@inbook{1120141997, author={Kim E. Barrett and Susan M. Barman and Scott Boitano and Heddwen L. Brooks}, title={Origin of the Heartbeat & the Electrical Activity of the Heart}, booktitle={Ganong’s Review of Medical Physiology, 25e}, publisher={McGraw-Hill Education}, address={New York, NY}, year={2018}, url={accesspharmacy.mhmedical.com/content.aspx?aid=1120141997} }