬁ laments, producing tension, while others are in a
detached portion of their cycle.
illustrates the chemical and physical events
during the four steps of the cross-bridge cycle. In a resting
muscle ﬁ ber, the cytoplasmic calcium concentration is low, and
the myosin cross-bridges (M) cannot bind to actin (A). The
cross-bridges, however, are in an energized state produced by
splitting ATP, and the hydrolysis products (ADP and inor-
ganic phosphate) are still bound to myosin. This energy stor-
age in myosin is analogous to the storage of potential energy
in a stretched spring.
Cross-bridge cycling is initiated when calcium enters the
cytoplasm (by a mechanism that will be described shortly).
The cycle begins with the binding of an energized myosin
cross-bridge to a thin ﬁ lament actin molecule (step 1):
A + M · ADP · P
A · M · ADP · P
Cross-bridges in the thick ﬁ laments bind to actin in the thin
ﬁ laments and undergo a conformational change that propels the
thin ﬁ laments toward the center of a sarcomere. (Only a few of the
approximately 200 cross-bridges in each thick ﬁ lament are shown.)
ATP binding sites
Actin binding sites
(a) The heavy chains of myosin molecules form the core of a thick ﬁ lament. The myosin molecules are oriented in opposite directions in either
half of a thick ﬁ lament. (b) Structure of thin ﬁ lament and myosin molecule. Cross-bridge binding sites on actin are covered by tropomyosin.
The two globular heads of each myosin molecule extend from the sides of a thick ﬁ lament, forming a cross-bridge.