Muscle
255
including the stomach, intestines, urinary bladder, uterus, blood vessels, and airways in the lungs.
Contraction of the smooth muscle surrounding hollow organs may propel the luminal contents through
the organ, or it may regulate internal fl ow by changing the tube diameter. In addition, small bundles
of smooth muscle cells are attached to the hairs of the skin and iris of the eye. The autonomic nervous
system, hormones, autocrine/paracrine agents, and other local chemical signals control smooth muscle
contraction. Some smooth muscles contract autonomously, however, even in the absence of such signals.
In contrast to skeletal muscle, smooth muscle is not normally under voluntary control.
Cardiac muscle is the muscle of the heart. Its contraction propels blood through the circulatory system.
Like smooth muscle, it is regulated by the autonomic nervous system, hormones, and autocrine/
paracrine agents, and it can undergo spontaneous contractions.
Although there are signifi cant differences among these three types of muscle, the force-generating
mechanism is similar in all of them. This chapter will describe skeletal muscle fi rst, followed by smooth and
cardiac muscle. Cardiac muscle, which combines some of the properties of both skeletal and smooth muscle,
will be described in more depth in Chapter 12 in association with its role in the circulatory system.
SECTION A
Skeletal Muscle
Structure
The most striking feature seen when viewing
skeletal mus-
cle
through a microscope is a distinct series of alternating light
and dark bands perpendicular to the long axis. Because
cardiac
muscle
shares this characteristic striped pattern, these two types
are both referred to as
striated muscle.
The third basic muscle
type,
smooth muscle,
derives its name from the fact that it lacks
this striated appearance.
Figure 9–1
compares the appearance
of skeletal muscle cells to cardiac and smooth muscle cells.
Nuclei
Striations
(a)
Skeletal muscle
Muscle fiber
Connective
tissue
Branching
Intercalated disc
(b)
Cardiac muscle
(c)
Smooth muscle
Nucleus
Glycogen
Striations
Nuclei
Muscle cells
Figure 9–1
Comparison of skeletal muscle (a) to cardiac (b) and smooth (c) muscle. Both skeletal and cardiac muscle have a striated appearance. Cardiac
and smooth muscle cells tend to have a single nucleus, while skeletal muscle fi bers are multinucleated.
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