Sinoatrial node
Low magnification micrograph of a sinoatrial node - central on image and surrounding the (sinuatrial) nodal artery (on lumen in the image), a branch of the right coronary artery - and its surrounding tissue. The SA node abuts cardiac myocytes (of the right atrium) on its deep aspect (left of image) and adipose on its superficial (epicardial) aspect (right of image). H&E stain.
Interior of right side of heart. (SA node not labeled, but region visible at left, near crista terminalis.
Latin	nodus sinuatrialis
Artery	sinuatrial nodal artery
MeSH	Sinoatrial+Node

The sinoatrial node (also commonly spelled 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, and thus the generator of sinus rhythm. 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. Though they possess some contractile filaments, they do not contract.

It was first described in 1907 by Arthur Keith and Martin Flack.^[1]

[edit] Role as a pacemaker

Although all of the heart's cells have the ability to generate the electrical impulses (or action potentials) that trigger cardiac contraction, the sinoatrial node 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.

In the absence of extrinsic neural and hormonal control, cells in the SA node, situated in the upper right corner of the heart, will naturally discharge (create action potentials) at about 60-100 beats/minute.^[2] 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.^{[citation needed]} These cells form the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum. If the AV node also fails, Purkinje fibers are capable of acting as the pacemaker. The reason Purkinje cells do not normally control the heart rate is that they generate action potentials at a lower frequency than the AV or SA nodes.

[edit] Histology

The sinoatrial node is submyocardial at the lateral aspect of the junction of the superior vena cava and right atrium. Its deep aspect abuts cardiac myocytes belonging to the right atrium. Its superficial aspect is covered by adipose tissue. The SA node fibres vaguely resemble cardiac myocytes; however, they are thinner, squiggly and stain less intensely (on H&E) than cardiac myocytes.

[edit] Innervation

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). This makes the SA node susceptible to autonomic influences.

• Stimulation of the vagus nerves (the parasympathetic fibers) causes a decrease in the SA node rate (thereby decreasing the heart rate). Parasympathetic fibers cannot change the force of contraction, however,because they only innervate the SA node and AV node (which control heart rate only)***(NOTE:This is NOT true.This study http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2278718/ shows how parasympathetic nervous system,through the action of vagus nerve,definitely exerts a negative inotropic effect upon the heart)
• Stimulation via sympathetic fibers causes an increase in the SA node rate (thereby increasing the heart rate and force of contraction). Sympathetic fibers can increase the force of contraction because in addition to innervating the SA and AV nodes, they innervate the atria and ventricles themselves.

[edit] Blood supply

In the majority of individuals, 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.

[edit] Sinus node dysfunction

Sinus node dysfunction describes an irregular heartbeat caused by faulty electrical signals of the heart. When the heart's sinoatrial node is defective, the heart’s rhythms become abnormal – either too fast, too slow, or a combination.^[3]

[edit] Additional images

Heart; conduction system

Schematic representation of the atrioventricular bundle

[edit] See also

• Cardiology
  • Cardiac pacemaker
  • Electrical conduction system of the heart
• Heart block
• Sinus bradycardia
• Sinus tachycardia

[edit] References

[edit] External links

• sinuatrial+node at eMedicine Dictionary
• SUNY Figs 20:06-01 - "The conduction system of the heart."
• Diagram at gru.net
• thoraxlesson4 at The Anatomy Lesson by Wesley Norman (Georgetown University) (thoraxheartinternalner)
• http://www.healthyheart.nhs.uk/heart_works/heart03.shtml