Sinoatrial node

From Wikipedia, the free encyclopedia

Sinuatrial node
Image:Gray493.png
Interior of right side of heart. (SA node not labeled, but region visible at left, near crista terminalis.
Image:Gray501.png
Schematic representation of the atrioventricular bundle of His.
Latin	nodus sinuatrialis
MeSH	Sinuatrial+Node
Dorlands/Elsevier	n_09/12577255

The sinuatrial node (abbreviated SA node or SAN, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiac myocytes. They possess some contractile filaments, though they do not contract.

1 Role as a pacemaker
2 Innervation
3 Blood supply
4 See also
5 External link

[edit] Role as a pacemaker

Although all of the heart's cells possess the ability to generate the electrical impulses (or action potentials) that trigger cardiac contraction, the sinuatrial node is what normally initiates it, simply because it generates impulses slightly faster than the other areas with pacemaker potential. Because cardiac myocytes, like all muscle cells, have refractory periods following contraction during which additional contractions cannot be triggered, their pacemaker potential is overridden by the sinoatrial node.

Cells in the SA node will naturally discharge (create action potentials) at about 70-80 times/minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.

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. These cells form the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum.

[edit] Innervation

The SA node is richly innervated by vagal and sympathetic fibers. This makes the SA node susceptible to autonomic influences.

Stimulation of the vagus nerve causes decrease in the SA node rate (thereby causing decrease in the heart rate).
Stimulation via sympathetic fibers causes increase in the SA node rate (thereby increasing the heart rate).

[edit] Blood supply

In the majority of patients, the SA node receives blood from the right coronary artery, meaning that a myocardial infarction occluding it will cause ischaemia in the SA node unless there is a sufficiently good anastomosis from the left coronary artery. If not, death of the affected cells will stop the SA node from triggering the heartbeat.