Mass of specialized cardiac muscle cells ("pacemaker cells") that spontaneously depolarize to initiate rhythmic contraction of the heart.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 805 (https://doi.org/10.1515/pac-2015-1202)