A small mass of specialized cardiac muscle cells in the mammalian heart, found in the wall of the right atrium near the opening for the vena cava. The cells initiate and maintain the heartbeat—their rhythmic electrical activity stimulates contractions of the atria, which in turn cause contractions of the ventricles (see atrioventricular node). The electrical characteristics of the pacemaker cells result from the interplay of their various ion channels. These allow spontaneous and repeated generation of action potentials, which spread to neighbouring cardiac muscle cells via gap junctions between neighbouring cells. In particular, the change in polarity during each action potential depends on an influx of calcium ions (Ca²⁺) through voltage-gated calcium channels, in contrast to the sodium ion channels in other cardiac muscle cells, neurons, and skeletal muscle. The interval between the action potentials of the pacemaker cells governs the rate of heartbeat, and is regulated by the autonomic nervous system. The sympathetic neurotransmitter noradrenaline speeds up heartbeat, whereas acetylcholine, released by parasympathetic nerve endings, has the opposite effect. Similar pacemakers occur in the hearts of other vertebrates.

2. An electronic or nuclear battery-charged device that can be implanted surgically into the chest to produce and maintain the heartbeat. These devices are used when the heart’s own pacemaker is defective or diseased.