Download Origin and Conduction of the Heart Beat

Origin and Conduction of the Heart Beat
The sinus venosus of lower vertebrates has become incorporated into the right atrial walls and is known
as the
or (sino-atrial node [SA node]) in
mammals. This structure contains a group of nerve cells near the junction of
and
known as the
or
with an intrinsic rhythmic rate of 40 to 60 beats per minute. This is the area of heart beat initiation. A
wave of nervous excitation in the SA node causes the atria to
,
forcing blood through the atrio-ventricular valves [AV valves]. The time for all the left and right
muscles to contract is approximately 0.09 seconds, effectively stimulating all the atrial muscles at the
same time. As this wave of nervous excitation approaches the atrio-ventricular junction, it collects to
form the
-
[
],
located in the
of the heart. From this, fibres pass along the septum forming the atrio-
ventricular bundle or
, which divides into left
and right branches. Each branch gives rise to a network of nervous conducting fibres called
which are made up of cells high in glycogen. Nerve
impulses from the Purkinje fibres pass down the septum separating the left and right ventricles, then up
the sides to the apex. The delay rate from start to finish is approximately 0.06 seconds, effectively
stimulating all the ventricular muscle at the same time.
Instantaneous excitation of both atria and ventricles is important in the proper functioning of
the heart, allowing maximal blood flow. Any errors in this nervous transmission can lead to a 20-30%
decrease in the effectiveness of the ventricles.