Sinusknotens

The Sinusknoten, also known as the sinoatrial node (Sinusknoten in German), is the heart’s natural pacemaker. It is a small cluster of specialized cardiac cells located in 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 node’s cells exhibit automaticity, meaning they depolarize spontaneously to trigger action potentials. The pacemaker potential is driven by gradual depolarization during phase 4, largely due to the funny current (If) through HCN channels, combined with calcium entry through L-type and T-type channels. This automatic firing sets the intrinsic heart rate, which on a resting adult typically ranges from about 60 to 100 beats per minute. The impulse generated in the Sinusknoten propagates through atrial myocardium via interatrial pathways to the atrioventricular (AV) node, providing a brief delay before ventricular activation.

Autonomic nervous system regulation allows rapid adaptation of heart rate. Sympathetic stimulation (norepinephrine) increases the rate and the slope of phase 4 depolarization, while parasympathetic input (vagal, acetylcholine) slows the rate by enhancing potassium currents and reducing calcium entry.

Clinical relevance includes sinus rhythm as the normal baseline. Sinus node dysfunction can cause bradycardia, pauses, or tachycardia-bradycardia syndrome and may require monitoring or pacing. If the Sinusknoten fails to pace properly, subsidiary pacemakers in the AV node or other conducting tissues can assume control, producing escape rhythms.