sinoatrial (SA) node

(noun)

impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm

Related Terms

Examples of sinoatrial (SA) node in the following topics:

  • Electrical Events

    • Cardiac contraction is initiated in the excitable cells of the sinoatrial node by both spontaneous depolarization and sympathetic activity.
    • The heart's activity is dependent on the electrical impulses from the sinoatrial (SA) node, and the atrioventricular (AV) node, which form the intrinsic conduction system of the heart.
    • The SA node is a bundle of nerve cells located on the outer layer of the right atria.
    • The SA node impulses also travel to the AV node, which stimulates ventricular contraction.
    • The normal firing rate in the AV node is lower compared to the SA node because it slows the rate of neural impulses.

  • The Cardiac Cycle

    • The internal pacemaker starts at the sinoatrial (SA) node, which is located near the wall of the right atrium.
    • Electrical charges spontaneously pulse from the SA node, causing the two atria to contract in unison.
    • The pulse reaches a second node, the atrioventricular (AV) node, between the right atrium and right ventricle, where it pauses for approximately 0.1 seconds before spreading to the walls of the ventricles.
    • The signal is initiated at the sinoatrial valve.
    • Summarize the caridac cycle and explain the role of the SA node and the AV node in regulating the its rhythm

  • Agonists, Antagonists, and Drugs

    • ACh binds to muscarinic receptors (M2) that are found principally on cells comprising the sinoatrial (SA) and atrioventricular (AV) nodes.
    • Increases in vagal activity to the SA node decreases the firing rate of the pacemaker cells by decreasing the slope of the pacemaker potential decreasing heart rate.
    • Similar electrophysiological effects also occur at the AV node.
    • However, in this tissue, these changes are manifested as a reduction in impulse conduction velocity through the AV node.
    • Working as a nonselective muscarinic acetylcholinergic antagonist, atropine increases firing of the sinoatrial node (SA) and conduction through the atrioventricular node (AV) of the heart, opposes the actions of the vagus nerve, blocks acetylcholine receptor sites, and decreases bronchial secretions.

  • Electric Activity in the Heart

    • Electric energy stimulating the heart occurs in the sinoatrial node, the heart's pacemaker, and is transmitted partially by Perkinje fibers.
    • The sinoatrial node (also commonly spelled sinuatrial node, abbreviated SA node) is the impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm.
    • If the SA node does not function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become the heart's pacemaker.
    • In contrast, the SA node outside of parasympathetic control can fire a rate of almost 100 beats per minute.
    • High magnification micrograph of sinoatrial node tissue and an adjacent nerve fiber.

  • Arrhythmia

    • These cells are found in the conduction system of the heart and include the SA node, AV node, Bundle of His, and Purkinje fibers.
    • The sinoatrial node is a single specialized location in the atrium which has a higher automaticity (a faster pacemaker) than the rest of the heart and, therefore, is usually responsible for setting the heart rate and initiating each heart beat.
    • In normal electrical activity, each heart beat originates as an electrical impulse from a small area of tissue in the right atrium of the heart called the sinus node or sino-atrial node or SA node.
    • Electrical mpulse arising in SA node, traversing atria to AV node, then entering ventricle.
    • Rhythm originating at or above AV node constitutes supraventricular tachycardia (SVT).

  • Cardiac Output

    • The heart rate is determined by spontaneous action potential generation in the sinoatrial node and the conduction in the atrioventricular node.
    • Sympathetic nervous system activation will stimulate the SA and AV nodes to increase the heart rate, which will increase cardiac output.
    • Parasympathetic nervous system activation will conversely act on the SA and AV nodes to decrease the heart rate, which will decrease cardiac output.
    • For the SA node, the rate of depolarization is altered, while the AV node's rate of conduction is altered by these autonomic nerve stimulation.

  • Anatomy of the Heart

    • The sino-atrial (SA) and atrioventricular (AV) nodes are bundles of nerve fibers that form this conduction system.
    • These nodes can send impulses to the heart without central nervous system stimulation, but may be influenced by nervous stimulation to alter heart rate.
    • Cardiac muscle tissue is distinct from skeletal or smooth muscle because it pumps involuntarily based on conduction from the AV and SA nodes.

  • Fibrous Skeleton of the Heart

    • It also surrounds the coronary arteries and AV node.
    • It also surrounds the coronary arteries and AV node.
    • The SA node lies on the top of the heart, while the AV node is located at the center of the fibrous skeleton, which acts as the only path through which electricity can flow through the heart.
    • The separation allows the AV node and AV bundle to delay the wave of depolarization, such that the atria can contract and assist in ventricular filling before the ventricles themselves depolarize and contract.

  • Layers of the Heart Walls

    • During ventricular contraction, the wave of depolarization from the SA and AV nodes moves from within the endocardial wall through the myocardial layer to the epicardial surface of the heart.
    • The Purkinje fibers are located just beneath the endocardium and send nervous impulses from the SA and AV nodes outside of the heart into the myocardial tissues.

  • Electrocardiogram and Correlation of ECG Waves with Systole

    • The P wave is the first wave on the ECG because the  action potential for the heart is generated in the sinoatrial node, located on the atria, which sends action potentials directly through the Bachman's bundle to depolarize the atrial muscle cells.
    • The QRS complex represents action potentials moving from the AV node, through the bundle of His and left and right branches and Purkinje fibers into the ventricular muscle tissue.
    • It generally occurs with myocardial infarcations and heart failure, and is thought to be caused by action potentials that re-enter the AV nodes from the muscle tissue and induce rapid, irregular, weak contractions of the heart that fail to pump blood althogether.