The structure indicated is the sinoatrial node

The conduction system of the heart is responsible for the coordinated contraction of the atria and ventricles and is comprised of the following main structures:

• Sinoatrial node
• Atrioventricular node
• Atrioventricular bundle of His
• Right and left bundle branches
• Purkinje fibres

The sinoatrial node (sinoatrial node), often referred to as the SA node is the pacemaker of the heart located in the right atrium and is responsible for generating normal sinus rhythm.

The sinoatrial node is located lateral to the junction where the superior vena cava enters the right atrium. Electrical signals generated from the sinoatrial node spread across the atria to the atrioventricular node, which is located close to the opening of the coronary sinus in the right atrium.

It receives parasympathetic (cranial nerve X – vagus nerve) and sympathetic innervation (T1-4 spinal nerves). Parasympathetic activation decreases sinoatrial firing and thus decreases heart rate. Sympathetic activation does the opposite, causing increase sinoatrial firing and increases heart rate.