Sinoatrial block Sinoatrial block A sinoatrial block is a disorder in the normal rhythm of the heart, known as a heart block, that is initiated in the sinoatrial node. The initial action impulse in a heart is usually formed in the sinoatrial node (SA node) and carried through the atria, down the internodal atrial pathways to the atrioventricular node (AV) node. In normal conduction, the impulse would travel across the bundle of His (AV bundle), down the bundle branches, and into the Purkinje fibers. This would depolarize the ventricles and cause them to contract. In an SA block, the electrical impulse is
Sinoatrial node superior vena cava and the inferior vena cava. The cells of the SA node are spread out within a mesh of connective tissue, containing nerves, blood vessels, collagen and fat. Immediately surrounding the SA node cells are paranodal cells. These cells have structures intermediate between that of the SA node cells and the rest of the atrium. The connective tissue, along with the paranodal cells, insulate the SA node from the rest of the atrium, preventing the electrical activity of the atrial cells from affecting the SA node cells. The SA node cells are smaller and paler than the surrounding