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
Orbital node terms αναβιβάζων and καταβιβάζων were also used for the ascending and descending nodes, giving rise to the English words "anabibazon" and "catabibazon". For the orbit of the Moon around Earth, the plane is taken to be the ecliptic, not the equatorial plane. The gravitational pull of the Sun upon the Moon causes its nodes to gradually precess westward, completing a cycle in approximately 18.6 years. Orbital node An orbital node is either of the two points where an orbit intersects a plane of reference to which it is inclined. A non-inclined orbit, which is contained in the reference plane, has