Cardiac Muscle Tissue Forms
the Muscular Wall of the Heart
LEARNING OBJECTIVES
1.
Describe
the structure of cardiac
muscle tissue.
2.
Compare
cardiac muscle, skeletal
muscle, and smooth muscle.
T
he heart is composed mostly of car-
diac muscle tissue. This tissue has
some of the characteristics seen in
skeletal muscle and some of those
seen in smooth muscle tissue, creating a unique
type of muscular tissue.
Cardiac muscle has a structure that is
intermediate between skeletal muscle and
smooth muscle (Figure 6.11). Cardiac cells
are larger than smooth muscle cells but smaller than
skeletal muscle cells. Like skeletal muscle, they have
cylindrical myofibrils, sarcomeres, T-tubules, and a
fairly
well-developed
sarcoplasmic
reticulum.
Like
single-unit smooth muscle cells, cardiac cells commu-
nicate with one another through gap junctions in the
intercalated disc
(in-TER-ka-lat-ed)
An irregular trans-
verse thickening of
sarcolemma where
two cardiac cells
come together. The
intercalated disc is
composed of desmo-
somes, which hold
cardiac muscle fibers
together, and gap
junctions, which aid in
conduction of muscle
action potentials from
one fiber to the next.
intercalated discs between them. Cardiac
cells are unique in that they are often branched.
Cardiac cells respond to signals from special-
ized heart muscle cells called
pacemaker cells
, which
in turn are influenced by the autonomic nervous
system. Pacemaker cells are
autorhythmic
; they
can contract on their own, without nervous sys-
tem stimulation. Via pacemaker cells, the auto-
nomic nervous system can speed up or slow down
the contraction of cardiac cells as needed. Unlike
other types of muscle cells, cardiac cells can pro-
duce only twitch contractions, and they do so at
a rate of 70 to 80 contractions per minute. The
actions of cardiac muscle are discussed more fully
in Chapter 11.
1.
What
is the function of intercalated discs?
2.
What
is the source of action potentials in car-
diac muscle? in skeletal muscle?
S tru ctu re o f cardiac m uscle cells • Figure 6.11
Cardiac muscle cells are smaller
than skeletal muscle fibers,
but larger than smooth ones.
Cardiac Muscle Tissue Forms the Muscular Wall of the Heart 167
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