Computational Intelligence in Cardiology and Heart Disease Diagnosis - Computational Intelligence in Biomedical Engineering - page 144

Pericardial cavity

Parietal
pericardium

Endocardium

Myocardium Epicardium

FIGURE 4.2
A sketch of the heart layers showing the epicardium, myocardium, and
endocardium.

bones. This layer may contain some Purkinje fibers, which conduct electrical
impulses. The middle endocardium layer is the thickest, composed of more
regularly arranged collagen fibers containing a variable number of elastic fibers
compactly arranged in the deepest part of the layer. Finally, the innermost
layer is composed of flat endothelial cells, which form a continuous lining with
the endothelial cells lining the blood vessels entering and emerging from the
heart. The thickness of the endocardium is variable, being thickest in the atria
and thinnest in the ventricles, particularly, the left ventricle. This increased
thickness is almost entirely due to a thicker fibroelastic middle layer.
The myocardium layer forms the bulk of the heart and consists of contractile
elements composed of specialized striated muscle fibers called cardiac muscle .
The cardiac muscle is slightly different from the skeletal muscle. Similar to
skeletal muscle, the cardiac muscle is striated, has dark Z lines, and possesses
myofibrils, which contain actin and myosin filaments (see Chapter 5). These
filaments slide over one another using a similar cross-bridge action observed in
skeletal muscle contraction. In addition to this, the cardiac muscle has several
unique properties such as the capability to contract without nerve impulse
stimulation (intrinsic contraction). Another striking microscopic feature of
these cells is branching patterns and the presence of more than one nuclei in
the muscle cell. The individual cells are separated from one another at the
ends by intercalated discs, which are specialized cell junctions between cardiac
muscle cells. This natural arrangement provides a low-resistance layer, which
allows a faster conduction of action potentials along the cells. The resistance
is so low that ions move freely through the permeable cell junction allowing
the entire atrial or ventricular chamber to function as a single cell. This single