The rhythm of heart is its ability to contract spontaneously at a regular rate. S.A node generates electrical current which causes heart to contract at regular intervals and sets the hate rate of heart from 60-100 bpm. S.A node is a primary pacemaker of the heart. A.V node is present just below the S.A node and electrically connects the atrial and ventricle chambers. S.A node and A.V node both are instrumental in cardiac conduction, which powers the cardiac cycle by firing electrical impulses.
What is S.A Node?
S.A node or sinoatrial node is known as primary pacemaker of the heart and it is responsible for the initiation of the cardiac cycle. It generates spontaneous electrical impulses which travels through the conducting system of the heart and enables heart to contract. The rate at which these electrical impulses fires from S.A node is always regular in a normal heart. The rate of these electrical impulses is due to the nerves innervating the sinoatrial node. S.A node is located in the right atrium lateral to the sinus venarum, the junction where superior vena cava enters into the right atrium. S.A node is embedded in the myocardium. S.A node is innervated by fibers of parasympathetic nervous system (10th cranial nerve. CN X: Vagus nerve) and some fibers of sympathetic nervous system (T1-T4 spinal nerves). Vagus nerve stimulation decreases the rate of S.A node and stimulation of sympathetic nervous system increases the rate of S.A node. The rate of electrical impulses from S.A node will naturally discharge upward of 100 beats/minute.
What is A.V Node?
A.V node or atrioventricular node is known as pacesetter of the heart and it receives impulses from S.A node. A.V node is also part of electrical conduction system of the heart that coordinates its activity with S.A node. It connects atria and ventricle electrically. A.V node is an oval mass of myogenic fibers between atria and ventricles of the heart, lies in posterioinferior region of interatrial septum near the opening of coronary sinus and conducts normal electrical impulses from atria to the ventricles. A.V node sets the rhythm of the heart contractions. The normal firing rate of A.V node without stimulation is 40-60 times/minute. There is a delay of 0.12s in A.V node. This delay is very important because it ensures that the atria have ejected all the blood into the ventricles before the ventricles start to contract. A.V node have a unique property that is decremental conduction in which more frequently the A.V node is stimulated the slower it conducts. There are two different pathways through A.V conduction take place: the first “pathway” has slow conduction velocity with shorter refractory period and the second “pathway” has faster conduction velocity with longer refractory period.
Key Differences between SA Node and AV Node
- SA node is called as pacemaker of the heart while A.V node is called as pacesetter of the heart.
- SA node is located in the right atrium lateral to sinus venarum, the junction where superior vena cava enters into the right atrium while A.V node is located in posterioinferior region of interatrial septum near the opening of coronary sinus.
- SA node generates the cardiac impulses while A.V node intensifies and relays the cardiac impulses that are being generated by S.A node.
- SA node is stimulated by both parasympathetic and sympathetic nervous systems while A.V node is stimulated by S.A node.
- Firing rate of S.A node is 60-100 times per minute while firing rate of A.V node is 40-60 times per minute.