The natural pacemaker of the heart. The sinoatrial
(SA) node consists of a cluster of specialized muscle cells within the wall
of the right atrium (upper chamber) of the
heart. Without any external influence, these cells emit electrical impulses
at a rate of 100 per minute, which initiate the contractions (beats) of
the heart. External control by the vagus
nerve reduces the rate to about 70 per minute. Various hormones
and nervous system activities can
affect the node, causing it to emit impulses at a different rate, thus slowing
down or speeding up the heart. See heart
Spread of the impulse
From the sinoatrial nerve, the waves of contraction spread over both atria
and then to the atriaventricular node serving the ventricles.
The spread of excitation over the two atria is fairly slow; the spread over
the ventricles is rapid.