Photomicrograph of a sheet of smooth muscle cells. Note the spindle
shape, single nucleus, and lack of striations.
Thick and thin ﬁ laments in smooth muscle are arranged in slightly
diagonal chains that are anchored to the plasma membrane or to
dense bodies within the cytoplasm. When activated, the thick and
thin ﬁ laments slide past each other, causing the smooth muscle ﬁ ber
to shorten and thicken.
Smooth and Cardiac Muscle
Having described in depth the mechanisms underlying skel-
etal muscle function, we now turn our attention to the other
muscle types, beginning with smooth muscle. Two character-
istics are common to all smooth muscles: they lack the cross-
striated banding pattern found in skeletal and cardiac ﬁ bers
(which makes them “smooth”), and the nerves to them are
derived from the autonomic division of the nervous system
rather than the somatic division. Thus, smooth muscle is not
normally under direct voluntary control.
Smooth muscle, like skeletal muscle, uses cross-bridge
movements between actin and myosin ﬁ laments to gener-
ate force, and calcium ions to control cross-bridge activity.
However, the organization of the contractile ﬁ laments and the
process of excitation-contraction coupling are quite different
in smooth muscle. Furthermore, there is considerable diversity
among smooth muscles with respect to the excitation-contrac-
tion coupling mechanism.
Structure of Smooth Muscle
Each smooth muscle cell is spindle-shaped, with a diameter
between 2 and 10 μm, and length ranging from 50 to 400 μm.
They are much smaller than skeletal muscle ﬁ bers, which are
10 to 100 μm wide and can be tens of centimeters long (see
Figure 9–1). Skeletal ﬁ bers are sometimes large enough to run
the entire length of the muscles they are found in, whereas
many individual smooth muscle cells are generally intercon-
nected to form sheetlike layers of cells (
muscle ﬁ bers are multinucleate cells that are unable to divide
once they have differentiated; smooth muscle cells have a sin-
gle nucleus and have the capacity to divide throughout the
life of an individual. A variety of paracrine agents can stimu-
late smooth muscle cells to divide, often in response to tissue
Just like skeletal muscle ﬁ bers, smooth muscle cells have
thick myosin-containing ﬁ laments and thin actin-containing
laments. Although tropomyosin is present in the thin ﬁ
ments, the regulatory protein troponin is absent. The thin
laments are anchored either to the plasma membrane or to
cytoplasmic structures known as
functionally similar to the Z lines in skeletal muscle ﬁ bers.
that the ﬁ laments are oriented slightly
diagonally to the long axis of the cell. When the ﬁ ber short-
ens, the regions of the plasma membrane between the points
where actin is attached to the membrane balloon out. The
thick and thin ﬁ laments are not organized into myoﬁ brils, as
in striated muscles, and there is no regular alignment of these
ﬁ laments into sarcomeres, which accounts for the absence of
a banding pattern. Nevertheless, smooth muscle contraction
occurs by a sliding-ﬁ lament mechanism.
The concentration of myosin in smooth muscle is only
about one-third of that in striated muscle, whereas the actin
content can be twice as great. In spite of these differences,
the maximal tension per unit of cross-sectional area devel-