Sinuatrial node
Nodus sinuatrialis
- Eponym: Flack's node; Node of Keith-Flack; Koch's node
Definition
The sinuatrial node (also commonly spelled sinoatrial node, abbreviated SA node or SAN, also called the sinus node) is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm.
The sinoatrial node is a group of cells positioned in the wall of the right atrium just lateral to the junction where the superior vena cava enters the right atrium. The SA node is located in the tissue beneath the myocardium. Its deep aspect abuts cardiac myocytes belonging to the right atrium, while its superficial aspect is covered by adipose tissue. It extends between 1 and 2 cm on the right from the crest of the right auricle and runs posteroinferiorly into the upper part of the terminal groove. SA node fibres are specialized cardiomyocytes that vaguely resemble cardiac myocytes; however, although they possess some contractile filaments they do not contract as robustly. Additionally, SA node fibers are measurably thinner, more tortuous and stain less intensely (with an H&E stain) than cardiac myocytes.
The SA node is richly innervated by parasympathetic nervous system fibers (CN X: vagus nerve) and by sympathetic nervous system fibers (T1-4, spinal nerves).
The SA node receives blood supply from the SA node artery (ramus nodi sinuatrialis). Anatomical dissection studies have shown that this supply may be branch of the right coronary artery in the majority (about 60-70%) of hearts, and a branch of the left coronary artery (usually the left circumflex artery) in about 20-30% of hearts.
