The normal sinus rhythm is the rhythm and rate of impulses generated by certain muscle cells as a signal to the upper heart chambers (atria) to contract.
The heart contains a cluster of specialized muscle cells that act as the heart's natural pacemaker. This cluster is called the sinoatrial node, or SA node. The SA node generates the electrical signals that cause the upper heart chambers (atria) to contract. At rest, an adult's SA node normally sends about 60 to 100 impulses per minute.