The sinuatrial node of the heart is the first component of the conducting system of the heart. It is the pacemaker for all of the contractile tissue of the heart. It develops at the junction of the primitive atrium of the heart with the absorbed part of the sinus venosus. Therefore, it is sited in the right atrium along a line between the base of the right auricle and junction with the superior vena cava to the superior part of the crista terminalis.
Typically, it is 1-2 cm long and 0.3-0.5 cm wide. Its shape has variously been described as elliptical and semi-circular. It is sited within the myocardial cell layer, but it is further from the the endocardium than epicardium.
Histologically, the centre of the SAN is characterised by stellate or P-cells. These make multiple contacts with surrounding cells and are characterised by minimal organelles and contractile machinery within their cytoplasm. The surrounding cells show features akin to both stellate cells and contractile myocytes - these are transitional cells. The artery of the sinuatrial node traverses the node; it is purported that it may serve a physiological function by conveying aortic pressure to nodal tissue.
The sinuatrial node initiates impulses at a resting frequency in adults of about 70/minute. Its rate of depolarisation can be modified by its neuronal contacts - sympathetic nerves are positively chronotropic, parasympathetic are negatively chronotropic.
The wave of depolarisation of the sinuatrial node passes to the atrioventricular node by either direct conduction through the atria, conduction through specialised internodal pathways, or a combination of both routes.
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