Sensitive tissue in the right atrium wall that begins the heartbeat:

Sinoatrial node

The sinoatrial node is found at the upper part of the right atrium's wall and it has a very vital role in regulating heartbeat. The cells in the sinoatrial node are specialized to generate electrical impulses, so this structure in the heart acts as a natural pacemaker. The impulses generated from the sinoatrial node then travel on to the atrioventricular node, from where ventricular and atrial contractions are carried out depending on how much of the signal the atrioventricular node lets through.