<Tr> <Th> MeSH </Th> <Td> D002037 </Td> </Tr> <Tr> <Td_colspan="2"> (edit on Wikidata) </Td> </Tr> <P> A bundle branch block is a defect of the bundle branches or fascicles in the electrical conduction system of the heart . </P> <P> The heart's electrical activity begins in the sinoatrial node (the heart's natural pacemaker), which is situated on the upper right atrium . The impulse travels next through the left and right atria and summates at the atrioventricular node . From the AV node the electrical impulse travels down the bundle of His and divides into the right and left bundle branches . The right bundle branch contains one fascicle . The left bundle branch subdivides into two fascicles: the left anterior fascicle and the left posterior fascicle . Other sources divide the left bundle branch into three fascicles: the left anterior, the left posterior, and the left septal fascicle . The thicker left posterior fascicle bifurcates, with one fascicle being in the septal aspect . Ultimately, the fascicles divide into millions of Purkinje fibres, which in turn interdigitise with individual cardiac myocytes, allowing for rapid, coordinated, and synchronous physiologic depolarization of the ventricles . </P>

What is a bundle block of the heart