112
UNIT 2
Basic Tissues
Figure 6-12.
Transverse section of smooth muscle of the trachea.
EM,
3
14,750
Although contraction of smooth muscle is produced by a calcium-mediated interaction between
actin
and
myosin ±
laments
similar
to that described in Figure 6-4C, there are signi±
cant differences in the structure of smooth muscle cells and striated (skeletal and
cardiac) muscle cells. Actin and myosin ±
laments are clearly visible (
see inset
), but their arrangement is not as orderly as in skeletal
muscle. Actin ± laments are anchored to the cell walls at
dense plaques
or at
dense bodies
in the interior of the cell (see Fig. 6-13B).
The actin ± laments contact myosin ± laments to produce contraction, but the organization is more random and more changeable
than in skeletal or cardiac muscle. Smooth muscle has the property of being able to produce relatively constant contractile force
over a greater range of cell lengths than striated muscle. Skeletal muscle, for example, cannot produce maximum contraction force
when it is fully extended because there is not suf± cient overlap between the actin and myosin ±
laments. This property of smooth
muscle is important in organs such as the stomach and uterus where strong contraction may be needed when the organ is distended
and the muscle cells already considerably stretched. Some smooth muscles have the ability to remodel their contractile architecture
in response to different conditions of muscle extension.
Intermediate ±
laments
provide mechanical and structural integrity for many
types of cells, including smooth muscle. They are composed primarily of the proteins vimentin and desmin. Lack of these proteins
impairs the contractility of smooth muscle. Contraction of smooth muscle can be initiated by
neural signals
(e.g., iris, respiratory
system),
mechanical stretch
(e.g., gut, urinary tract),
electrical signals
traveling from one smooth muscle ±
ber to another via gap
junctions (e.g., gut, respiratory system), or
hormones
in the blood stream (e.g., respiratory system, uterus). The calcium necessary
to initiate contraction enters the cell from the
extracellular space
rather than from the
sarcoplasmic reticulum
as in striated muscle.
Smooth muscle ± bers have a poorly developed
sarcoplasmic reticulum
and no T-tubule system at all. Cup-shaped indentations in the
sarcolemma (
caveolae
) may play a role in sequestering calcium.
Actin filaments
Myosin filament
Dense body
Intermediate filament
Mitochondrion
Dense plaque
Sarcoplasmic
reticulum
Caveolae
Caveolae
Caveolae
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