Download Architecture of fibers of the working myocardium and

Architecture of fibers of the working myocardium and the sequence of excitation of heart ventricles
of a pig
A.S.Gulyaeva, I.M.Roshchevskaya, M.P.Roshchevsky
Institute of Physiology, Komi Science Center, Ural Division, Russian Academy of Sciences,
50, Pervomayskaya st., Syktyvkar, 167982, Komi Republic, Russia.
e-mail: [email protected]
ABSTRACT
The ventricular fibre architecture in pig has been studied by the method of level-by-level splitting
of muscular bundles. In both ventricles, the muscular fascicles of the myocardium are arranged in three
preferential directions through the ventricular mass. We differentiated three different layers of fibres:
superficial, middle, and deep. The fibres of the superficial layer have spiral direction. The superficial
fibres invaginate into the apex cordis and form the deep layer. The middle layer in both pig ventricles is
represented by circumferential fibres, which are absent both on the apex of the right and left ventricles.
Depending on the depth of location orientation of fibres is changed and differentiated in both ventricles.
The fibers of the deep layer of the right ventricle are arranged obliquely on the free wall, while on the
interventricular septum – lengthways the apical- basal heart axis. In the left ventricle the fibres of the
deep layer are going spirally from the apex to the base of the heart. The sequence of depolarization of
pig's heart ventricles was investigated and the comparison of orientation of the fibres of the working
myocardium to the character of the excitation wave propagation was carried out.
1. INTRODUCTION
Architecture of the conducting system and working myocardium determines the succession of
excitation and contraction of heart ventricles. Conducting system takes the leading role in the
chronotopography of the excitation process in the heart of warm-blooded animals and man. Heart
ventricles of man and predators are characterized by “successive” type of activation. In ungulates (pig)
the spatial organization of Purkinje fibres is a structural basis of polyfocal excitation of heart ventricles6.
Information on the architecture of the working myocardium in animals of various classes is
fragmentary. The heart of man2, 4 and dog7 is studied in greater detail. Spatial organization of fibres
distribution in heart ventricles in pig is weakly studied.
The problem of similarity or difference of the architecture of the working myocardium of pig and
man is yet unsolved. In this relation the aim of our investigation is to study the architecture of the
ventricular fibers structure for comparative-physiological assessment of the heart function.
2. METHODS
The architecture of the working myocardium in 12 pigs of both sexes aged 8-9 months is studied.
We used the method of layer-by-layer splitting of muscular fibers3. Hearts were boiled in running water
for an hour. The direction of fibres of heart ventricles was photographed with the aid of digital video
camera and then used the system of image analysis (Ista-VideoTest, St.Petersburg). Orientation of
muscular fascicles was described relative to the longitudinal heart axis.
The succession of depolarization of heart ventricles in pig was studied by way of multi-channel
synchronous cardioelectrotopography. Cardiopotentials were recorded from the ventricular epicardium
and intramural myocardial layers in pigs anaesthetized by sodium thiopental. Measurements were carried
out by 128-channel system of simultaneous potential registration. On the basis of the obtained
experimental data the three-dimensional patterns of ventricles' depolarization wavefronts were
reconstructed.
3. RESULTS
The criterion of layers separation was different direction of muscular fibers. Three layers were
discovered: superficial (subepicardial), middle (intramural) and deep (subendocardial).
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Superficial layer of fibres is common for both ventricles. Muscular fibres begin from fibrous
skeleton at the base of the heart, spirally twist clockwise and form a curl at the left ventricle apex. Orifice
of pulmonary artery is surrounded with a bundle of fibres attached to the fibrous skeleton. At the ventral
side the superficial fibers of heart ventricles go obliquely from right to left continuously crossing the
anterior interventriclular groove (Fig. 1a). At the dorsal side of ventricles the bundles of muscular fibers
going from the posterior edge of the mitral valve go obliquely from left to right (Fig. 1b). The angle of
inclination relative to the longitudinal heart axis is less. After crossing the posterior interventricular
groove they intersect with fibres, beginning from the posterior edge of tricuspidal valve that go down
from the base to the apex. In the area of the right ventricle apex the subepicardial fibers branch and give
the beginning to the fibres of the deep layer.
