Page 316 - Atlas of Histology with Functional Correlations

SECTION 3 Smooth Muscle

Smooth muscles have a wide distribution in the body and predominantly line the

visceral hollow organs and blood vessels. In digestive tract organs, the uterus,
ureters, and other hollow organs, smooth muscles occur in large sheets or layers.
In the dermis of the skin, smooth muscles are associated with hair follicles.
Zonula adherens bind the cells, and the numerous gap junctions provide
functional coupling between individual smooth muscle cells.

Under a light microscope, smooth muscle appears as elongated individual
fibers with fusiform shapes of slender fascicle bundles. Individual muscle fibers
are also shorter than the skeletal muscles and exhibit a single central nucleus.
Connective tissue surrounds individual muscle fibers as well as muscle layers. In

the blood vessels, smooth muscle fibers are arranged in a circular pattern, where
they control blood pressure by altering luminal diameters. In intestines, smooth
muscles are arranged in concentric layers around the organs.

Individual smooth muscle fibers contain contractile actin and myosin
filaments; however, they are not arranged in the regular, cross-striated patterns

that are visible in both the skeletal and cardiac muscle fibers (Fig. 8.14). Instead,
actin and myosin course obliquely throughout the cell in the form of a lattice
network that crisscrosses the sarcoplasm. As a result of the irregular distribution

of contractile elements, these muscle fibers appear smooth, or nonstriated. The
actin filaments attach to dense bodies, structures that are unique to smooth
muscles. The dense bodies are either scattered throughout the cytoplasm or
attached to the cytoplasmic side of the cell membrane. The intermediate and
actin filaments attach to the dense bodies in the cytoplasm and the dense bodies

in the cell membrane. The dense bodies also contain α-actinin and other
accessory Z-disc proteins such as desmin that are similar to the Z discs of the
skeletal and cardiac muscles. Another characteristic feature of smooth muscle

fibers is the presence of numerous vesicular invaginations of the cell membrane
that look like the endocytotic or pinocytotic vesicles in other cells. These are the
caveolae and are believed to function like the T tubules of skeletal muscles that
transmit the stimulatory signals to the interior of the muscle fibers for
contraction.

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