Sinuatrial node

The sinoatrial (SA) node is the primary pacemaker of the heart, a specialized structure responsible for initiating each heartbeat through spontaneous electrical impulses. It is located in the terminal crest at the confluence of the cranial vena cava, sinus venarum cavarum, and auricular orifice.

The SA node is a three-dimensional intramural fibrotic structure with a crescent shape, measuring approximately 8 to 21.5 mm in length along its vertical axis in humans. Although the intrinsic anatomical length is only 1 to 2 cm, the functional dynamic range of SA node activation can extend over a larger area, reaching up to 3 cm or more. 

The SA node contains specialized pacemaker cells (P cells or "typical nodal" cells) at its center, surrounded by transitional cells, fibroblasts, endothelial cells, and extracellular scaffolding. These pacemaker cells are electrically insulated from surrounding atrial myocardium by strands of connective tissue, blood vessels, fibrotic tissue, and fat, which protect them from the suppressive effects of hyperpolarization from adjacent myocytes. The SA node connects to the atria through one or more discrete sinoatrial conduction pathways (exit pathways). 

The SA node generates spontaneous electrical activity through a "coupled-clock" mechanism involving both membrane ion channels (voltage clock) and rhythmic calcium release from the sarcoplasmic reticulum (calcium clock). This automaticity is modulated by autonomic nervous system input—sympathetic innervation from T1-T4 spinal nerves and parasympathetic innervation from the vagus nerve.