conduction system cell area called
the 
sinoatrial
node (SA node). It is horseshoe shaped and located just
beneath
the epicardium. 
On the posterior part of the right atrial wall, adjacent to
the junction of the superior vena cava and the right atrium, is a
specialized conduction system cell area called
the
sinoatrial
node (SA node). It is horseshoe shaped and located just
beneath
the epicardium.
Although the depolarization of the SA node is the first step of the cardiac cycle, it does not produce enough energy to be recorded by the electrocardiograph.
The depolarization wave spreads down these specialized
cardiac conduction system fibers over the atria by way of
internodal and interatrial pathways known as 
Bachmann's
bundle.
These specialized fibers transmit impulses six times faster than
do ordinary cell-to-cell interconnections. These pathways trigger
the contraction of other cells in the atrium more rapidly than if
the depolarization wave followed the cell-to-cell
interconnections.
The SA node 
sets the heart rate at 72 beats per
minute
rather than the 60 beats per minute intrinsic to the atria alone
or the 20-40 beats per minute intrinsic to the ventricals alone.