The automatic rhythmicity of the heart is its ability to contract spontaneously and at a regular rate. The impulse of contraction of the heart originates in the sino atrial node (SA node) situated in the right atruim close to the point of entry of the vena cava. SA node consists of a small number of diffusely oriented cardiac fibres, possessing few myofibrils, and a few nerve endings from the autonomic nervous system.It is also known as the pace maker of the heart.
Atrioventricular Node or AV node: It is compact half oval mass of myogenic fibres which lies in the posterior septal wall of right atrium near the opening of coronary sinus. It receives impulse from SA node and transmits it to ventricles. Direct transmission of impulse from SA node to ventricles is not possible due to the absence of continuity of muscles between atria and ventricles and the presence of fibrous connective tissue of atrioventricular septum. Transmission from SA node stimulates AV node to generate fresh impulse for ventricles. Therefore, AV node is also called pacesetter.
SA node vs AV node
|SA node (Sinoatrial node)||AV node (Atrioventricular node)|
|Sinoatrial node is located in superior lateral wall opening of superior vena cava.||AV node is present in the posterior septal wall of right of right atrium near the atrium near the opening of coronary sinus.|
|It is longer.||It is shorter.|
|SA node is ellipsoid flattened.||It is half oval in outline|
|It generates the cardiac impulse.||It relays and intensifies the cardiac impulse.|
|SA node transmits impulse directly to the two atria.||Atrioventricular node carries the impulse to the two ventricles through AV bundle, its branches and terminal strands.|
|It is influenced by autonomic nervous system.||It is influenced by SA node.|
|SA node acts as pacemaker.||AV node functions as pacesetter.|
|Function: Autorhythmic fibres initiate cardiac action potentials, which set basic pace for heart rate and conduct throughout both atria.||Receives action potentials from SA node and passes them to atrioventricular(AV) bundle|