sinusknoten

The Sinusknoten, or SA node, is the heart’s natural pacemaker. It comprises specialized pacemaker cells located in the wall of the right atrium near the opening of the superior vena cava. Its primary role is to generate rhythmic electrical impulses that initiate each heartbeat.

The SA node produces spontaneous action potentials through automaticity, with cells gradually depolarizing during diastole (phase 4). The impulses originate here and propagate through the atrial myocardium via gap junctions to the atrioventricular node and the rest of the conduction system, coordinating atrial contraction and serving as the origin of the cardiac rhythm.

Cellular activity is supported by pacemaker currents, including the 'funny' current (If) mediated by HCN channels, which contribute to the diastolic depolarization. The intrinsic firing rate of the SA node in adults typically ranges from about 60 to 100 beats per minute, though this rate is continually modulated by the autonomic nervous system.

Autonomic regulation serves to adapt heart rate to physiological needs. Sympathetic stimulation increases the rate and enhances conduction through β-adrenergic pathways, while parasympathetic (vagal) activity decreases the rate, primarily by acetylcholine acting on muscarinic receptors.

Clinical relevance includes maintenance of normal sinus rhythm; dysfunction can lead to sinus bradycardia, sinus tachycardia, or sick sinus syndrome, which may require clinical evaluation or intervention. Historically, the SA node was described in the early 20th century by Keith and Flack.