The normal sinus rhythm is the 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, the SA node normally sends 60 to 100 impulses per minute (the normal sinus rhythm).