SA node

Sinoatrial node
Heart conduction system, with the sinoatrial node labeled at the top right.
Details
Precursor	Cardiac mesoderm
System	Cardiac conduction system
Artery	Sinoatrial nodal artery
Identifiers
Latin	nodus sinuatrialis
TA98
TH	{{#property:P1694}}
TE	{{#property:P1693}}
FMA	{{#property:P1402}}
Anatomical terminology

The sinoatrial node (SA node), also known as the sinus node, is a group of specialized cardiac muscle cells located in the right atrium of the heart. It is the primary pacemaker of the heart, responsible for initiating the electrical impulses that dictate the heart's rhythm.

Structure

The SA node is a small, ellipsoid structure situated in the upper wall of the right atrium, near the entrance of the superior vena cava. It is composed of pacemaker cells that are capable of generating spontaneous electrical activity. These cells are smaller than typical cardiac muscle cells and have fewer contractile fibers.

Function

The primary function of the SA node is to generate electrical impulses that propagate throughout the heart, initiating the heartbeat. The SA node sets the pace for the heart rate, typically between 60 to 100 beats per minute in a resting adult. This intrinsic rhythm is modulated by the autonomic nervous system:

• The sympathetic nervous system increases the heart rate by releasing norepinephrine, which acts on beta-1 adrenergic receptors in the SA node.
• The parasympathetic nervous system, primarily through the vagus nerve, decreases the heart rate by releasing acetylcholine, which acts on muscarinic receptors.

Blood Supply

The SA node receives its blood supply from the sinoatrial nodal artery, which can arise from either the right coronary artery or the left circumflex artery, depending on the individual's coronary circulation dominance.

Clinical Significance

Dysfunction of the SA node can lead to various cardiac arrhythmias, such as:

• Sinus bradycardia: A slower than normal heart rate.
• Sinus tachycardia: A faster than normal heart rate.
• Sick sinus syndrome: A group of arrhythmias caused by malfunction of the SA node.

In some cases, an artificial pacemaker may be required to maintain an adequate heart rate if the SA node is unable to function properly.

Also see

• Atrioventricular node
• Cardiac conduction system
• Electrocardiogram
• Heart rate
• Pacemaker (artificial)