Chapter 9
Muscles and Muscle Tissue
307
9
allow smooth muscles to transmit action potentials from fiber
to fiber.
Some smooth muscle fibers in the stomach and small intes-
tine are
pacemaker cells
: Once excited, they act as “drummers”
to set the pace of contraction for the entire muscle sheet. Tese
pacemakers have fluctuating membrane potentials and are
self-excitatory, that is, they depolarize spontaneously in the ab-
sence of external stimuli. However, neural and chemical stimuli
can modify both the rate and the intensity of smooth muscle
contraction.
Contraction in smooth muscle is like contraction in skeletal
muscle in the following ways:
Actin and myosin interact by the sliding filament mechanism.
Te final trigger for contraction is a rise in the intracellular
calcium ion level.
A±P energizes the sliding process.
During excitation-contraction coupling, the tubules of the
SR release Ca
2
1
, but, as mentioned above, Ca
2
1
also moves into
the cell from the extracellular space via membrane channels. In
all striated muscle types, calcium ions activate myosin by bind-
ing to troponin. In smooth muscle, calcium activates myosin
by interacting with a regulatory molecule called
calmodulin
,
a cytoplasmic calcium-binding protein. Calmodulin, in turn,
interacts with a kinase enzyme called
myosin kinase
or
myosin
light chain kinase
which phosphorylates the myosin, activating
it
(Figure 9.28)
.
As in skeletal muscle, smooth muscle relaxes when intracel-
lular Ca
2
1
levels drop—but getting smooth muscle to stop con-
tracting is more complex. Events known to be involved include
calcium detachment from calmodulin, active transport of Ca
2
1
into the SR and extracellular fluid, and dephosphorylation of
myosin by a phosphorylase enzyme, which reduces the activity
of the myosin A±Pases.
Tere are no striations in smooth muscle, as its name indi-
cates, and therefore no sarcomeres. Smooth muscle fibers do
contain interdigitating thick and thin filaments, but the myosin
filaments are a lot shorter than the actin filaments and the type
of myosin contained differs from skeletal muscle. Te propor-
tion and organization of smooth muscle myofilaments differ
from skeletal muscle in the following ways:
Thick filaments are fewer but have myosin heads along
their entire length.
Te ratio of thick to thin filaments is
much lower in smooth muscle than in skeletal muscle (1:13
compared to 1:2). However, thick filaments of smooth mus-
cle contain actin-gripping myosin heads along their
entire
length,
a feature that makes smooth muscle as powerful as a
skeletal muscle of the same size. Also, in smooth muscle the
myosin heads are oriented in one direction on one side of the
filament and in the opposite direction on the other side.
No troponin complex in thin filaments.
As in skeletal mus-
cle, tropomyosin mechanically stabilizes the thin filaments,
but smooth muscle has no calcium-binding troponin com-
plex. Instead, a protein called
calmodulin
acts as the calcium-
binding site.
Thick and thin filaments arranged diagonally.
Bundles of
contractile proteins crisscross within the smooth muscle cell
so they spiral down the long axis of the cell like the stripes
on a barber pole. Because of this diagonal arrangement, the
smooth muscle cells contract in a twisting way so that they
look like tiny corkscrews (Figure 9.27b).
Intermediate filament–dense body network.
Smooth mus-
cle fibers contain a lattice-like arrangement of noncontrac-
tile
intermediate filaments
that resist tension. Tey attach at
regular intervals to cytoplasmic structures called dense bod-
ies (Figure 9.27). Te
dense bodies
, which are also tethered
to the sarcolemma, act as anchoring points for thin filaments
and therefore correspond to Z discs of skeletal muscle.
Te intermediate filament–dense body network forms a
strong, cable-like intracellular cytoskeleton that harnesses
the pull generated by the sliding of the thick and thin fila-
ments. During contraction, areas of the sarcolemma between
the dense bodies bulge outward, making the cell look puffy
(Figure 9.27b). Dense bodies at the sarcolemma surface also
bind the muscle cell to the connective tissue fibers outside the
cell (endomysium) and to adjacent cells. Tis arrangement
transmits the pulling force to the surrounding connective tis-
sue and partly accounts for the synchronous contractions of
most smooth muscle.
Contraction of Smooth Muscle
Mechanism of Contraction
In most cases, adjacent smooth muscle fibers exhibit slow, syn-
chronized contractions, the whole sheet responding to a stimu-
lus in unison. Tis synchronization reflects electrical coupling
of smooth muscle cells by
gap junctions
, specialized cell con-
nections described in Chapter 3. Skeletal muscle fibers are elec-
trically isolated from one another, each stimulated to contract
by its own neuromuscular junction. By contrast, gap junctions
Intermediate
filament
Dense bodies
Nucleus
Caveolae
(a) Relaxed smooth muscle fiber (note that gap junctions connect
adjacent fibers)
(b) Contracted smooth muscle fiber
Dense bodies
Nucleus
Gap junctions
Figure 9.27
Intermediate filaments and dense bodies of
smooth muscle fibers harness the pull generated by myosin
cross bridges.
Intermediate filaments attach to dense bodies
throughout the sarcoplasm.
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