1. SAN: It is also known as Sino Atrial Node. It is present in the right atrium near the opening of superior Vena Cava. It is also known as Pacemaker because electrical impulse initiates from this part of the heart.

2. AVN: It is also known as Atrio Ventricular Node. It is present in the right atrium near the base of the inter atrial septum. It is also known as Pacesetter. It causes a delay between the atrial and ventricular excitation. This delay ensures that the blood is completely pumped into the ventricles by the atria before the ventricles contract.

3. Bundle of His: It is a part of the electrical conduction system. It transmits impulses received from the AV node to ventricles.

4. Purkinje Fibres: The branching of bundle of His give rise to small fibres known as Purkinje fibres. These fibres transmit impulse to the ventricular muscles.