The human heart uses electrical signals to maintain and initiate the regular heartbeat in a living person; incorrect conduction or interference from external sources can lead to mild or serious symptoms depending upon the location of the blockage and how severely conduction is being blocked. Conduction is initiated by the sinoatrial node ("sinus node" or "SA node"), and then travels to the atrioventricular node ("AV node") which also contains a secondary "pacemaker" that acts as a backup for the SA nodes, then to the bundle of His and then via the bundle branches to the point of the apex of the fascicular branches. Blockages are therefore classified based on where the blockage occurs - namely the SA node ("Sinoatrial block"), AV node ("AV block" or AVB), and at or below the bundle of His ("Intra-Hisian" or "Infra-Hisian block" respectively). Infra-Hisian blocks may occur at the left or right bundle branches ("bundle branch block") or the fascicles of the left bundle branch ("fascicular block" or "Hemiblock"). SA and AV node blocks are each divided into three degrees, with second-degree blocks being divided into two types (written either "type I or II" or "type 1 or 2"). The term "Wenckebach block" is also used for second degree type 1 blocks of either the SA or AV node; in addition second-degree blocks type 1 and 2 are also sometimes known as " Mobitz 1" and "Mobitz 2".