General Principles of Anatomy and Cellular Electrophysiology

The electrical activity of the heart is governed by the sinoatrial (SA) node (also known as the node of Keith and Flack) (Fig. 1.1), a microscopic structure located in the right atrium, at the junction between the atrium and the superior vena cava. It is made of pacemaker cells, which are intrinsically capable of generating rhythmic electrical impulses at rates that normally range from 60 to 100 beats per minute (bpm). The electrical current that originates here is propagated to the atria along preformed pathways known as the internodal tracts: the anterior internodal tract (known as Bachmann’s bundle), which includes a branch for the left atrium; the middle internodal tract (Wenckebach’s bundle); and the posterior internodal tract (Thorel’s bundle).