sinusknutan

Sinusknutan, also known as the sinoatrial node (SA node), is the heart’s natural pacemaker. It is a small region of specialized cardiac tissue located in the right atrium, near the entrance of the superior vena cava. The node contains pacemaker cells that generate rhythmic electrical impulses and set the pace for heart rate. Impulses from the SA node spread through the atria to the atrioventricular node and then into the His-Purkinje system, coordinating orderly atrial and ventricular contraction. The SA node typically fires at about 60–100 beats per minute in healthy adults, though the rate is modulated by autonomic input and circulating factors.

Anatomically, the SA node consists of pacemaker and transitional cells and is usually supplied by a branch of the right coronary artery, with variation among individuals. Its automaticity arises from spontaneous depolarization during phase 4 of the pacemaker action potential, aided by ionic mechanisms such as the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and the funny current (If).

Regulation occurs through the autonomic nervous system. Sympathetic stimulation increases heart rate via norepinephrine, while parasympathetic stimulation via the vagus nerve decreases rate through acetylcholine. Other factors, including temperature and circulating hormones, can influence pacing.

Clinical significance includes sinus node dysfunction, which can cause bradycardia or tachy-brady syndrome and other rhythm disturbances. In some cases, severe dysfunction requires the implantation of a pacemaker. The discovery and study of the sinus node were pivotal in understanding cardiac conduction and rhythm generation in the early 20th century.