Middle layer of circumferential muscles is found in both heart ventricles in pig. We singled out
high-lying and low-lying muscular bundles. High-lying circumferential fibers continuously go along the
dorsal and lateral surface of both ventricles; while from the ventral side of the left ventricle they go deep
forming the interventricular septum.
At the right ventricle apex the circumferential fibres of the middle layer are not available. The
transition of the superficial layers of the right ventricle to the deep ones is observed in this area.
At the dorsal side the low-lying muscular fibres are not continuous. The fibres of the right and left
ventricle form the interventriclular septum and corresponding free walls. In the right ventricle the fibres
go transversally to the longitudinal heart axis. In the left ventricle the low-lying fibres penetrate deep and
twist as a spiral. Spires of the spiral approach in the medial part of the interventriclular septum and go
away in the direction of the apex and the base of the free wall reminding in form the truncated cone
(Fig.2). At the left ventricle apex the middle layer of fibres is also absent.
The fibres of the superficial and low-lying middle layer participate in the formation of the deep
layer. Deep muscular bundles from the side of the interventricular septum are oriented along the
longitudinal heart axis, while from the side of the free wall – obliquely (Fig. 3a). Reaching the ventricle
base the bundles of muscular fibres attach to the fibrous ring of the tricuspidal valve.
In the left ventricle the muscular fibres of the superficial and low-lying middle layers combine
and, twisting inside, give the beginning to the deep fibres. From the apex to the base the fibres of the deep
layer spirally rise relative to the longitudinal heart axis (Fig. 3b) and attach to the fibrous ring of the
mitral valve.
The excitation wave in heart ventricles in pig spreads from multiple foci of depolarization located
in subendocardial and intramural myocardial layers.
From multiple intramural foci of depolarization the excitation wave spreads successively. The
multi-focused excitation of heart ventricles in pig results in depolarization of myocardium during a short
time period. The excitation wave in the intramural layers of heart ventricles spreads not only from
endocardium to epicardium, but also in the reverse direction – from subepicardial and intramural zones of
depolarization to endocardium. Multiple foci of depolarization occurring on subepicardium are caused by
both the excitation wave coming to subepicardium from deeper layers and the activation from the
subepicarial Purkinje fibres. There are plenty of areas of subepicardial activation located on both heart
ventricles.
4. DISCUSSION
Analysis of the direction of superficial fibers has shown that on the anterior surface of pig’s heart
the ventricles are oriented as in man from right to left, but differ by the angle of inclination relative to the
longitudinal heart axis4. On the posterior surface the difference in the direction of fibers is more clearly
expressed: in man the fibres on the posterior surface go continuously from left to right. The difference in
the direction of muscular fibres maybe related to different form of heart ventricles. It is known that in pig
greater part of the heart is formed the left ventricle, which means that the interventricular septum is
located more to the right, while the apex is composed of entirely left ventricular musculature. In man the
left ventricle is less dominant, as a result the interventricular septum occupies a more central position
within the section, while the heart apex is formed of both right and left ventricles1.
It was shown that the middle layer in both ventricles of pig’s heart is represented by the
circumferential fibres. However, we have different information on the availability of middle layer in the
right ventricle. Some authors distinguish a thin sheet of fibres between the superficial and the deep
layers2, others do not discover it5.
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Our investigation has shown that the left ventricle of pig’s heart has more complexly twisted
spires of the spiral, their form reminding the truncated cone, unlike that of man.
We revealed that in pig’s heart deep muscular bundles of the left ventricle and deep fibres of the
free wall of the right ventricle are oriented obliquely from the apex to the base, and from the side of the
interventricular septum of the right ventricle – longitudinally, while in both ventricles of man’s heart deep
fibres are oriented along the longitudinal heart axis2, 5.
The carried out investigation has shown that pig’s heart ventricles are characterized by essential
differences in the orientation of the working fibres of superficial, middle and deep layer as compared to
that in man.
ACKNOWLEDGMENTS
The work is supported by the grant of the scientific school of the academician M.P.Roshchevsky
Sci.Sch.-759.2003.4, Russian Science Support Foundation for I.M.Roshchevskaya.
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