Anatomy · Heart & blood vessels

Sinoatrial node

Discovered 1907 by Keith and Flack

Heart · natural pacemaker

A small crescent of specialised cardiac muscle cells in the wall of the right atrium, at the junction of the superior vena cava and the atrium near the upper end of the crista terminalis. It depolarises spontaneously faster than any other part of the conducting system, so it sets the heart rate, normally 60–100 beats a minute at rest. It was discovered in 1907 by Arthur Keith and Martin Flack.

How to spot them

▸About 1–2 cm long, banana- or crescent-shaped, just under the epicardium at the top of the right atrium
▸Lies at the upper end of the sulcus terminalis, where the superior vena cava enters
▸Supplied by the sinoatrial nodal artery, a branch of the right coronary artery in about 60% of people
▸First in the chain: SA node, atrial muscle, AV node, bundle of His, bundle branches, Purkinje fibres

What to know

The natural pacemaker of the heart, in the wall of the right atrium where the upper great vein enters

Sinoatrial node.

Discovered in 1907 by an anatomist and a medical student in a mole's heart, then found in humans

Sinoatrial node — Arthur Keith and Martin Flack.

Its cells depolarise spontaneously, setting a resting rate of about 60 to 100 beats a minute

Sinoatrial node — automaticity, pacemaker potential.

Stimulation of the vagus nerve slows it, while sympathetic nerves speed it up

Sinoatrial node — resting vagal tone keeps the rate near 70.

Faulty function of this structure, sick sinus syndrome, is treated by implanting an artificial pacemaker

Sinoatrial node.

The impulse it fires reaches the atrioventricular node after a brief delay, then travels down the bundle of His

Sinoatrial node — start of the conduction pathway.

Its blood comes from a branch of the right coronary artery in about 60% of people, the rest from the circumflex

Sinoatrial node — the sinoatrial nodal artery.

Key ideas

Hierarchy of pacemakers: SA node about 60–100/min, AV node about 40–60, Purkinje fibres about 20–40. The fastest wins.
The impulse spreads through the atria by ordinary myocytes and internodal pathways to the AV node, where it is delayed about 0.1 s before ventricular contraction.

Quick-fire facts

•Location: superior part of the crista terminalis, at the junction of the SVC and right atrium.
•Discovered by Arthur Keith and Martin Flack in 1907.
•Intrinsic rate is about 100 per minute; parasympathetic (vagal) tone slows it to about 60–80 at rest.
•Blood supply: SA nodal artery, from the right coronary artery in roughly 60% and the circumflex in about 40%.
•Sinus rhythm on an ECG means each P wave is generated by the SA node.
•The AV node, described by Sunao Tawara and Ludwig Aschoff in 1906, lies in the triangle of Koch.
•Sick sinus syndrome and sinus arrest are common indications for a permanent pacemaker.
•Adenosine transiently blocks the AV node; sympathetic stimulation acts through beta-1 receptors.

Drill it — 7 cards

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Sources

OpenStax — 19.2 Cardiac Muscle and Electrical Activity — Anatomy and Physiology 2e
TeachMeAnatomy — The Conducting System of the Heart