The sinoatrial node is the impulse-generating tissue located in the right atrium of the
heart, and thus the generator of normal sinus rhythm.
Structure The sinoatrial node is a group of cells positioned
on the wall of the right atrium, near the entrance of the superior vena cava.
These cells are specialized cardiomyocytes.
Though they possess some contractile filaments, they do not contract robustly.
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 measurably thinner,
more tortuous and stain less intensely than cardiac myocytes.
Innervation The SA node is richly innervated by parasympathetic
nervous system fibers and by sympathetic nervous system fibers.
This unique anatomical arrangement makes the SA node susceptible to distinctly paired and
opposed autonomic influences.
Stimulation of the vagus nerves causes a decrease in the SA node rate.
Parasympathetic fibers cannot change the force of contraction, however, because they only
innervate the SA node and AV node***(NOTE: This statement is not entirely accurate.
The following study: http:www.ncbi.nlm.nih.govarticles demonstrates how the parasympathetic nervous
system, through the action of vagus nerve, exerts a negative inotropic effect upon the
heart) Stimulation via sympathetic fibers causes
an increase in the SA node rate.
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.
Blood supply The SA node receives blood supply from the
SA node artery.
Anatomical dissection studies have shown that this supply may be branch of the right coronary
artery in the majority of hearts, and a branch of the left coronary artery in about 20-30%
of hearts.
Rarer variants may include blood supply from both right and left coronary arteries or two
branches of the right coronary artery.
Function
Pacemaking Although all of the heart's cells have the
ability to generate the electrical impulses that trigger cardiac contraction, the sinoatrial
node normally initiates it, simply because it generates impulses slightly faster than
the other areas with pacemaker potential.
Cardiomyocytes, like all muscle cells, have refractory periods following contraction during
which additional contractions cannot be triggered; their pacemaker potential is overridden by
the sinoatrial or atrioventricular nodes.
In the absence of extrinsic neural and hormonal control, cells in the sinoatrial node, situated
in the upper right corner of the heart, will naturally discharge upwards of 100 beats/minute.
Because the sinoatrial node is responsible for the rest of the heart's electrical activity,
it is sometimes called the primary pacemaker.
Clinical significance 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.
Occlusion of the arterial blood supply to the SA node can therefore cause ischaemia
and cell death in the SA node.
This can disrupt the electrical pacemaker function of the SA node, and can result in
sick sinus syndrome.
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, 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.
History The SAN was first described in 1907 by Arthur
Keith and Martin Flack.
Additional images
See also This article uses anatomical terminology;
for an overview, see anatomical terminology.
Cardiology Cardiac pacemaker
Electrical conduction system of the heart
Heart block Sinus bradycardia
Sinus tachycardia References
External links sinuatrial+node at eMedicine Dictionary
Anatomy figure: 20:06-01 at Human Anatomy Online, SUNY Downstate Medical Center - "The
conduction system of the heart."
Diagram at gru.net thoraxlesson4 at The Anatomy Lesson by Wesley
Norman http:www.healthyheart.nhs.ukheart03.shtml