Cardiac muscle 2

a. Transverse portion, which occurs
between the ends of tandem cells, contains
specialized junctions for attachment: fascia
adherens and desmosomes. Since actin
filaments attach to transverse portions of IDs at the ends of the cells, they serve the function of Z lines.
b. Longitudinal (lateral) portion,
which occurs between lateral cells, contains gap
junctions and desmosomes. Unlike skeletal
muscle, cardiac muscle cells are electrically coupled
to each other through gap junctions, which is
important for cardiac myocyte rhythmicity.
5. Costameres, which appear to be an
important cellular specialization for lateral
attachment of myofibrils to adjacent sarcolemmal
membranes in skeletal muscle, have also been
described in cardiac muscle (Pardo, JV et al. J Cell Biol
97:1081-1088, 1983). They encircle cardiac myocytes
perpendicular to the long axis of the cell and are
localized at the I bands of adjacent sarcomeres.
However, unlike their discontinuity at the Z lines in
skeletal muscle (see above), they are apparently
associated with Z lines in cardiac muscle. Perhaps
this is related to the fact that T tubules, which are
specializations of the sarcolemma, occur at the Z
lines in cardiac muscle, but not in skeletal muscle.

6. Cardiac muscle myofibrils are different
from those found in skeletal muscle. Due to an
important role of extracellular calcium in cardiac
muscle contraction, the SR is not as extensive and
well developed in cardiocytes as in skeletal muscle
myocytes. Thus, cardiac myofilaments are not
completely segregated into discrete myofibrils by the
SR as in skeletal muscle. Rather, cardiac myofibrils
occur as continuous fields that are only partially
divided by discontinuous profiles of SR and other
portions of sarcoplasm. It should be noted that this
distinction is only visible at the EM level. At the LM
level, typical myofibrils appear to occur. Finally,
unlike skeletal myofibrils, cardiocyte myofibrils vary
in length due to the fact that each cell has a variable
branching morphology.
7. Types of cardiocytes. Based upon both
structural and functional characteristics, three types
of cardiac myocyte are identified.
a. Contractile cardiocytes, also referred
to as “working myocytes”, are the typical cells of the
myocardium that tirelessly contract about 3 billion
times during an average human lifespan.
b. Conductile cardiocytes are the cells
that make up the fibers of the cardiac conduction
system (see below). They are specialized for
conducting the depolarization currents rather than
for contraction. In this respect they resemble nerve
cells in function. Two types of conductile fibers can be identified: narrow diameter fibers and wide
diameter fibers, which are also called Purkinje
fibers.
c. Endocrine cardiocytes. As a result of
electron microscopy, it has been known for a long
time that certain atrial cardiocytes contain unique
cytoplasmic “bodies” (Kisch, B. Exp Med Surg 14:99-112,
1956), which were subsequently called atrial-specific
granules (Jamieson, JD & Palade, GE. J Cell Biol 23:151-172,
1964). These granules have been found to contain
peptide hormones with natriuretic, diuretic, and
vasorelaxant activities.