108
UNIT 2
Basic Tissues
Cardiac Muscle
Figure 6-8A.
Organization of cardiac muscle—a branching
network of interconnected muscle cells.
Cardiac muscle f
bers split and branch repeatedly and join other
muscle f
bers end to end to Form an
anastomosing
network oF
contractile tissues. In contrast to skeletal muscle, cardiac muscle
f bers contract and relax spontaneously. The
intercalated disks
at the boundaries between f
bers contain gap junctions, which
permit electrical depolarization to move directly and rapidly
From one myocyte to the next. The
sympathetic
and
parasym-
pathetic innervation
oF the heart serves to
increase
or
decrease
the rhythm oF contraction rather than to command individual
contractions as the peripheral nervous system does For
skeletal
muscle. This modulation oF heart rate occurs via a system that
includes the
sinoatrial
and
atrioventricular (AV) nodes
and
specialized, highly conductive muscle f
bers
(AV bundle
and
Purkinje ±
bers)
that connect the AV node with the contractile
myocytes (see ±igs. 9-2 and 9-4A).
D. Cui J.Lynch
Single nucleus
in each fiber
Intercalated disks
Fibers branch
and anastomose
A
Endomysium
Nuclei
Nuclei
Endomysium
Capillary
Capillary
Fibroblast
Fibroblast
Nuclei
Capillary
Fibroblast
C
Figure 6-8C.
Cardiac muscle, transverse section.
H&E,
3
272; inset
3
418
Cardiac muscle ±
bers
(
myocytes
) are elliptical or lobulated
in transverse section. Each f
ber has a single nucleus, which
is irregular in shape and centrally located in the f ber. Many
capillaries
traverse the tissue, and the
endomysium
is typi-
cally more prominent than in skeletal muscle. The
inset
shows nuclei oF myocytes and
± broblasts
at higher power,
with a capillary in the lower right quadrant (
arrow
).
Intercalated disks
B
Figure 6-8B.
Cardiac muscle, longitudinal section.
H&E,
3
272; inset
3
418
Cardiac muscle
is like skeletal muscle in that it is
striated
.
Actin and myosin f
laments are arranged into sarcomeres,
with A bands, I bands, H bands, and Z lines (see ±ig. 6-9).
However, cardiac muscle is diFFerent in several respects.
Actin and myosin f
laments are not arranged in discrete
myof
brils.
Cardiac muscle ± bers
are much shorter than
skeletal muscle f
bers and typically split into two or more
branches (
thin arrows
). The branches are joined, end to
end, by
intercalated disks
(
thick arrows in inset
) and Form a
meshwork oF muscle f
bers. Each f
ber has a single, centrally
located nucleus. Cardiac muscle tissue is highly vascularized
and contains many more mitochondria than other muscle
types, owing to its constant activity and resulting high meta-
bolic requirements.
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