sinoatrial node

The sinus node (also called sinoatrial node, SAN) is a small strip of specialized cardiac muscle about 3 millimeters wide, 15 millimeters long, and 1 millimeter thick. The SA node has pacemaker activity that initiates the heart beat.The SA node is located in the superior posterolateral wall of the right atrium immediately below and slightly lateral to the opening of the superior vena cava. The cells in the SA node have almost no contractile muscle function, unlike the surrounding atrial muscle fibers. However, the sinus nodal cells connect directly with the atrial muscle fibers so that any action potential that begins in the sinus node spreads immediately into the surrounding atrial muscle.The term sinus venosus (SV) has sometimes been used to refer to the part of the developing heart's venous pole that has pacemaker activity. Eventually, in the adult heart, pacemaker activity is confined to only the small SAN.

Image source: Figure 5 in Three-dimensional and molecular analysis of the venous pole of the developing human heart.

Definition source: Guyton Medical Physiology Textbook