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Chapter 9
muscle leads to some degree of depolarization and a contrac-
tile response, action potentials do not occur in most multi-
unit smooth muscles. Circulating hormones can increase or
decrease contractile activity in multiunit smooth muscle, but
stretching does not induce contraction in this type of muscle.
The smooth muscles in the large airways to the lungs, in large
arteries, and attached to the hairs in the skin are multiunit
smooth muscles.
It must be emphasized that most smooth muscles do
not show all the characteristics of either single-unit or multi-
unit smooth muscles. These two prototypes represent the two
extremes in smooth muscle characteristics, with many smooth
muscles having overlapping characteristics.
Cardiac Muscle
The third general type of muscle, cardiac muscle, is found only
in the heart. Although many details about cardiac muscle will
be discussed in the context of the cardiovascular system in
Chapter 12, a brief explanation of its function and how it com-
pares to skeletal and smooth muscle is presented here.
Cellular Structure of Cardiac Muscle
Cardiac muscle combines properties of both skeletal and
smooth muscle. Like skeletal muscle, it has a striated appear-
ance due to regularly repeating sarcomeres composed of
myosin-containing thick fi laments interdigitating with thin
fi laments that contain actin. Troponin and tropomyosin are
also present in the thin fi lament, and they have the same
functions as in skeletal muscle. Cellular membranes include
a T-tubule system and associated calcium-loaded sarcoplasmic
reticulum. The mechanism by which these membranes inter-
act to release calcium is different than in skeletal muscle, how-
ever, as will be discussed shortly.
Like smooth muscle cells, individual cardiac muscle cells
are relatively small (100 μm long and 20 μm in diameter) and
generally contain a single nucleus. Adjacent cells are joined
end-to-end at structures called
intercalated disks,
within
which are desmosomes that hold the cells together and to
which the myofi brils are attached (
Figure 9–39
). Also found
within the intercalated disks are gap junctions similar to those
found in single-unit smooth muscle. Cardiac muscle cells also
are arranged in layers and surround hollow cavities, in this
case the blood-fi
lled chambers of the heart. When muscle in
the walls of cardiac chambers contracts, it acts like a squeezing
fi st and exerts pressure on the blood inside.
Excitation-Contraction Coupling
in Cardiac Muscle
As in skeletal muscle, contraction of cardiac muscle cells
occurs in response to a membrane action potential that propa-
gates through the T-tubules, but the mechanisms linking that
excitation to the generation of force exhibit features of both
skeletal and smooth muscles (
Figure 9–40
). Depolarization
during cardiac muscle cell action potentials is in part due to an
infl
ux of Ca
2+
ions through voltage-gated channels. These cal-
cium channels are known as
L-type calcium channels,
and
are modifi ed versions of the DHP receptors that act as the
voltage-sensor in skeletal muscle cell excitation-contraction
coupling. Not only does this entering calcium participate in
depolarization of the plasma membrane and cause a small rise
in cytosolic calcium concentration, but it also serves as a trig-
ger for the release of a much larger amount of calcium from
the sarcoplasmic reticulum. This occurs because ryanodine
receptors in the cardiac sarcoplasmic reticulum lateral sacs are
calcium channels, but rather than being opened directly by
voltage as in skeletal muscle, they are opened by the bind-
ing of trigger calcium in the cytosol. Once cytosolic calcium
is elevated, thin fi lament activation, cross-bridge cycling, and
force generated occur by the same basic mechanisms described
for skeletal muscle (review Figures 9–8 and 9–9).
Thus, even though most of the calcium initiating car-
diac muscle contraction comes from the sarcoplasm reticu-
lum, the process—unlike that in skeletal muscle—is dependent
on the movement of extracellular calcium into the cytosol.
Contraction ends when the cytosolic calcium concentration is
Striations
Nucleus
Intercalated discs
(a)
Gap
junction
Desmosome
(b)
Sarcolemma
Nucleus
Intercalated
discs
Cardiac muscle
cell
Mitochondrion
Figure 9–39
Cardiac muscle. (a) Light micrograph. (b) Cardiac myocytes and
intercalated discs.
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