CHAPTER 6
Muscle
113
Figure 6-13B.
Schematic diagram of the contractile mechanism of
smooth muscle.
Actin f
laments (
red
) in smooth muscle are anchored in
dense plaques
in the cell walls, and the corresponding myosin f laments (
green
) are
suspended between two or more actin f
laments. This arrangement is
much less orderly than the arrangement oF actin and myosin in skeletal
and cardiac muscles. ±urthermore, the organization is, to some extent,
dynamic and can be rearranged in response to changing demands on
the muscle. The cells are attached to each other via junctions with the
collagen and elastin f
bers in the extracellular matrix.
Figure 6-13A.
Smooth muscle ± bers in the tunica
media of a medium artery.
EM,
3
4,260; inset
3
6,530
The smooth muscle f
bers in this illustration are cut
obliquely to their long axis. The thickenings oF the cell
membrane, which are termed
dense plaques
, can be
clearly seen. Actin myof
laments are attached to these
structures and the Force oF the actin-myosin contrac-
tion is transmitted to the cell wall at these points. Actin
myof laments are also sometimes anchored to
dense
bodies
within the cytoplasm. In some types oF smooth
muscles (e.g., in the intestine), the myof
laments seem
to be arranged randomly, in a crisscross Fashion. In
other types (e.g., in the airway), myof laments seem to
be arranged in parallel as diagramed in ±igure 6-13B.
Smooth muscle cells transmit Force From one to another
via the
extracellular matrix
and, in some cases, via tight
junctions between the cell membranes oF adjacent cells.
The extracellular matrix in smooth muscle is composed
oF elastin, collagen, and other elements, but in contrast
to other tissues, it is secreted by the myocytes themselves
rather than by f broblasts. In some tissues, such as in
elastic arteries and in the airway, smooth muscle f
bers
may, with age or disease, gradually lose their ability to
contract and become more and more like f
broblasts.
Lumen of artery
Tunica intima
Endothelial cell
Nucleus
Extracellular matrix
Internal elastic membrane
Dense body
Dense plaque
Caveolae
Caveolae
Caveolae
Caveolae
Dense body
Sarcolemma
Dense plaque
Nucleus
A
B
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