Muscles and Muscle Tissue
allow smooth muscles to transmit action potentials from ﬁber
Some smooth muscle ﬁbers in the stomach and small intes-
: 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 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
, but, as mentioned above, Ca
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
a cytoplasmic calcium-binding protein. Calmodulin, in turn,
interacts with a kinase enzyme called
light chain kinase
which phosphorylates the myosin, activating
As in skeletal muscle, smooth muscle relaxes when intracel-
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
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
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
acts as the calcium-
Thick and thin ﬁlaments arranged diagonally.
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.
cle ﬁbers contain a lattice-like arrangement of noncontrac-
that resist tension. Tey attach at
regular intervals to cytoplasmic structures called dense bod-
ies (Figure 9.27). Te
, 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
, 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
(a) Relaxed smooth muscle fiber (note that gap junctions connect
(b) Contracted smooth muscle fiber
Intermediate ﬁlaments and dense bodies of
smooth muscle ﬁbers harness the pull generated by myosin
Intermediate ﬁlaments attach to dense bodies
throughout the sarcoplasm.