Heart contraction is from
neuronal stimulus ( it can be
by the autonomous
system). In the there are
pacemaker that trigger by
the action potentials
begin the muscle . These
cells are at two special
of the heart: sinoatrial node
(SA ) located in the
portion of the atrium and the
node (AV node)
near the interatrial .

The action potentials by
depolarization of SA node cells
cell to cell the
atria producing atrial contraction.
The depolarization also propagates
the AV node then transmits the
impulse to the
through specialized conduction
of the interventricular (the
bundle of ) and then to Purkinje
fibers of ventricle walls causing
contraction. (The atrial
precedes the ventricular
for blood to the ventricles
before ventricular contraction.)

The of the SA makes
the atria and then the
relax too.