Why the SA Node is the pacemaker of the heart
SA Node discharges more rapidly with the faster spread of impulse to the rest of the heart via conducting tissue of heart see below. Due to this rest of the tissue can not take over as a pacemaker. In absence of impulse coming from SA node then subsidiary node viz AV node can become the pacemaker of the heart.
Pacemakers of the heart (SA node and AV Node)have prepotential, which declines after every impulse thus it triggers the next impulse.
Following hyperpolarization from previous impulse Ih channel (hyperpolarization channel) also called funny channel If open, causing Na influx. This forms the first part of membrane prepotential. When it reaches threshold ICa channel open. both T(transient) and L(long-lasting). This opening of T channel forms prepotential. L channel opening forms actual impulse
Conducting tissue of the heart.
1. SA Node
2. AV Node
3. Inter nodal atrial pathways
4. Bundle of His
5. Purkinje fibers
SA node (Sinoatrial node)
located at junction of Superior vena cava and right atrium
AV Node(atrioventricular node)
Located at right posterior portion of interatrial septum
Inter Nodal pathways
Three in number anterior, middle (a tract of Wenkebach), and posterior (a tract of Thorel)
Bachman bundle is branch anterior pathway connecting the right and left atrium