Biodynamics of vertebrate circulation. Electrical activity
The vertebrate heart is myogenic (rhythmic contractions
are an intrinsic property of the cardiac muscle cells
themselves). Pulse rate varies widely in different vertebrates,
but it is generally higher in small animals, at least
in birds and mammals. Each chamber of the heart has
its own contraction rate. In the frog, for example,
the sinus venosus contracts fastest and is the pacemaker
for the other chambers, which contract in sequence and
at a decreasing rate, the conus being the slowest. In
birds and mammals, where the sinus venosus is incorporated
into the right atrium at the sinoauricular node, the
latter is still the pacemaker and the heartbeat is initiated
at that point. Thus, the evolutionary history of the
heart explains the asymmetrical pattern of the heartbeat.
In the frog each contraction of the heart begins with
a localized negative charge that spreads over the surface
of the sinus venosus. In lower vertebrates, the cardiac
muscle cells themselves conduct the wave of excitation.
In birds and mammals, however, special conducting fibres
(arising from modified muscle cells) transmit the wave
of excitation from the sinoauricular node to the septum
between the auricles, and then, after a slight delay,
down between and around the ventricles. The electrical
activity of the heart can be recorded; the resulting
pattern is called an electrocardiogram.
