Pacemaker

Pacemaker, small mass of cells in the right atrium of the heart that gives rise to the electrical impulses that initiate contractions of the heart; also called sinoatrial (S-A) nodes. Under abnormal circumstances, other cardiac tissues may assume the pacemaker role. The artificial pacemaker is a battery-powered device that controls the beating of the heart by a series of rhythmic electrical discharges. If the electrodes that deliver the discharges to the heart are placed on the outside of the chest, it is called an external pacemaker. If they are placed within the chest wall, it is called an internal pacemaker.

See also: Heart.