Structure of the Cardiac Musculature (Myocardium)

External musculature of the cor, simplified anteroinferior view. The
muscular walls of the ventriculi dexter and sinister have been windowed
to display the deeper fibers.
Note: The epicardium has been removed in a and b along with the
subepicardial fat. The coronary vessels are not shown in order to display
more clearly the cardiac surface grooves (sulci interventriculares
anterior and posterior).
The musculature of the atria is arranged in two layers, superficial
and deep. The superficial layer (shown here) extends over the atria
and is common to both, whereas each atrium has its own deep layer.
Looped and annular muscle fibers extend down to the atrioventricular
boundary and also encircle the ostia venarum. The ventricular
musculature has a complex arrangement, consisting basically of a
superficial (subepicardial), middle, and deep (subendocardial) layer.
The superficial layer joins apically with the deeper layers to form a
whorled arrangement of muscle fibers around the apex cordis (vortex
cordis). The ventriculus dexter, which is a low-pressure system, is less muscular than the ventriculus sinister and almost completely
lacks a middle layer. The subendocardial layer forms the tra-beculae carneae and mm. papillares.
The histological unit of the myocardium is the cardiac myocyte, a
specialized form of cardiac muscle cell. Unlike their electronically isolated
counterparts in skeletal muscle, cardiac myocytes form a syncytium
in which membrane depolarization and contraction spread in a
wave.
 Myocardial cross-sections perpendicular to the long axis of the cor,
viewed from above. c Schematic representation: The ventriculi in an
expanded state (diastole, left figure), and in a contracted state (systole,
right figure). d Transverse section through a specimen during
diastole.
All the sections clearly demonstrate the difference in thickness between
the left and right ventricular myocardia: The ventriculus sinister
is part of the high-pressure system, and therefore its myocardium
must generate a significantly higher pressure (120–140 mmHg
during ventricular contraction) than the ventriculus dexter (approximately
25–30 mmHg). The difference in thickness is most pronounced
during ventricular contraction (see c). Section d shows how
the coronary vessels and subepicardial fat fill the sulci in the cor.