Cardiac muscle

Cardiac muscle cells. Like skeletal muscle cells,
cardiocytes are elongated cells that contain
sarcomeres, exhibit transverse banding patterns, and
are surrounded by an external lamina. The
molecular basis for cardiocyte contraction is also
viewed to be the same. However, these two types of
striated muscle differ in several important ways.
1. Size. Cardiac cells are smaller than skeletal
myocytes. They are about 100-150 ìm in length and
range from 10-35 ìm in diameter.
2. Nucleus. Cardiocytes usually possess one
centrally-placed nucleus. However, it has been
demonstrated that the distribution of mononucleate,
binucleate, trinucleate, and tetranucleate human
cardiocytes is 74%, 25.5%, 0.4%, and 0.1%,
respectively (Olivetti, G et al. J Mol Cell Cardiol 28:1463-1477,
1996). Percentages did not change with age (26-93
yrs), myocardial hypertrophy, or ischemic
cardiomyopathy.
3. Arrangement: branching myofibers.
Unlike skeletal myocytes, cardiocytes are arranged
end-to-end so that a cardiac muscle fiber is
actually composed of numerous cells. Cardiocytes
also have branching ends so that they usually contact
more than two adjacent cells. Thus, cardiac
myofibers are branched.
4. Intercalated disks (IDs) are extensive
junctional structures that occur between cardiac
myocytes where they are attached to each other at
their ends. Because cardiac muscle fibers are
composed of individual myocytes that are arranged
in tandem, IDs are crucial for cardiac muscle
function by providing areas for 1) secure attachment
and 2) intercellular communication. In well stained
preparations, IDs can be clearly seen between cells at
the LM level. However, their constituent parts can
only be resolved at the EM level. Close inspection
shows that IDs are arranged as step-like structures
that follow the zigzag contours that occur at the ends
of cardiocytes. They consist of the following
components.