110
UNIT 2
Basic Tissues
Smooth Muscle
Figure 6-10A.
A representation of smooth muscle.
Smooth muscle
is similar to skeletal and cardiac muscle in that
contraction is produced by the interaction of
actin
and
myosin ± la-
ments
in the presence of Ca
++
. However, there are many differences.
Smooth muscle F
bers are short (15 to 500
μ
m) and
spindle shaped
and have single, centrally placed nuclei. Smooth muscle lacks the stri-
ations observed in skeletal and cardiac muscle because the arrange-
ment of the actin and myosin F
laments is not as orderly. Smooth
muscle is innervated by sympathetic and parasympathetic axons,
but transmitter molecules are released into the intercellular space at
swellings in the axon (
varicosities
), rather than at speciF
c neuromus-
cular junctions (“endplates”) as in skeletal muscle. The sarcolemmas
of some smooth muscles contain
gap junctions
that permit electrical
excitation to move directly from one F
ber to adjacent F
bers, there-
fore producing a moving wave of contraction.
T. Yang
Varicosity
Gap junction
Autonomic nerve fiber
A
Epithelium
Lumen
Lumen
Epithelium
Circular layer
Myenteric plexus
Connective tissue
Epithelium
Longitudinal layer
B
Figure 6-10B.
Smooth muscle, duodenum.
H&E,
3
117; upper inset
3
485; lower inset
3
259
In the gastrointestinal tract,
smooth muscle
is important for keeping food moving at the proper rate to enhance digestion, to permit
the absorption of nutrients, and to prepare waste to be expelled from the body. A low-power section through the duodenum of the
small intestine is shown (see also ±ig. 3-7A). The
lumen
of the intestine, with columnar
epithelium
specialized for absorption, is at
the top of the picture; beneath it is a layer of
connective tissue
. Bands of smooth muscles encircle the duodenum. A transverse section
through this
circular layer
is shown at higher power in the
upper inset
. The nuclei are scattered randomly through the section. Many
muscle F bers are cut through a portion of the F ber that does not contain a nucleus. A second layer of smooth muscle is oriented
along the length of the duodenum and here, it is cut longitudinally. The
lower inset
shows a longitudinal section at higher power.
Note the long, spindle-shaped nuclei. The smooth muscle of the gut is classiF ed as
visceral
or
unitary smooth muscle
and has many
gap junctions. Spontaneous waves of contraction move along the length of the gut, modulated by signals from pacemaker ganglia
or
plexuses
in the autonomic nervous system. One such plexus, a
myenteric plexus
, is visible in the low-power photomicrograph
(see also ±ig. 7-15B).
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