Chapter 18
The Cardiovascular System: The Heart
671
18
sheets of corrugated cardboard at dark-staining junctions called
intercalated discs
(in-ter
9
kah-la
0
ted;
intercala
5
insert) (Fig-
ure 18.12). Intercalated discs contain anchoring
desmosomes
and
gap junctions
(cell junctions discussed in Chapter 3). Te
desmosomes prevent adjacent cells from separating during con-
traction, and the gap junctions allow ions to pass from cell to
cell, transmitting current across the entire heart. Because gap
junctions electrically couple cardiac cells, the myocardium
be-
haves
as a single coordinated unit, or
functional syncytium
(sin-sit
9
e-um;
syn
5
together,
cyt
5
cell).
Large mitochondria account for 25–35% of the volume of car-
diac cells (compared with only 2% in skeletal muscle), a charac-
teristic that makes cardiac cells highly resistant to fatigue. Most
of the remaining volume is occupied by myofibrils composed of
fairly typical sarcomeres. Te sarcomeres have Z discs, A bands,
and I bands that reflect the arrangement of the thick (myosin)
and thin (actin) filaments composing them. However, in con-
trast to skeletal muscle, the myofibrils of cardiac muscle cells vary
greatly in diameter and branch extensively, accommodating the
abundant mitochondria between them. Tis difference produces
a banding pattern less dramatic than that seen in skeletal muscle.
Te system for delivering Ca
2
1
is less elaborate in cardiac
muscle cells. Te ± tubules are wider and fewer than in skel-
etal muscle and they enter the cells once per sarcomere at the
Z discs. (Recall that ± tubules are invaginations of the sarco-
lemma. In skeletal muscle, the ± tubules invaginate twice per
sarcomere, at the A band–I band junctions.) Te cardiac sar-
coplasmic reticulum is simpler and lacks the large terminal
cisterns seen in skeletal muscle. Consequently, cardiac muscle
fibers do not have
triads
.
Mechanism and Events of Contraction
Although both heart muscle and skeletal muscle are contractile
tissues, they have three fundamental differences:
Means of stimulation.
Each skeletal muscle fiber must be
stimulated to contract by a nerve ending, but some cardiac
muscle cells are self-excitable. Tese cells can initiate not
only their own depolarization, but that of the rest of the heart
as well, in a spontaneous and rhythmic way. We describe this
property, called
automaticity
, or
autorhythmicity
, in the
next section.
Organ versus motor unit contraction.
In skeletal muscle, im-
pulses do not spread from cell to cell. Only muscle fibers that
are individually stimulated by nerve fibers contract. Skele-
tal muscles usually contract with only some of the muscle’s
motor units activated. In contrast, in cardiac muscle, either
all fibers in the heart contract as a unit or the heart doesn’t
contract at all. Tis coordinated action occurs because gap
junctions electrically tie all cardiac muscle cells together into
a single contractile unit. Consequently, the depolarization
wave travels across the heart from cell to cell via ion passage
through the gap junctions.
Length of absolute refractory period.
Recall that the abso-
lute refractory period is the inexcitable period when Na
1
channels are still open or inactivated. In skeletal muscle
Homeostatic Imbalance
18.3
Blockage of the coronary arterial circulation can be serious and
sometimes fatal.
Angina pectoris
(an-ji
9
nah pek
9
tor-is; “choked
chest”) is thoracic pain caused by a fleeting deficiency in blood
delivery to the myocardium. It may result from stress-induced
spasms of the coronary arteries or from increased physical de-
mands on the heart. Te myocardial cells are weakened by the
temporary lack of oxygen but do not die.
Prolonged coronary blockage is far more serious because it
can lead to a
myocardial infarction (MI)
, commonly called a
heart attack
, in which cells
do
die. Since adult cardiac muscle
is essentially amitotic, most of the dead tissue is replaced with
noncontractile scar tissue. Whether or not a person survives a
myocardial infarction depends on the extent and location of the
damage. Damage to the le² ventricle—the systemic pump—is
most serious.
Check Your Understanding
5.
Which side of the heart acts as the pulmonary pump? The
systemic pump?
6.
Which of the following statements are true? (a) The left
ventricle wall is thicker than the right ventricle wall. (b) The
left ventricle pumps blood at a higher pressure than the right
ventricle. (c) The left ventricle pumps more blood with each
beat than the right ventricle. Explain.
7.
Name the two main branches of the right coronary artery.
For answers, see Appendix H.
Cardiac Muscle Fibers
Describe the structural and functional properties of cardiac
muscle, and explain how it differs from skeletal muscle.
Briefly describe the events of cardiac muscle cell
contraction.
Although similar to skeletal muscle, cardiac muscle displays
some special anatomical features that reflect its unique blood-
pumping role.
Microscopic Anatomy
Like skeletal muscle,
cardiac muscle
is striated and contracts
by the sliding filament mechanism. However, in contrast to the
long, cylindrical, multinucleate skeletal muscle fibers, cardiac
cells are short, fat, branched, and interconnected. Each fiber
contains one or at most two large, pale,
centrally
located nuclei
(Figure 18.12a)
. Te intercellular spaces are filled with a loose
connective tissue matrix (the
endomysium
) containing numer-
ous capillaries. Tis delicate matrix is connected to the fibrous
cardiac skeleton, which acts both as a tendon and as an inser-
tion, giving the cardiac cells something to pull or exert their
force against.
Skeletal muscle fibers are independent of one another both
structurally and functionally. By contrast, the plasma mem-
branes of adjacent cardiac cells interlock like the ribs of two
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