CHAPTER 6
Muscle
109
Figure 6-9.
Cardiac muscle.
EM,
3
24,800
Cardiac muscle
is similar to skeletal muscle in many respects. Both have similar arrangements of
actin
and
myosin f laments
that
interact to produce contraction. The actin F laments are anchored at Z lines, and myosin F
laments occupy a central position between
two successive Z lines. The structures between two Z lines form a
sarcomere
. The resulting
A band
,
I band
,
H band
, and
Z line
are
analogous in the two muscle types. However, there are several notable structural differences. The most obvious is that cardiac myo-
cytes are much shorter than are skeletal muscle F
bers and are joined to each other by complex structures called
intercalated disks
.
The intercalated disks are specialized regions of the sarcolemma that contain regions of
±ascia adherens
which bind the adjacent cells
together against the stress of contraction, and
gap junctions
which provide a path for the muscle action potential to travel directly
from one cell to the next. A single intercalated disk typically includes portions that are oriented transversely with respect to the
muscle F ber (1 and 3) and a portion that is oriented longitudinally (2). The path of this intercalated disk is indicated by the
red line
in the inset
.
Gap junctions
are found predominantly in the longitudinal sections. A second major difference is in the T-tubule system.
T tubules
(invaginations of the cell membrane) are prominent in cardiac muscle, although there is only one tubule per sarcomere
(located at the Z line) instead of two tubules per sarcomere (located at the A–I junctions) as in skeletal muscle. In addition, the
sarcoplasmic reticulum
is not as prominent in cardiac muscle and its function in contraction is not as well understood. Nevertheless,
the release of Ca
++
is critical to contraction, just as in skeletal muscle. The muscle action potential travels along the cell membrane
and T tubules and triggers the fl ow of Ca
++
into the cell from the extracellular space and from the sarcoplasmic reticulum. There
is also a slow leakage of Ca
++
into the muscle F bers that is responsible for the spontaneous contraction and relaxation rhythm of
isolated cardiac muscle. This natural rhythm is modiF
ed by
neuronal
(
autonomic
) and
hormonal infl uences
. Heart rate increases
during physical exercise or stress and decreases during periods of rest and sleep.
G
ap junctions
a
p
ju
n
c
tio
n
s
Intercalated disk
Intercalated disk
Intercalated disk
3
2
1
Gap junctions
Adherens junction
A
d
h
e
r
e
n
s
ju
n
c
tio
n
Adherens junction
I band
I b
a
n
d
I band
H band
H
b
a
n
d
H band
A band
A
b
a
n
d
A band
Z line
Z
lin
e
Z line
T-tubules
T-tubules
T-tubules
Mitochondria
Mitochondria
Mitochondria
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