Final answer:
Sinus rhythm is the normal, rhythmic beating of the heart, established by the pacemaker cells in the sinoatrial node. Electrical signals cause contraction of the atria and ventricles. Autonomic nervous system and endocrine system also influence heartbeat.
Step-by-step explanation:
The normal, rhythmical beating of the heart is called sinus rhythm. It is established by the heart's pacemaker cells in the sinoatrial node. Electrical signals from the pacemaker cells travel to the atria and cause them to contract.
Then the signals travel to the atrioventricular node and from there to the ventricles, causing them to contract. Electrical stimulation from the autonomic nervous system and hormones from the endocrine system can also influence heartbeat.