Essential Cell Biology 5th edition

myofibrils
602 CHAPTER 17 Cytoskeleton
(A)
Z disc
overlap
region
sarcomere ~2.5 µm
Figure 17–41 Sarcomeres are the contractile units of muscle.
(A) Detail of the electron micrograph from Figure 17–40 showing two
myofibrils; the length of one sarcomere and the region where the actin
and myosin filaments overlap are indicated. (B) Schematic diagram
of a single sarcomere showing the origin of the light and dark bands
seen in the microscope. Z discs at either end of the sarcomere are
attachment points for the plus ends of actin filaments. The centrally
located thick filaments (green) are each composed of many myosin-II
molecules. The thin vertical line running down the center of the thick
filament bundle in (A) corresponds to the bare regions of the myosin
filaments, as seen in Figure 17−38B. (A, courtesy of Roger Craig.)
(B)
thin filament
(actin)
Z disc
thick filament
(myosin-II)
Z disc
of a muscle-specific form of myosin-II. The myosin filaments (the thick
filaments) are centrally positioned in each sarcomere, whereas the more
slender actin filaments (the thin filaments) extend inward from each end
of the sarcomere, where they are anchored by their plus ends to a structure
known as the Z disc. The minus ends of the actin filaments overlap
the ends of the myosin filaments (Figure 17–41).
ECB5 E17.41/17.41
The contraction of a muscle cell is caused by a simultaneous shortening
of all the cell’s sarcomeres, which is caused by the actin filaments
sliding past the myosin filaments, with no change in the length of either
type of filament (Figure 17–42). The sliding motion is generated by myosin
heads that project from the sides of the myosin filament and interact
with adjacent actin filaments (see Figure 17–39). When a muscle is stimulated
to contract, the myosin heads start to walk along the actin filament
in repeated cycles of attachment and detachment. During each cycle, a
myosin head binds and hydrolyzes one molecule of ATP. This causes a
series of conformational changes that move the tip of the head by about
5 nm along the actin filament toward the plus end. This movement,
repeated with each round of ATP hydrolysis, propels the myosin molecule
unidirectionally along the actin filament (Figure 17–43). In so doing, the
myosin heads pull against the actin filament, causing it to slide against
sarcomere
myosin filament
actin filament
(A)
plus end
Z disc
minus end
CONTRACTION
RELAXATION
minus end
plus end
Z disc
(B)
Figure 17–42 Muscles contract by a sliding-filament mechanism. (A) The myosin and actin filaments of a sarcomere overlap with
the same relative polarity on either side of the midline. Recall that actin filaments are anchored by their plus ends to the Z disc and
that myosin filaments are bipolar. (B) During contraction, the actin and myosin filaments slide past each other. Although the filaments
themselves remain the same length, the sarcomere to which they belong shortens. The sliding motion is driven by the myosin heads
walking toward the plus ends of the adjacent actin filaments (Movie 17.11).