Jaarboek no. 89. 2010/2011 - Koninklijke Maatschappij voor ...

Natuurkundige voordrachten I Nieuwe reeks 89
Ontwikkeling van het bouwplan van het hart
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to generate and conduct the depolarizing impulse.
In a seminal account Davies, Anderson and Becker,
in 1983, have scrutinized many anatomical, developmental,
clinical, as well as historical aspects of the
conduction system of the heart. The conduction
system is defined as the system that initiates and
conducts the sinus impulse. It would comprise sinus
node, atrioventricular node, atrioventricular bundle,
bundle branches and their ramifications. The
myocardium of the atrial and ventricular chambers
has not been classified as component parts of the
conduction system. This strict dichotomy between
conduction system and chamber myocardium was
and still is the ruling view.
The cardiac electrical impulse is generated in the
sinus node, rapidly propagated through the atrial
myocardium towards the atrioventricular node,
where the propagation of the depolarizing impulse
is delayed. The impulse is then rapidly propagated
via the atrioventricular bundle, bundle branches
and their ramifications, finally resulting in the fast
depolarization of the ventricular myocardium.
The whole sequence of cardiac electrical activity
can be registered by ECG, which explicitly includes
the atrial and ventricular chamber myocardium as
fast-conducting elements. A curious aspect is that,
although all myocardial components are involved
in the sequence of electrical activity resulting in
the ECG, some components are dubbed conduction
system and others not. Following the conventional
dichotomy mentioned above, the slow conducting
nodal tissues and the fast conducting bundle
branches belong to the conduction system and the
fast-conducting myocardium of the chambers not.
In fact the force-producing, but well-conducting
myocardium of the atrial and ventricular chambers
are excluded from the conduction system. Although
a tenable point of view, the subdivision between
conduction system and chamber myocardium is
not intrinsically logically consistent. This is particularly
confusing in the developing heart in which the
separate structures are not recognizable, and yet an
ECG can be derived.
Admittedly, the other side of the coin is considering
the entire heart a single conduction system,
which is true but not very useful either. A preferred,
but not realistic option would be banning the word
conduction system and just indicating what we
mean, i.e. nodal tissues, atrioventricular bundle,
bundle branches, their ramifications, and atrial and
ventricular chamber myocardium.
In view of the above-mentioned inconsistencies
in terminology, it may not be surprising that the
cardiac conduction system and its development in
particular, have always been surrounded by controversy.
Much of this controversy is in fact confusion
that rests in the perception evoked by the words
‘conduction system’, ‘nodes’ and ‘chamber (working)
myocardium’ that intuitively are related to conduction,
pacemaking and force production, respectively.
These contentions are not entirely true.
Paraphrasing Becker’s and Anderson’s statement
of 1984: ‘cardiac embryology: a help or a hindrance?’,
cardiac anatomy has proven to be a hindrance rather
than a help in understanding normal cardiac development.
Moreover, the whole field has been mystified
by a jungle of terms that range from ‘the cardiac specialized
tissues’ to ‘the conducting tissues’ as if nodal
cells would be more specialized and conduct faster
than cells of the chamber myocardium. This confusing
terminology has deleteriously distracted attention
from the true understanding of cardiac design and its
evolutionary and developmental origin.
In this chapter we have tried to present in simple
terms what we regard as the proper biological context
for the essentialities of cardiac design, for all
one hears about the ‘cardiac, primitive, specialized
conducting tissues’.
Simple circuitries
The circulatory system is made of muscle pumps
and transporting vessels. In practice nature uses
two arrangements to make those muscle-pumping
devices. One version is the one also used by the
intestine. It uses peristalsis as the driving force.
The other version is the one present in adult vertebrates
which has developed chambers and one-way
valves (see next section). In the peristaltic version a
wave of contractions runs along the muscle mantle
that enfolds a blood vessel, and pushes ahead the